CELL CYCLE AND CELL DIVISION (MITOSIS)

Introduction :

Growth and reproduction are characteristics of cells, indeed of all living organisms. All cells reproduce by dividing into two, with each parent cell giving rise to two daughter cells each time they divide. These newly formed daughter cells can themselves grow and divide, giving rise to a new cell population that is formed by the growth and division of a single parental cell and its progeny. In other words, such cycles of growth and division allow a single cell to form a structure consisting of millions of cells.

Cell division is of two types :

(1) Mitosis

(2) Meiosis

MITOSIS

Term mitosis was proposed by Flemming. Mitosis produces genetically identical cells, which are similar to mother cell.

Cause of mitosis :

(i) Kern plasm theory: Hertwig proposed kern plasm theory. According to this theory mitosis occurs due to disturbance in Karyoplasmic Index (KI) or Nucleocytoplasmic ratio of cell.

Karyoplasmic Index: 

• Karyoplasmic Index of small cell is high as they have less cytoplasm. Nucleus efficiently controls the activity of cytoplasm in small cells.

• In a large cell nucleus fail to control the activity of cytoplasm. To attain the control of nucleus on metabolism a large cell divides into two cells.

(ii) Surface-volume Ratio:

• Surface volume ratio is also considered as a cause of cell division. When a cell grows in size its volumes increases more than its surface. So a stage will reach when the surface area becomes insufficient to draw the material. At such critical stage, division of cell started.

CELL CYCLE

• Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place.

• All these processes, i.e.cell division, DNA replication, and cell growth. hence, have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes.

• The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.

• Although cell growth (in terms of cytoplasmic increase) is a continuous process.DNA synthesis occurs only during one specific stage in the cell cycle.

• The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control.

PHASES OF CELL CYCLE

• A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours.

• Yeast can progress through the cell cycle in only about 90 minutes. The time period of cell cycle is varied from organism to organism and also from cell type to cell type.

Cell cycle involves two stages -

(1) Interphase 

(2) Division phase/M-phase

1. Interphase :- This is phase between two successive M-phase. In interphase cell grows in size and prepares itself for next division. Interphase is most active phase of cell cycle. The interphase last more than 95% of the duration of cell cycle.

• A series of metabolic changes occurs during interphase in cell. These changes were not visible under microscope, So some scientist termed interphase as resting phase. It is the time during which cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.

Howard and Pelc classified interphase into three sub stages:- 

(i) G1-phase or Pre DNA synthesis phase (1st Gap phase)

• G1 phase corresponds to the interval between mitosis and initiation of DNA replication. During G1, phase the cell is metabolically active and continuous grows.
• During G1, most of cell organelles increases in cell and cell rapidly synthesizes different types of RNA and proteins. Due to availability of protein, synthesis of new protoplasm takes place in cell ard it starts growing in size. Cell grows maximum in G1 stage.

(ii) S- phase (DNA synthesis phase):

• Replication of nuclear DNA and synthesis of histone protein takes place in s-phase. Replication of cytoplasmic DNA may occur in any stage of cell cycle.

• During this time the amount of DNA per cell doubles. If the  Initial Amount of DNA is denoted as 2C then It increases to 4C. However, there is no increase in the chromosome number: if the cell has diploid or 2n number of chromosomes at G1, even after S-Phase the number of chromosomes remains the same, i.e., 2n.

• S-Phase marks the phase of DNA replication and chososome duplication (DNA content in chromosome become double).

• In animal cells, during the S-phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.

G2- phase (2nd Gap phase) or Post DNA synthesis phase (Pre mitosis phase)

Actual preparation (Final preparation) of M-phase occurs during this phase. Special materials required for M-phase are synthesized in G1 phase. eg. Tubulin protein. -(Required for formation of spindle fibres). Cell growth continues.

G0 phase-

•  Some cells in the adult animals do not appear to exhibit division (e.g. heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death.These cells that do not divide further exit G1 phase to enter an inactive  stage called quiescent stage (G0) of the cell cycle.

• Cells in this stage remain metabolically active but no longer proliferate (divided) unless called on to do so depending on the requirement of the organism.

Checkpoints of cell cycle :

• Cell cycle is running by a group of special proteins "Cyclins and Cdks (MPF). (Nurse, T.Hunt & Hartwell 2001 studies on saccharomyces)

• Cell cycle is running by a group of special proteins "Cyclins and Cdks.

• The activity of enzymes, known as cyclin dependant kinases. (Cdk's) regulates the cell cycle. Kinase is an enzyme that removes a phosphate group from ATP & add to another protein. The kinases involved in the cell cycle are called Cdks because they are activated when they combined with key protein called cyclin.

• At some check points a kinase enzyme combines with cyclin & this moves the cell cycle forwardly.
• G2-M transition is triggered by maturation promoting factor (MPF) formed by M-cyclin + CDK2.

2. Division phase :

Division phase or M-phase or mitotic phase lasts for only about an hour in the 24 hour duration of cell cycle of a human cell.

The M-phase represents the phase when the actual cell division or mitosis occurs.

In animals, mitotic cell division is restricted or only seen in diploid somatic cell except in some social insects. Against this, the plants can show mitotic division in both haploid and diploid cells.

This is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division.

Though for convenience mitosis has been divided into four stages of nuclear division, it is very essential understand that cell division is a progressive process and very clear-cut cannot be drawn between various stages.

The M-phase start with nuclear division, corresponding to the separation of daughter chromosome (Karyokinesis) and usually ends with division of cytoplasm (cytokinesis).

Mitosis is divided into the following four stages :
  *Prophase *Metaphase *Anaphase *Telophase 

1. Prophase :

Prophase which is the first stage of karyokinesis of mitosis follows the S And G2 phases of interphase.

In the S and G2 phase the new DNA molecules formed are not distinct but Intertwined.

Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation.

The centriole, which had undergone duplication during S-phase of interphase, now begins to move towards opposite poles of the cell.

Formation of astral ray occurs due to gelation of proteins around centrioles in animal cells.

Anastral and Amphiastral Mitosis: In higher plants, centrioles are absent and no asters are formed. Mitosis without asters is known as anastral mitosis. In animals, the asters are present and the mitosis is described as amphiastral or astral mitosis.

The completion of prophase can thus be marked by the following characteristic events:

•  Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.

• Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres forms mitotic apparatus.

• Cell at the end of prophas when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and nuclear envelope.

(2) Metaphase

The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell.

By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope.This then, is the stage at which morphology of chromosomes is most easily studied.

