RNA: Chemical Nature, Types and Features
Chemical Nature:
RNA is a single stranded mixed polymer of four types of ribotides linked together by 3′, 5 –
phosphodiester bonds. They are adenylate (AMP), guanylate (GMP), cytidylate (CMP) and
uridylate (UMP).
RNA is predominantly single stranded nucleic acid. However, it may fold back on itself to
form an antiparallel duplex structure called a hairpin which consists of a base-paired stem
(like A-DNA) and a loop of unpaired bases.
RNA came first in evolution, having both genetic and catalytic properties. Later DNA
replaced RNA as more stable molecule for genetic information and proteins carried out the
catalytic role while RNA acts as the intermediate between the two.
Size:
RNA molecule is much smaller in size than DNA. It consists of up to 12,000 nucleotides
whereas DNA consists of up to 4.3 million nucleotides.
Location:
RNA found in both prokaryotic and eukaryotic cells. In eukaryotic cell RNA found in
cytoplasm as well as in nucleus. In the cytoplasm it occurs freely as well as in the ribosomes
while in the nucleus it is present in association with chromosomes. RNA also found in
matrix of mitochondria and stroma of chloroplast.
Types of RNA:
On the basis of function, RNA is of two types, viz. (i) genetic RNA and (ii) non-genetic RNA.
(i) Genetic or Genomic RNA:
It occurs in riboviruses and viroids. It is single stranded in TMV, HIV, Influenza viruses etc.
while double stranded in Reovirus. In TMV genetic RNA is (+) RNA strand that directs the
synthesis of a (-) RNA strand. Then the (-) RNA strand serves as the template for the
synthesis of a large number of (+) RNA strands. In retroviruses like HIV, Rous sarcoma
viruses the genetic RNA is the (+) strand that directs the synthesis of DNA by reverse
transcription.
(ii) Non-Genetic RNA:
