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)