At This stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the microtubules) to the chromosomes that are moved into position at the centre of the cell.

Hence, the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.

Chromosomal fibres (discontinous/kinetochore which run from pole to centromere) and supporting fibres (continous/non-kinetochore, which run from pole to pole) arrange in cell.

Centromere lies at equator and arms of chromosomes remain directed towards poles.

The key features of metaphase are:

• Spindle fibres attach to kinetochores of chromosomes.
• *Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.

(3) Anaphase

Centromere of each chromosome splits simultaneously lengthwise (division of centromere). Sister chromatids separate from each other and separated each chromatid is now reffered to as individual chromosome.

Number of chromosome become double in cell.

As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.

The two new daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubule shorten due to depolymerisation of tubulin protein towards kinetochoric end. Because these microtubules are attached at the centromere region, the centromeres are pulled ahead of the arms. (Pulling).

The cell elongates as the nonkinetochore microtubules lengthen.

• Anaphase stage is characterised by the following key events:

1. Centromeres split and chromatids separate.
2. Chromatids (now referred as chromosomes) move to opposite poles.

(4) Telophase (Reverse prophase):

 At the beginning of the final stage of karyokines, i.e.  telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. The Individual chomosomes can no longer be seen and chromatin material tends to collect at each of the two poles. This in the stage which shows the following key events:

• Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
• Nuclear envelope develops around the chromosome dusters at each pole forming  two daughter nuclei.
• Nucleolus, golgi complex and ER reform.

CYTOKINESIS

Mitosis accomplishes not only the segregation of duplicated chromosome into daughter nuclei (Karyokinesis) but the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis at the end of which cell division gets completed.

In animals cytokinesis occurs by constriction & furrow formation. Microtubules and microfilaments arrange on equator to form midbody and at the periphery of the equator a contractile ring is formed that is made up of actin and myosin protein. 

Due to interaction between actin and myosin ring contract, thus a furrow forms from outside to inside in cell. Furrow deepens continuously and ultimately a cell divides into two daughter cells. In animals cytokinesis occurs in centripetal order.

Cytokinesis in plants takes place by cell plate formation because constriction is not possible due to presence of the rigid cell wall. Many golgi vesicles and spindle microtubules arrange themselves on equator to form phragmoplast. Fragmentes of ER may also deposit in phragmoplast. Membrane of golgi vesicles fuse to for a plate like structure called cell plate. Golgi vesicles secrete calcium and magnesium pectate. Further cell plate is modified into middle lamella. In plants, cytokinesis occurs in centrifugal order (cell plate formation is from center to periphery).

In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g. liquid endosperm in coconut).

SIGNIFICANCE OF MITOSIS

Development of an organism occurs by mitosis. Every organism starts its life from a single cell i.e. zygote. Repeated mitosis in zygote leads to the formation of the whole body.

The growth of multicellular organisms is due to mitosis.

Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.

A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the living of the gut, and blood cells are being constantly replaced.

Mitotic divisions in the meristematic tissues - the apical (primary meristem) and the lateral cambium (secondary meristem), result in a continue growth of plants throughout their life.

MODIFICATIONS OF MITOSIS

Free nuclear division :- Karyokinesis is not followed by cytokinesis as a result of which multinucleated condition arises.

Endomitosis :- This is duplication of chromosomes without division of nucleus. Endomitosis leads to polyploidy.

i.e. Increase in number of set of chromosomes. Colchicine induces polyploidy in plants. Colchicine is a mitotic poison as it arrests the formation of spindle fibres. 

Endoreduplication :- Endoreduplication is a modification of endomitosis. The polytene chromosomes are formed by the process of endoreduplication. In endoreduplication, the chromatids replicate but do not get seperated. This process is also known as polyteny.

Note :

AMITOSIS : It is a simple method of cell division which is also called direct cell division. In this division there is no differentiation of chromosomes and spindle. The nuclear envelope does not degenerate. The nucleus elongates and constricts in the middle to form two daughter nuclei. This is followed by a centripetal constriction of the cytoplasm to form two daughter cells, eg. Prokaryotes and Some unicellular eukaryotes.

CELL CYCLE AND CELL DIVISION (MEIOSIS)

Introduction :

Growth and reproduction are characteristics of cells, indeed of all living organisms. All cells reproduce by dividing into two, with each parent cell giving rise to two daughter cells each time they divide. These newly formed daughter cells can themselves grow and divide, giving rise to a new cell population that is formed by the growth and division of a single parental cell and its progeny. In other words, such cycles of growth and division allow a single cell to form a structure consisting of millions of cells.

Cell division is of two types :

(1) Mitosis

(2) Meiosis

MEIOSIS

"Term meiosis" was proposed by Farmer and Moore.

The specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis.

Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation restores the diploid phase. Meiosis occurs during gametogenesis, leads to the formation of haploid gametes.

The key features of meiosis are as follows:

Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.

Meiosis I:-

Heterotypic division or reduction division. It leads to reduction in chromosome numbers. Division of chromosome does not occurs in meiosis-1 land only segregation of homologous chromosomes takes place.

Meiosis-1 is initiated after the parental chromatids have replicated to produce identical sister chromatids at the S-Phase.

Meiosis-1 involves pairing of homologous chromosomes and recombination between non sister chromatids of homologous chromosome.

Meiosis II :

This is a homotypic division or equational division. It does not leads to any change in chromosome number.

Division of chromosome or centromere occurs during meiosis II. 

Four haploid cells are formed at the end of meiosis ll. All the four daughter cells produced by meiosis are genetically different from each other and also differ from the mother cell.

In meiosis, division of nucleus takes place twice but division of chromosome occurs only once in meiosis-Il.

Meiotic events can be grouped under the following phases :

Meiosis I                         Meiosis II 

      

Prophase I                       Prophase II

Metaphase I                    Metaphase II

Anaphase I                      Anaphase II

Telophase I                     Telophase  II

Interphase - same as in mitosis.

 Stages of meiosis I

1. Prophase -1:

Typically longer and more complex when compared to prophase of mitosis. Prophase l  is classified in five substages based on chromosomal behaviour :

(a) Leptotene = Chromatin threads condense to form chromosomes. Chromosomes are longest & thinest

Chromosomes become gradually visible under the light microscope.

All the chromosomes in nucleus remain directed towards centrioles, so group of chromosomes in nucleus appears like a bouquet. (Bouquet stage).

