RNA : General structure, types and functions

It is a nucleic acid found in the nucleus as well as in the cytoplasm.

There are two main types of RNA

i. Genetic RNA [acts as genetic material of some viruses] and

ii. Non genetic RNA.[functions for the protein synthesis in most of the organisms]

 

Structure of RNA

Generally the RNA molecule is a single stranded structure.

It may be simple and straight or may be variously folded or coiled upon itself.

Chemically it is a poly nucleotide molecule.

Components of RNA are

1. Ribose sugar.

2. Phosphate as phosphoric acid

3. N-bases.

 

The nitrogen bases present in RNA are

1. Purines: Adenine and Guanine.

2. Pyrimidines: Uracil and Cytosine (In RNA thymine Is replaced by uracil)

The strand of RNA is a long chain of nucleotides joined by phospho di-ester linkages.

There are four different types of nucleotides present as A,G U. and C nucleotides.

RNA strand has two ends. 3' end and 5' end.

In the RNA molecule

  • if coiling or folding occurs then base pairing may occur between complementary bases as ; A = U or U = A and GC or CG
  • the base pairing is not fixed and definite.
  • Therefore, purine : pyrimidine ratio may or may nol be 1:1.

 

Types of non genetic RNA:

There are three types of non genetic RNA.

They play important role in protein synthesis in both pro and eukaryotic cells.

These are (1) m-RNA. (2) r-RNA and (3) t-RNA.

All the three types of RNA are produced on DNA template by the process called Transcription

 

m- RNA or Messenger RNA:

  1. The m-RNA includes is always simple and straight without any fold.
  2. It carries message for protein synthesis from DNA to ribosomes.
  3. About 3-5% of the total cellular RNA is of this type.
  4. It has molecular weight up to 5,00,000.
  5. It has two ends. 5' end and 3' end.
  6. A sequence of three nucleotides on m-RNA is called codon.
  7. Each 'triplet' codon on m-RNA codes for specific amino acid.
  8. This is called m-RNA language or genetic code or cryptogram.
  9. The codon present at 5' end of m-RNA is called start codon or initiation codon. which is usually AUG in some cases it may be GUG.
  10. The codon at 3' end is stop or non sense codon, which is either UAA. UAG or UGA.
  11. Each codon on m-RNA species a particular amino acid. e.g. the start codon AUG. specifies methionine or f-methionine; GGG species glycine. etc..
  12. Stop codons do not specify any amino acid hence are called nonsense codons.
  13. They are also called terminators as the synthesis of polypeptide chain stops and termination of protein synthesis occurs.

 

Thus m-RNA carries coded information about the sequence of amino acids in the polypeptide chain, from DNA lo ribosomes. during protein synthesis.

 

mRNA.png

 

r-RNA or Ribosomal RNA:

  • It is a single- stranded structure which is variously folded upon itself.
  • In the folded regions it may show pairing between complementary bases.
  • It remains associated with ribosomes permanently.
  • About 80% of the tola! cellular RNA is of this type.
  • The molecular weight ranges between 40,000 to 100,000.
  • It provides proper binding site for m-RNA.
  • It orients the m-RNA molecule in such a way that all the codons are properly read.

rRNA.png

 

 

t-RNA or Transfer RNA

The t-RNA molecule is single- stranded.

About 10-20% of the total cellular RNA is of this type.

It is the smallest of all the RNA types containing 73-93 nucleotides

Molecular weight ranging between 23,000-30,000.

It shows two patterns of folding upon itself.

The structure of t-RNA can be explained by two models,

- Hair pin model (proposed by Hoagland) and

- Clover leaf model (proposed by Holley)

  • In hair pin model of t-RNA. by folding there is formation of

1. one loop having a triplet of unpaired bases called anticodon.

2. The 5' end has G-nucleotide while at 3' end there is a sequence of CCA nucleotides.

  • The clover leaf model of t-RNA shows presence of three arms namely

1. DHU arm - amino acyl binding loop

2. middle arm - anticodon loop

3. T¥C arm - ribosomal binding loop

tRNA.png

  • The anticodon loop has anticodon which is a triplet of unpaired nucleotides.
  • These anticodons present on t- RNA are complementary to codons present on the mRNA (anticodons are also referred to as nodoc).
  • In addition. it also shows a small lump called variable lobe or extra arm.
  • Like the hair-pin t- RNA it has G nucleotide at 5' end and CCA nucleotides at 3" end

 

t-RNA is also called s-RNA (soluble) because it cannot be easily separated even by ultra centrifugation technique.

It emits specific type of amino acid at CCA end to the ribosomes during protein synthesis.

It places the required amino acid properly in the sequence and translates the coded message of m-RNA in terms of amino acids.

 

Important Terms - RNA

Codogen - It is the smallest possible sequence (triplet) of nucleotides present on the DNA strand which can specify one particular amino acid.

Codon - It is the smallest possible sequence (triplet) of nucleotides present on m-RNA strand which can specify one particular amino acid.

Anticodon - It is a triplet of nucleotides present on the anticodon loop t-RNA which is complementary to the codon of mRNA.