RIBO NUCLEIC ACID (RNA)

RIBO NUCLEIC ACID (RNA)

Structure of RNA is fundamentally same as DNA, but there are some differences. The differences are as follows:- 

1. In place of De-oxyribose sugar in DNA, there is present Ribose sugar in RNA.

2. In place of nitrogen base Thymine in DNA, there is present Uracil in RNA.

3. RNA is made up of only one polynucleotide chain i.e. RNA is single stranded.

Exception:-

RNA found in Reo-virus is double stranded, i.e. it's has two polynucleotide chains.

Types of RNA:

1. Genetic RNA or Genomic RNA- in some viruses RNA works as genetic material and it transfers informatiots form one generation to next generation.

Eg. Reo virus, TMV, QB bacteriophage.

2. Non genetic RNA- mainly of 3 types:-

a. r-RNA

b. t-RNA

c. m-RNA

RNA functions as adapter, structural and in same cases as a catalyst (Ribozyme).

1. Ribosomal RNA (r-RNA)

 

• This RNA is 80% of the cells total RNA.
• r-RNA was discovered by Kuntze.
• It is found in ribosomes and it is produced in nucleolus.
• It is the most stable form of RNA.
• These are present in 80s type of ribosomes in Eukaryotic cells. Their subunits are 60s and 40s. In 60s sub unit of ribosome three types of r-RNA are found - 5s,5.8s,28s.
• 40s sub unit of ribosome has only one type of r-RNA i.e 18s.
• So 80s ribosome has total 4 types of r-RNA.
• Prokaryotic cells have 70s type of ribosomes and it's subunits are 50s and 30s.
• 50s Subunit of ribosome contain 2 types of r-RNA i.e 5s and 23s.
• 30s sub unit of ribosome has 16s type of r-RNA. 
• So 70s ribosome has total 3 types of r-RNA.

Function:-

At the time of protein synthesis, r-RNA provides attachment site to t-RNA and m-RNA and attaches them on the ribosomes.

It attaches t-RNA to the larger subunit of ribosome and m-RNA to smaller subunit of ribosome.

2. Transfer RNA (t-RNA)

• It is 10-15% of total RNA.
• It is synthesized in the nucleus by DNA.
• It is also known as soluble RNA (sRNA).
• It is also known as Adapter RNA.
• It is the smallest RNA (4s).

Function:- at the time of protein synthesis it acts as a carrier of amino-acids.

Structure:- the structure of t-RNA is most complicated. 

• Holley presented Clover leaf model of its structure. In two dimensional structure the t-RNA appears clover leaf like but in three dimensional structure (by Kim) it appears inverted L-shaped.
• The structure of tRNA looks like a clover 
• leaf but in actual structure, the tRNA is compact molecule which looks like inverted "L".
• There are present three nucleotides in a particular sequence at 3' end of t- RNA and that sequence is CCA.
• All the 5' ends i.e. last ends are having G (guanine). 
• 3' end is known as Acceptor end.
• t-RNA accepts amino acids at acceptor points. Amino acid binds to 3' end by its -COOH group.
• The molecule of t- RNA is folded and due to folding some complementary nitrogenous bases come across with each other and form hydrogen bonds.
• There are some places where hydrogen bonds are not formed, these places are known as loop.

Loops:-

There are some abnormal nitrogenous bases in the loops, that is why hydrogen bonds are not formed.

e.g. 1. Inosine(l) 

        2. Pseudouracil  (ψ)

        3. Dihydrouridine (DHU)

(A) T ψ C Loop or Attachment loop -

 This loop connects t- RNA to the large subunit of ribosome. 

(B) Recognition Loop (Anticodon loop)- This is the most specific loop of t-RNA and different types of tRNA are different due to this loop. There is a specific sequence of three nucleotides called Anticodon, is present at the end of this loop.

t-RNA  recognizes its place on m-RNA with the help of Anticodon.

(C) Dihydrouridine (DHU)

• It is also known as Amino-acyl synthetase recognition loop. Amino-acyl synthetase is a specific type of enzyme. The function of this enzyme is to activate a specific type of amino acid. after activation this enzyme  attaches the amino acid to the 3' end of t-RNA.
• There are 20 types of enzymes for 20 types of amino acids.
• The function of DHU loop is to recognize this specific Aminoacyl synthetase enzyme.

3. Messenger RNA (mRNA) :- 

• The m-RNA is 1-5% of the cells total RNA.
• The name m-RNA was given by Jacob and Monad.
• The m- RNA is produced by genetic DNA in the nucleus.
• It is least stable RNA.

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