(b)  Zygotene or Synaptotene - Zygotene is characterized by pairing of homologous. chromosomes (Synapsis). Pairs of homologous chromosomes are called Bivalents or tetrads. However these are more clearly visiblr at  next stage (pachytene). A structure develops in between homologous chromosomes. Which is termed as synaptonemal complex.

 The 1st two stages of prophase I is relatively short lived compared to the pachytene.

( c) Pachytene (Thick thread) - Due to increased attraction, homologous chromosomes tightly coil around each other. Both the chromatids of each chromosome become distinct and are called sister chromatids.

During this stage, the four chromatids of each bivalent chromosome become distinct and clearly appeared as tetrad.

Recombination nodules between nonsister chromatids of homologous pair develop and these non sister chromatid exchange their parts. i.e. crossing over.

Crossing over leads to recombination of genetic material on the two chromosomes.

Crossing over is an enzyme mediated process and the enzyme involved is called recombinase (Endonuclease+ligase).

Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.

(d) Diplotene - The begining of diplotene is recognised by dissolution of synaptonemal complex. Homologous chromosomes start repulsing each other so X-shape structures.appeared called chiasmata.

Diplotene may last long up to months or years in oocytes of some vertebrates (Dictyotene).

(e) Diakinesis - It is final stage of meiotic prophase I. Marked by terminalization of chiasmata (Chiasmata open in zip like manner).

Chromosome are fully condensed and meiotic spindle is assembled to prepare the homologous chromosome for separation.

Centrioles move towards the opposite poles.

By the end of diakinesis nucleolus disappear and the nuclear envelope also breaks down.

Diakinesis represents transition to metaphase.

2. Metaphase I:

Bivalents arrange on equator (congression) of cell to form metaphase plate. The microtubules (spindle fibres) from the opposite poles of the spindle attach to the pair of homologous chromosome with one kinetochore of each chromosome.

Two types of spindle fibres appear in the cell:

(i) Chromosome / Kinetochore Spindle fibres

(ii) Supporting/Continuous /non-kinetochore Spindle fibres

3. Anaphase I:

Due to shortening of kinetochore/chromosomal fibres homologous chromosomes segregate from each other and move towards the opposite poles. Sister chromatids remain associated at their centromeres (i.e. chromosomes remain in double chromatid stage).

Anaphase I is characterised by segregation or disjunction of chromosomes. Division of centromere is absent.

4. Telophase I :

The nuclear membrane and nucleolus reappear. Although in many case the chromosomes do undergo some dispersion, but they do not reach the extremely extended state of the interphase nucleus.

Cytokinesis follows telophase-l and a diploid (2n) cell divides into two haploid (n) daugther cells. This is called as dyad of cells.

Interkinesis :- Gap between meiosis l and meiosis II is called Interkinesis. Preparations of meiosis ll occur during interkinesis. It is like interphase of mitosis but replication of DNA is absent in interkinesis.

Interkinesis is generally short lived. Interkinesis is followed by prophase-Il, a much simpler prophase than prophase-l.

Stages of Meiosis - II

1. Prophase II :

Meiosis ll dis initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis l, meiosis ll  resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.

2. Metaphase II

At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.

3. Anaphase II:

It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell by shortening of microtubules attached to kinetochores.

4. Telophase II:

Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e. four haploid daughter cells.

Significance of Meiosis :

(1) Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process (per se paradoxically) results in reduction of chromosome number by half.

(2) It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.

NOTE : Prophase-I further subdivide into five phases based on the chromosomes behaviour.

• Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organism.
• Meiosis involves pairing of homologous chromosomes and recombination between them.
• Chiasmata formation is the result of crossing over.
• Meiosis increases the genetic variability in the population of organism from one generation to next.

CHROMOSOME

GENERAL INTRODUCTION :

At the time of cell division the chromatin material get condensed to form chromosomes, thus chromosome is highly condensed form of the chromatin. Chromosomes are not visible during interphase stage but during different stages of cell division, cells show structured chromosomes in place of the nucleus.

Chromosomes can be best studied at metaphase stage because size of chromosomes is the shortest during metaphase (Shape of chromosome is studied at Anaphase stage)

The number of chromosomes in a gamete is called "Genome" or "A complete set) of chromosomes inherited as a unit from one parent is known as genome.

A single human cell has approximately two metre long thread of DNA distributed among its 46 (23 pairs) chromosomes.

TYPES OF CHROMOSOMES ON THE BASIS OF POSITION OF CENTROMERE

i) Telocentric - When centromere is terminal or Located at the top of chromosome.

ii) Acrocentric- When the centromere is sub-terminal or located near the tip.

iii) Metacentric - When the centromere is located at mid of the chromosome

iv) Sub metacentric - When the centromere located near centre or mid point of chromosome

The ratio of length of the long arm to the short arm of a chromosome is called arm ratio A ratio is maximum in acrocentric chromosome.

Karyotype is external morphology of all Chromosomes of a cell which is specific for each species of living organisms. Karyotype can be studied in metaphase of mitosis. Karyotype includes the number of chromosomes, relative size, position of centromere, length of the arms, secondary constrictions and banding patterns.

Idiogram :-Diagrammatic representation of Karyotype. In idiogram chromosomes are arranged in decreasing order of size. Sex chromosomes are placed in last. Idiogram is specific for every species.

STRUCTURE OF CHROMOSOME

1. Chromatid - At metaphase stage each chromosome is consist of two cylindrical structures - called chromatids Both sister chromatids are Joined together by a common centromere. A chromosome, may have single chromatid (in Anaphase or Telophase) or two chromatids in prophase and metaphase).

•  Each chromatid is consist of a single long thread of DNA associated with histone. Non histone proteins and RNA are also present

2. Centromere:

Each chromosome (at prophase or metaphase) is consist of two chromatids. Both the chromatids of a chromosome are joined or connected by a structure called Centromere. At centromere two protein discs are present which is called Kinetochore.

Kinetochores constitute the actual site of attachement of spindles to chromosomes during cell division.

At the region of centromere the chromosome is comparatively narrower than remaining part of chromosome thus it is termed as Primary constriction.

3. Secondary constriction : Besides primary constrictions, other constriction may also choose which are knoWn as secondary constriction. These constriction are non staining and foundata constant location.

Secondary constriction is also known as NOR Nucleolar organizer region) (3.14.15.21.22 chromosomes in human)

4. Satellite : Part of chromosome remains after the NOR Is known as chromosomes satellite.

5. Telomere : Chromosomes have polarity and polar ends of chromosomes are known as Telomeres.

 Telomere prevents fusion of one chromosomes to other chromosome. Telomere rich in Guanine base.(5 TTAGGG-3).

