It is a nucleic acid found in the nucleus as well as in the cytoplasm. These are two main types of RNA.
i) Genetic RNA and
ii) Non genetic RNA.
The genetic RNA acts as genetic material of some viruses. The 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 polynucleotide molecule.
Components of RNA are ribose sugar, phosphate as phosphoric acid and n-bases.
The nitrogen bases present in RNA are as Purines: Adenine and Guanine. 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 of coiling or folding occurs then base pairing may occur between complementary bases as; A = U or U = A and G = C or C = G.
In the RNA molecule the base pairing is not fixed and definite. Therefore, purine :pyrimidine ratio may or may not 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 m-RNA, r-RNA and t-RNA.
All the three types of RNA are produced on DNA template by the process called transcription.
m-RNA or Messenger RNA :
The m-RNA molecule is always simple and straight without any fold. It carries message for protein synthesis from DNA to ribosomes. About 3-5% of the total cellular RNA is of this type. It has molecular weight up to 5,00,000. It has two ends, 5' end and 3' end.
A sequence of three nucleotides on m-RNA is called codon. Each 'triplet' codon on m-RNA codes for specific amino acid. This is called m-RNA language or genetic code or cryptogram. The codon present at 5' end of m-RNA is called start codon or initiation codon. It is usually AUG, in some cases it may be GUG. The codon at 3' end is stop or non sense codon. It is either UAA, UAG or UGA. Each codon on m-RNA specifies a particular amino acid, e.g. the start codon AUG, specifies methionine or f-methionine, CGG specifies glycine, etc. Stop codons do not specify any amino acid hence are called nonsense codons. They are also called terminators as the synthesis of polypeptide chain stops and termination occurs.
Thus m-RNA carries coded information about the sequence of amino acids in the polypeptide chain, from DNA to ribosomes, during protein synthesis.
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 total cellular RNA is of this type. The molecular weight ranges between 40,000-1,00,000.
It provides proper binding site for m-RNA.
It orients the m-RNA molecule is such a way that all the codons are properly read.
t-RNA or Transfer RNA :
The t-RNA molecule is also 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 only with molecular weight ranging between 23,000-30,000. If 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 one loop having a triplet of unpaired bases called anticodon. 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 DHU arm, middle arm and TΨC arm. These arms have loops at their ends such as amino acyl binding loop, anticodon loop and ribosomal binding loop respectively. The anticodon loop has anticodon which is a triplet of unpaired nucleotides. The anticodons present on t-RNA are complementary to codons present on the m-RNA (anticodons are also referred to as codon). 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 carries 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 acid.