The nitrogen bases are – Adenine and guanine (purines) and cytosine and uracil (pyrimidines). Molecular structure of RNA: Here also the back bone is made up of alternating sugar and phosphate. The single chain folds to enable the base pairs to meet here and there to form the steps. The pairing is between A-U and C-G. Occasionally the RNA may be double stranded but does not form a helix like DNA. Singie stranded RNA is the genetic material in plant viruses, poliomyletis viruses etc. Double stranded (non helical) RNA occurs as the genetic material in reoviruses.
Types of RNA: The following are the types of RNA found (1) Genetic RNA (2) Ribosomal RNA (3) Messenger RNA (4) Soluble or Transfer RNA.
1. 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.
2. 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 bases on sedimentation and molecular weight. These are (a) rRNA with molecules weight over a million, e.
g. 21 s-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. 5srRNA.
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 edimentation cefficient 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. The following are some of the general features of mRNA. 1. Cap: xhis is a blocked methylated structure found at the 5? end of RNA. 2.
Non coding region: This follows the cap and consists of 10-100 nucleotides. There are no codes for proteins in this region. 3. Initiating codon: This region begins the coding 4. Coding region: Codes for proteins and has about 1500 nucleotides. 5. Termination codon: The coding region is followed by a terminating codon, which stops translation.
4. Transfer RNA (rRNA 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 coefficient 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 with the 3? end always terminating in a C-C- A sequence. Some bases are paired.
The function of tRNA is to carry aminoacids to mRNA during protein synthesis. Each aminoacid couples with a specific tRNA. Since the protein molecule consists of about 20 types of amino acids, there must at least be 20 types of tRNAs. But there could be many more, sinced there are two tRNAs for every amino acid in several cases.