Enzyme Telomerase synthesize telomere part of chromosome which is a Ribonucleoprotein Telomeres of chromosomes becomes shorter during ageing process.

SPECIAL TYPE OF CHROMOSOMES

Salivary gland chromosome :- This type of chromosome was discovered by E.G. Balbiani, in Chironomia larva.

This chromosome is called Polytene chromosome, because number of chromatids are very high

Swollen areas present at some places in polytene chromosome, which are called as Balbiani rings or puffs. These puffs helps in synthesis of RNA and proteins.

Salivary gland chromosome concerns with metamorphosis and moulting process of insect larva.

 Lamp brush chromosome :- Discovered by Flemming and Ruckert from oocytes of vertebrates (Amphibia) during diplotene stage of cell division. These chromosomes look like lamp - brush, thus called as lamp brush chromosomes.

Axis of lamp brush chromosome is consist of DNA, while matrix is consist of RNA & proteins.

Lamp brush chromosome is concerned with "Vitellogenesis" (Yolk formation)

RIBOSOME (ENGINE OF CELL) and OTHER ORGANELLES

Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid RNA) and proteins and are not surrounded by any membrane.

Except mammalian RBC all living cells have ribosomes. (Both prokaryotes & Eukaryotes) Ribosomes are smallest cell organelles.

 Ribosomes are also called as "Organelle within organelle" and "Protein factory of cell"

Types of Ribosomes :

1) Eukaryotic ribosomes :- 80S- Occur in cytoplasm of eukaryotic cells.

2) Prokaryotic ribosomes :- 70 S- Occur in cytoplasm and associated with plasma membrane of prokaryotic cell. Their size is 15 to 20 nanometre.

70 S ribosome also present in mitochondria and chloroplast of eukaryotes.

S=Svedberg unit or Sedimentation rate. It indirectly is a measure of density and size.

Each ribosome composed of two subunits i.e, larger and smaller subunits

80 S - 60 S+ 40s 70 S - 50 S + 30 S

Magnesium ion is essential groups of for the binding the ribosome subunits. Mg form ionic bond with phosphate r- RNA of two subunits. Minimum 0.001 M Mg concentration is required for structural formation of ribosomes.

Chemical Composition of Ribosomes :

70 S 60% r- RNA + 40% proteins 80 S - 60 S - 40 S - 40% r-RNA + 60% proteins rRNA 28S, 5.8 S, 5 S rRNA 18S 50 S - rRNA 23S, 5S 30 S = RNA 16 S

At the time of protein synthesis, several 70 S ribosomes become attached to m-RNA with the help of smaller subunits. This structure is called polyribosome or polysome vs Ribosome Large subunit (50s) contains peptidyl transferase enzyme (23S rRNA) which helps in the formation of peptide bond during protein synthesis. This is an example of Ribozyme. (Noller 1992)

CYTOSKELETON

An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton The cytoskeleton in a cell are Involved in many functions such as mechanical support motility maintenance of the shape of the cell.

MICROTUBULES

Microtubules are composed of protein, Tubulin (Size 25 nm).

In plants microtubules often found associated with cell wall. Probably these transport cell wall material from Golgi body to outside of cell. During cell division these microtubules form spindle fibers.

MICROFILAMENTS

They are composed of contractile protein Actin which concern with muscle contraction, Microtubules and microfilament are part of cytoskeleton-base of cell. [Size 6-7 nm)

INTERMEDIATE FILAMENT

Intermediate filaments has size/diameter in between microfilaments and microtubules. These filaments form basket like structure around the nucleus. [Size 8-12 nm]

CILIA AND FLAGELLA

Cilia (sing. cilia) and flagella (sing: flagellum) are hair-like outgrowths of the cell membrane. Cilia are smal structures which work like oars, causing the movement of either the cell or the surrounding fluid Flagella are comparatively longer and responsible for cell movement. The bacteria (prokaryotic cell also possess flagella but these are structurally different from that of the eukaryotic flagella

The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane.

Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array

(9 doublet + 2 singlet)

Arms of A tubules consist of an enzymatic protein dynein similar to myosin of muscle cells. Dynein have ability of hydrolysis of ATP & Liberates energy for ciliary and flagellar movement.

The central tubules are connected by bridges and is also enclosed by a central sheath, which is one of the tubules of each peripheral doubles by radial spoke. Thus there are nine radial

connected to spokes, The peripheral doublets are also interconnected by lakers. Both the cilium & flagella emerge from centriole like structure called the basal bodies or blepharoplasty.

CENTROSOME and CENTRIOLES

Centrosome is absent in higher plants.

Centrosome containing twa centrioles (diplosome) located just outside the nucleus and lie at right angle 1909 each other. Each centriole is surrounded by amorphous pericentriolar materials.

Centrioles are membraneless cylindrical structure which exhibicari wheel structure in transverse section Centriole mainly consist of 9 evenly spaced peripheral triplet fibrils of tubulin. These triplets are linke with the help of A-C linker.

The central part of the centriole is proteinaceous and called the hub, which Is connected with peripher triplets by radial spokes made of protein (9 + 0 arrangement) Centrioles are self duplicating units.

Function :

In animal cells, centrioles play important role in cell division by arranging spindle fibres between two poles of cell. The location of centrioles during cell division decides the plane of division. The plane of division is always at right angle to the spindle.

Centrioles form the basal body of cilia or flagella.

MICRO-BODIES

These are many, membrane bound minute vesicle contain various enzyme that are present in both plant and animal cells.

1) Peroxisomes and ribosomes :

These are found in both plant and animal cells. Peroxisomes contain catalase enzyme which is concerned with peroxide (HO Metabolism. Catalase degrade the H,O into water and oxygen

In plants, peroxisomes occurs in cells of green tissues and concerned with photorespiration (glycolate pathway).

Peroxisomes are also involved in B-oxidation of fatty acids,

(2) Glyoxysomes :

Glyoxysomes occurs only in plants especially in fatty seeds (castor seed, ground nut seed etc.).

Glyoxysomes are considered as a highly specialised peroxisomes Glyoxylate acid cycle takes place in glyoxysomes. This cycle convert fats into carbohydrates.

NUCLEUS

INTRODUCTION:

Nucleus as a cell organelle was first described by Robert Brown as early as 1831. Later the material of the nucleus stained by the basic dyes (Acetocarmine was given the name chromatin by Flemming.

"Nucleus is double membrane bound dense protoplasmic body, which controls all cellular metabolism and encloses the genetic information of cell".

Nucleus is consider as controller or director of cell. Importance of nucleus in control of heredity. growth and metabolism was Largest alga). experimentally proved by Hammerling Experiment was an Acetabularia a single cell

Generally eukaryotic cell contain at least one nucleus but nucleus is absents in mature phloem sieve tube elements and mature RBCs of mammals.

Dikaryotic (Paramoecium) and multikaryotic cells are also known.

STRUCTURE OF INTERPHASE NUCLEUS :

i) Interphase nucleus : Nucleus of cell when it is not dividing

ii) Nuclear membrane or karyotheca.

iii) Nuclear matrix/Nucleoplasm/Karyolymph/Sarcoplasm.

iv) Chromatin net

v) Nucleolus/little nucleus/Ribosome factory

i) Nuclear membrane : 

• Electron microscope has revealed that the nuclear envelope, which consists of two preme ne n a space between (10 to 50 um) called the perinuclear space. These actors a s a batter between the material present.inside the nucleus and that the cytoplasm.

• The intermembrane usually remains continuous with the endoplasmic reticulum and also beschloss

• At a number of places the nuclear envelope is interrupted by minute pores, which ats lormed bu the Son of two membranes These nuclear pores otet passages through which movement END and protein molecules takes place in both directions between the nucleus and the cytoplasm.

• The nucleus he nuclear pore, is guarded by a octagonal discold structure of nucleoplasmin protein.

• The inner side of Inner nuclear membrane is lined by nuclear lamina This structure is formed by filaments of lamin protein.

ii) Nucleoplasm or Karyolymph :

Nucleoplasm or Nuclear sap is a ground substance of nucleus, which is a complex number of chemicals in nucleotides, nucleosides. ATP proteins & enzymes Chromatin net and nucleolus are components of nucleoplasm.

A) Chromatin net :- (Term Given by Fleming)

•  Interphase ucles has a loose and indistinct network of nucleoprotein fibers called chromatin, which Embedded in nucleoplasm. Chromatin net is mainly formed of DNA and histone per complexes.

• Chromatin fibres contain genetic information and condensed to form chromosomes during cell division.

•  During different stages of cell division cells how structured chromosomes in place of nucleus.

•  Chemically chromatin consists of DNA, RNA Histone protein (asic proteins, rich in arginine and in and non histone proteins:

Chromatin net has two type of chromatins:

A) Euchromatin : This is lightly stained and diffused part of chromatin. Which is transcriptionally or genetically more active.

B) Heterochromatin : This is dark stained thick and condensed part genetically less active or inactive chromatin.

i) Constitutive heterochrómatin:- Occurs in all cells in all stages eg. centromeric region.

ii) Facultative heterochromatin :- Occurs in some cells in some stages ag barr body in females,

B) Nucleolus :-

The nucleoli are spherical and membraneless structure so that the content of nucleous is continuous with the rest of the nucleoplasm.

It is a site for active ribosomal RNA (r-RNA) synthesis.

Nucleolus usually attached to chromatin (or chromosomes) at specific site called Nucleolar organiser region/NOR.

Nucleolus is called Ribosome factory of cell.

• Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.

Plastids

Plastids are found in all plant cells and in euglenoides. These are easily observed under the microscope as the sone specific pigments, thus imparting specific colours to the plants. Based on the presence or absence and type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.

TYPES OF PLASTIDS 

Chromoplasts : In chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives yellow, orange or red colour to the part of the plant. Chlorophyll either absent or occur in very less amount Chromoplasts occurs mainly in pericarp and petals. Red colour of tomatoes is due to the red pigment "Lycopene" of chromoplasts.

• Chromoplasts occur in petals but colour in petals is mainly due to water soluble pigments which are found in cell sap. eg. Anthocyanin

Chloroplasts :- The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.

Leucoplasts:- The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients.

Amyloplasts store carbohydrates (starch), e.g. potato: elaioplasts store oils and fats whereas the elaioplasts store proteins. Pigments and lamellar structure absents in Leucoplasts Generally occurs in non green and underground plant cells.

• Different types of plastids may transform from one form to another. Because genetic material is similar

Number, Shape & Size of chloroplasts :

• Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves.

• Number varies from 1 per cell of the Chlamydomonas a green alga.to 20-40. T cell in the mesophyll.

• These are lens-shaped, oval spherical discoid. or even ribbon shaped,

• Length and width are also variable.

          Length= 5-10 m

           Width= 2-4 um

STRUCTURE OF CHLOROPLAST

Membrane : Like mitochondria the chloroplast are also double membrane bound Out of the two, the inner membrane is relative less permeable (Outer membrane contain porins

• The space limited by the inner membrane is caled the stroma.

Components of stroma:

(a) Thylakoids : In the stroma a number of organised flattened membranous sacs are present called thylakoids.

Thylakoids are arranged in stacks like the piles of coins called grana (singular granum or the intragranal thylakoids.

Each chloroplast! contains about 40-60 granum.

Stroma lamellae or Fret channel or Stroma thylakoids are flat membranous tubules connecting the thylakoids of the different granum.

The membrane of the thylakoids enclose a space called lumen.

Chlorophyll (photosynthetic pigments) are present in the thylakoids membrane.

A photosynthesis functional unit (Located in thylakoids membrane) contains of about 250 to 400 molecules of various pigments (Chl-a, Chl-b. Carotenes, Xanthophylls etc.) is called as Quantasome.

(b)Enzymes :  The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrate. die enzymes of Calvin cycle or Dark reaction and protein synthesis.

(c) DNA :Stroma contain small double-stranded circular DNA molecules.

(d) Ribosome The Ribosome of the chloroplast are smaller (70s) than the cytoplasmic ribosOmes (80s) Chloroplasts have their own genetie system & complete protein synthesis machinery (ds - DNA, RNA, Ribosome, Enzymes Amino acids) but enzymes for photosynthesis are synthesised by nucleus and chloroplast thus chloroplasts are also called as semi autonomous organelle of the cell

FUNCTION :

Photosynthesis : The chloroplasts trap the light energy of sun and transform it into the chemical energy in the form glucose.

BIOGENESIS 

(1)From Proplastid

(2)  From binary fission of pre-existing plastids.

 ORIGIN : Endosymbiotic origin by a cyanobacterium.

mitochondria

fig : Mitochondria

Number of mitochondria depends upon physiological activity of cell.

• One in Microasterias, Chlorella fusca (alga)
• All the mitochondria present in a cell are collectively called chondriome.
• Usually plant cells have fewer mitochondria as compared to animal cell.
• In higher animals maximum mitochondria are found in flight muscles of birds.
• Mitochondria are differ in size and shape and can make its shape sausage or cylindrical.
• Diameter 0.2-1.0 um (average 0.5 um), length 1.0-4.1 uum

Mitochondria is also named as - 

• Power house of cell or ATP-mill in cell
• Cell within cell
• Most busy and active organelle in cell Semi autonomous cell organelle,
• Endosymbionts of cell

STRUCTURE

• Mitochondria unless specifically stained are not easily visible under the microscope. Mitochondria are stained by Janus green B.
• Mitochondria is covered by double unit membrane, the outer membrane is smooth and inner one folds into several cristae. Outer membrane has more phospholipids (Phosphatidyl choline) and cholesterol as compared to inner membrane. Phospholipid in inner membrane is mainly phosphatidyl glycerol and Inner membrane have more protein
• The outer membrane and the inner membrane dividing its lumen distinctly into two aqueous.compartments.ie.. the outer compartment and the inner compartment. The inner compartment filled with a dense homogenous substance is called the matrix. The outer membrane forms the continuous limiting boundary of the organelle
• The two membranes have their own specific enzymes associated with the mitochondrial function
• Both membrane are separated by a space called peri mitochondrial (Intermembrane) space
• Inner membrane is folded into a number of finger like cristae.
•  In metabolically active mitochondria number of cristae is higher Many electron carrier cytochromes are arranged in a definite sequence in Inner membrane of mitochondria, which forms Electron transport system (ETS).
• Inner membrane is studded with pin head particles called oxysomes or elementary particles on particles or ATP Synthase. These particles first described by Fernandez Moran.
• Head of Oxysomes or F, is concerned with Oxidative phosphorylation (formation of ATP by energy of oxidation).
•  Mitochondrial matrix have enzyme for Krebs cycle (Aerobic respiration). Beside these enzymes matrix have a complete protein synthesis apparatus (Ribosome (70-s), DNA, few RNA's & enzymes) so mitochondria called as semi autonomous cell organelles.
• Single double stranded and circular naked DNA present in mitochondrial matrix.
•  Mitochondrial DNA is 1% of total DNA in a cell. It is rich in GC content
• Mitochondrial DNA can code the synthesis of some types of proteins. Rest of the proteins and enzymes of mitochondria are synthesized under the control of nuclear genes.
• Enzymes for replication and transcription of DNA like DNA- polymerase and RNA- polymerase are found in mitochondrial matrix.

FUNCTION OF MITOCHONDRIA:

Mitochondria are site of aerobic respiration and ATP production.

 

Biogenesis of mitochondria -

• New mitochondria arise from division of pre-existing mitochondria (Mitochondria divide By binary fission
• Endosymbiotic origin from prokaryotic cells.
• Type of DNA (DNA sequences, double stranded, circular, G-C rich).
• Type of ribosome (70s).
• Divide by binary fission.

Lysosome

These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus

The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes hydrolases - lipases, proteases, carbohydrases) optimally active at the acidic pH (pH=5). These enzymes. capable of digesting carbohydrates, proteins, lipids and nucleic acids.

fig : Lysosome

With the exception of mammalian RBC they were reported in all animal cells.

In plant cells large central vacuole functions as Lysosome. So in higher plants lysosomes are less frequent. But number of lysosomes is high in fungi (bcz their mode is absorpton). 

• Periplasmic Space - space between cell wall and cell membrane in bacteria.may play similar role.
• Lysosomes are spherical bag like structures, which is covered by single unit membrane. They are large sized in Phagocytes (WBC).
• Lysosomes are filled with 50 different type of digestive enzymes termed as Acid hydrolases digestion of all type of macromolecules.
• These acid hydrolases function in acidic medium (pH=5). Membrane of lysosome has an active H pump mechanism which produce acidic pH in lumen of lysosome.
• "Lysosomes are highly polymorphic.cell organelle. Because, lysosomes have different physiological states.

TYPES OF LYSOSOMES

1. Primary Lysosomes or storage granules. These lysosomes store enzyme Acid Hydrolases in the inactive form. (Enzymes synthesized on ribosomes in cytoplasm) these are newly formed lysosome
2. Digestive vacuoles or Heterophagosome - These lysosome formed by the fusion of primary lysosomes and phagosomes. These are secondary Lysosomes
3. Residual bodies :- Lysosomes containing undigested material are called residual bodies. These may be eliminated by exocytosis
4. Autophagic Lysosomes or autophagosomes :- Lysosomes containing cell organelles to be digested are known as Autophagosomes.

FUNCTIONS

(1) Intracellular digestion :-

a) Heterophagy :- This is digestion of foreign materials received in cell by phagocytosis and pinocytosis:

b) Autophagy :- Digestion of old or dead cell organelles. Autophagy also takes place during starvation of cell.

(2) Extracellular digestion :

Lysosomes of osteoclast (bone eating cells) dissolve unwanted part of bones.

(Extracellular digestion also occurs by fungal lysosomes)

(3) Cellular digestion (Autolysis) - Sometimes all lysosomes of a cell burst to dissolve the cell completely. (so Lysosome caled as suicidal bags of cell).

   Old cells are removed by autolysis. unwanted organs of embryo are destroyed by autolysis Cathepsin of lysosome digests the tail of tadpole of frog during metamorphosis.

Vacuoles

fig : Vacuoles

• The vacuole is the membrane-bound space found in the cytoplasm. It.contains water, sap, excretory product and other materials not useful for the cell.
• The vacuole is bound by a single membrane called tonoplast.
• In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell.
• In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole hence their concentration is significantly higher in the vacuole than in the cytoplasm.
• In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.

Golgi Body or Golgi Complex

Camillo Golgi (1898) first observed densely stained reticular structure near the nucleus. These were later named Gogi bodies after him.

Golgi body also named as :

Lipochondria ( rich in lipids)

Dictyosome (plant golgi body)

 The cytoplasm surrounding Golgi body have fewer or no other organelles. It is called Golgi ground substance or zone of exclusion.

