Types of RNA: The following are the types of RNA found -a) Genetic RNA b) Ribosomal RNA c) messenger RNA d) soluble or transfer RNA Genetic RNA: In certain viruses, RNA is the only genetic material, DNA being absent. This RNA is self replicating; it is called RNA dependent RNA synthesis. The viral RNA directly functions as a messenger and produces RNA polymerase enzyme as well as proteins.
Ribosomal RNA (rRNA): This is found in the ribosomes. It constitutes about 80% of total RNA of the cell. The base pairs of rRNA are complementary to that part of the nuclear DNA from where they are produced. rRNA consists of a single strand folding upon itself at some regions. The folded regions have paired bases, while the other regions have unpaired bases. Due to this in rRNA, there is no purine pyrimidine equality.
There are three types of rRNA based on sedimentation and molecular weight. These are a) rRNA with molecular weight over a million, e.g. 2 ls-29s rRNA, b) rRNA with molecular weight less than a million, e.g. 12s-18s rRNA, c) rRNA with very low molecular weight (40,000) e.g. 5s rRNA.
Messenger RNA (mRNA): mRNA carries the message from DNA to ribosomes during protein synthesis. It is about 3-5% of total RNA content of the cell.
mRNA has a molecular weight of about 5,00,000 with a sedimentation coefficient of 8s. This however is highly variable. mRNA is always single stranded and there is no base pairing, even though random coiling may be observed.
The mRNA molecule is produced from the coding strand and its base sequence is complementary to the coding strand.
Transfer RNA (tRNA or sRNA): The transfer RNA also called soluble RNA, is next in quantity to rRNA. It has a low molecular weight of about 25,000-30,000 and the sedimentation co efficient is 3.8s. It is made up of about 70-90 nucleotides.
tRNA is synthesised in the nucleus on a DNA template. Only a minute fraction of DNA (0.025%) codes for tRNA. The tRNA molecule consists of a single strand looped around itself withUhe 3′ end always terminating in a C- C-A sequence. Some bases are paired.