STRUCTURE :

• Golgi complex is made up of three parts - 
• (1) Cisternae :- These are flat disc shaped, sacs like structure many cisternae are arranged in a stack (parallel to each other). Diameter 0.5 um to 1.0 um. Dense opaque material inside cisternae is called Nodes.
• Varied number of cisternae are present in Golgi complex.
• The Golgi cisternae are concentrically arranged near the nucleus.
• Convex surface of cisternae which is towards the nucleus is called cis- face or forming face.
• Concave surface of cisternae which is towards the membrane is called Transface or maturing face
• The cis and trans faces of the organelle are entirely different but inner connected.
• (2) Tubules :- These are branched and irregular tube like structures associated with cisternae.
• (3) Vesicles :- Transition vesicle and Mature vesicle.

fig : Golgi Complex

FUNCTIONS OF GOLGI COMPLEX

Chief function of golgi body is secretion (export) of macromolecules

(1) Cell Secretion :- Chief function of golgi body is secretion (export) of macro molecules.

Secretion involve three steps:

(a) Golgi body receives the materials from E.R. through it's cis - face.

(b) These materials are chemically modified by golgi body. For e g. glycosylation or glycosidation of proteins and lipids takes place in golgi body and it yields glycoproteins and glycolipids.

(c) After chemical modifications materials are packed in vesicles. These vesicles are pinched off from trans face of golgi body and discharged out side the cell

• All the macromolecules which are to be sent out side the cell, move through the golgi body. So golgi body is termed as "Director of macromolecular traffic in cell or middle men of cell.

  The golgi apparatus principally performs the function of packaging materials, to be delivered either to the intra cellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face. This explains why the golgi apparatus remairis in close association with the endoplasmic reticulum.

A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans face. Golgi apparatus is the important site of formation of glycoproteins and glycolipids.

(2) Formation of Lysosome = It is collective function of golgi body and E.R.

(3) Synthesis of cell wall Material (Polysaccharide synthesis).

(4) Cell plate formation (Phragmoplast) during cell formation.

(5) Formation of acrosome during spermiogenesis. (formation of male gametes).

Endoplasmic Reticulum

    Electron microscopic studies of eukaryotic cells reveal the presence of a network of reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER) 

Components of E.R. :

(1) Cisternae. These are long flattened and unbranched units arranged in stacks.

(2) Vesicles - These are oval membrane bound structures.

(3) Tubules - These are irregular, often branched tubes bounded by membrane. Tubules may free or associated with cisternae.

• Structure of E.R. is like the golgi body but in E.R. cisternae, vesicles and tubules are isolated in cytoplasm and these do not form complex.
• Golgi body is localised cell organelle while ER is widespread in cytoplasm. Er is often termed as "System of Membranes''.

• ER divide the intracellular space into two distinct compartment i.e., Luminal (inside ER) and extra luminal (cytoplasm) compartments.

fig : E.R.

Modifications of E.R.(Endoplasmic Reticulum) :

(1)  Sarcoplasmic Reticulum (S.R.) :- These smooth E.R. occurs in skeletal and cardiac muscles. S.R. Stores Ca+2 and energy rich compounds required for muscle contraction.

(2)   Microsomes - These are pieces of ER with associated ribosomal particles. These can be obtained by fragmentation and high speed centrifugation of cell. They do not exist as such in the living cell. Scientist used microsome for invitro protein synthesis study.

FUNCTIONS OF E.R.(Endoplasmic Reticulum) :

(1) Mechanical support :- Microfilaments, Microtubules and E.R. forms endoskeleton of cell

(2) Intracellular exchange :-E.R. forms intracellular conducting system. Transport of materials in cytoplasm from one place to another may occurs through the E.R.

• At some places E.R. is also connected to P.M. Sober can secrete the materials outside the cell.

(3) Rough E.R. :- Provides site for the protein synthesis, because rough E.R., has ribosomes on its surface.

(4) Lipid Synthesis:-Lipids (cholesterol & phospholipids) synthesized by the agranular portion of E.R. (Smooth E.R.).

(5) ER also helps in the synthesis of lipoproteins and glycogen.

(6) Cellular metabolism ;- The membranes of the reticulum provides an increased surface for metabolic activities within the cytoplasm. 

(7) Formation of nuclear membrane :-Fragmented (Mainly rough ER )vesicles of disintegrated nuclear membrane and ER elements arranged around the chromosomes to form a new nuclear membrane during cell division.

Cytoplasm

Cytoplasm Definition

• Cytoplasm refers to the fluid that fills the cell, which includes the cytosol along with filaments, proteins, ions and macromolecular structures as well as the organelles suspended in the cytosol.
• In eukaryotic cells, cytoplasm refers to the contents of the cell with the exception of the nucleus. Eukaryotes have elaborate mechanisms for maintaining a distinct nuclear compartment separate from the cytoplasm. Active transport is involved in the creation of these subcellular structures and for maintaining homeostasis with the cytoplasm. For prokaryotic cells, since they do not have a defined nuclear membrane, the cytoplasm also contains the cell’s primary genetic material. These cells are usually smaller in comparison to eukaryotes, and have a simpler internal organization of the cytoplasm.

Structure of Cytoplasm

• The cytoplasm is unusual because it is unlike any other fluid found in the physical world. Liquids that are studied to understand diffusion usually contain a few solutes in an aqueous environment. However, the cytoplasm is a complex and crowded system containing a wide range of particles – from ions and small molecules, to proteins as well as giant multi protein complexes and organelles. These constituents are moved across the cell depending on the requirements of the cell along an elaborate cytoskeleton with the help of specialized motor proteins. The movement of such large particles also changes the physical properties of the cytosol.
• The physical nature of the cytoplasm is variable. Sometimes, there is quick diffusion across the cell, making the cytoplasm resemble a colloidal solution. At other times, it appears to take on the properties of a gel-like or glass-like substance. It is said to have the properties of viscous as well as elastic materials – capable of deforming slowly under external force in addition to regaining its original shape with minimal loss of energy. Parts of the cytoplasm close to the plasma membrane are also ‘stiffer’ while the regions near the interior resemble free flowing liquids. These changes in the cytoplasm appear to be dependent on the metabolic processes within the cell and play an important role in carrying out specific functions and protecting the cell from stressors.
• The cytoplasm can be divided into three components:

• The cytoskeleton with its associated motor proteins
• Organelles and other large multi-protein complexes
• Cytoplasmic inclusions and dissolved solutes

Cytoskeleton and Motor Proteins

• The basic shape of the cell is provided by its cytoskeleton formed primarily by three types of polymers – actin filaments, microtubules and intermediate filaments.
• Actin filaments or microfilaments are 7 nm in width and are made of double stranded polymers of F-actin. These filaments are associated with a number of other proteins that help in filament assembly and are also involved in anchoring them close to the plasma membrane. This cytoplasmic location helps the microfilaments become involved in rapid responses to signal molecules from the extracellular environment and produce cellular responses through signal transduction or chemotaxis. In addition, myosin, an ATP-based motor protein transmits cargo and vesicles along the microfilament and is also involved in muscle contraction.
• Microtubules are polymers of α and β tubulin, which form a hollow tube by the lateral association of 13 protofilaments. Each protofilament is a polymer of alternating α and β tubulin molecules. The inner diameter of a microtubule is 12 nm and its outer diameter is 24 nm.

• 
• Microtubule structure
• Microtubules radiate towards the periphery of the cell from microtubule organizing centers (MTOCs) located close to the nucleus, and provide structure and shape to the cell.
• 

• Fluorescent Cells
• This image shows the nucleus in blue, the actin filaments on the cell periphery are labeled red and the extensive microtubule network is marked green. The cytoplasm undergoes rapid reorganization during cell division with microtubules forming the spindle, which binds to chromosomes and segregates them into two daughter cell.

• 

• Kinetochore

• Similar to the previous image, chromosomes are stained blue and microtubules are green. Tiny red dots are kinetochores.
• Microtubules are involved in cytoplasmic transport, chromosome segregation and in forming structures such as cilia and flagella for cellular movement.
• Intermediate filaments are larger than microfilaments but smaller than microtubules and are formed by a group of proteins that share structural features. Though they are not involved in cell motility, they are important for cells to come together as tissues and to remain anchored to the extracellular matrix.

Organelles and Multi-protein Complexes

• Most eukaryotic cells have a number of organelles that provide compartments within the cytoplasm for specialized microenvironments. For instance, lysosomes contain a number of hydrolases in an acidic environment that is ideal for their enzymatic activity. These hydrolases are actively transported into the lyosome after being synthesized in the cytoplasm.  Mitochondria, while containing their own genome, also need many enzymes synthesized in the cytosol, which are then selectively moved into the organelle. These organelles are placed in specific locations due to the physical gel-like nature of the cytoplasm and by anchoring to the cytoskeleton.
• In addition, the cytoplasm also plays host to multi-protein complexes like the proteasome and ribosomes. Ribosomes are large complexes of RNA and protein that are important for the translation of mRNA code into amino acid sequences of proteins. Proteasomes are giant molecular structures about 20,000 kilodaltons in mass and 15 nm in diameter. Proteasomes are important for targeted destruction of proteins that are no longer needed by the cell.

Cytoplasmic Inclusions

• Cytoplasmic inclusions can include a wide range of biochemicals – from small crystals of proteins, to pigments, carbohydrates and fats. All cells, especially in tissue like the adipose, contain droplets of lipids in their triglyceride form. These are used to create cellular membranes and are an excellent energy store. Lipids can generate twice as many ATP molecules per gram when compared to carbohydrates. However, the process of releasing this energy from triglycerides in intensive in oxygen consumption and therefore the cell also contains stores of glycogen as cytoplasmic inclusions. Glycogen inclusions are particularly important in cells like the skeletal and cardiac muscle cells where there can be a sudden increase in demand for glucose. Glycogen can be quickly broken down into individual molecules of glucose and used in cellular respiration before the cell can obtain more glucose reserves from the body.
• Crystals are another type of cytoplasmic inclusion found in many cells, and have special function in cells of the inner ear (maintaining balance). Presence of crystals in cells of the testis appears to be linked with morbidity and infertility. Finally, the cytoplasm also contains pigments such as melanin, which lead to the pigmented cells of the skin. These pigments protect the cell and internal body structures from the deleterious effects of ultraviolet radiation. Pigments are also prominent in the cells of the iris that surround the pupil of the eye.
• Each of these components affects the functioning of the cytoplasm in different ways, making it a dynamic region that plays a role in, and is influenced by the cell’s overall metabolic activity.
• Functions of Cytoplasm
• The cytoplasm is the site for most of the enzymatic reactions and metabolic activity of the cell. Cellular respiration begins in the cytoplasm with anaerobic respriration or glycolysis. This reaction provides the intermediates that are used by the mitochondria to generate ATP. In addition, the translation of mRNA into proteins on ribosomes also occurs mostly in the cytoplasm. Some of it happens on free ribosomes suspended in the cytosol while the rest happens on ribosomes anchored on the endoplasmic reticulum.
• The cytoplasm also contains the monomers that go on to generate the cytoskeleton. The cytoskeleton, in addition to being important for the normal activities of the cell is crucial for cells that have a specialized shape. For instance, neurons with their long axons need the presence of intermediate filaments, microtubules, and actin filaments in order to provide a rigid framework for the action potential to be transmitted to the next cell. Additionally, some epithelial cells contain small cilia or flagella to move the cell or remove foreign particles through coordinated activity of cytoplasmic extrusions formed through the cytoskeleton.
• The cytoplasm also plays a role in creating order within the cell with specific locations for different organelles. For instance, the nucleus is usually seen towards the center of the cell, with a centrosome nearby. The extensive endoplasmic reticulum and Golgi network are also placed in relation to the nucleus, with the vesicles radiating out towards the plasma membrane.

Cytoplasmic Streaming

• Movement within the cytoplasm also occurs in bulk, through the directed movement of cytosol around the nucleus or vacuole. This is particularly important in large single celled organisms such as some species of green algae, which can be nearly 10 cm in length. 
• Cytoplasmic streaming is also important for positioning chloroplasts close to the plasma membrane to optimize photosynthesis and for distributing nutrients through the entire cell. In some cells, such as mouse oocytes, cytoplasmic streaming is expected to have a role in the formation of cellular sub-compartments and in organelle positioning as well.

Cytoplasmic Inheritance

   The cytoplasm plays hosts to two organelles that contain their own genomes – the chloroplast and mitochondria. These organelles are inherited directly from the mother through the oocyte and therefore constitute genes that are inherited outside the nucleus. These organelles replicate independent of the nucleus and respond to the needs of the cell. Cytoplasmic or extranuclear inheritance, therefore, forms an unbroken genetic line that has not undergone mixing or recombination with the male parent.

Related Biology Terms

• Chemotaxis – Movement of a cell in response to a chemical signal.
• Intermediate Filaments – Cytoskeletal components formed by a family of proteins sharing structural and functional features larger than actin fibers and smaller than microtubules.
• Kinesin – A group of motor proteins that can travel along a microtubule and are important for the movement of cellular components, especially during cell division.
• Syncytium – A multinucleated cell formed by the fusion of the plasma membrane of multiple cells. Syncytia can also be formed through the interconnections between cells containing specialized gap junctions, allowing the cells to behave synchronously as a single unit.