The following points highlight the four types of RNA. The types are: 1. Ribosomal RNA 2. Transfer RNA 3. Messenger RNA 4. Genetic RNA.
Type # 1. Ribosomal RNA (rRNA):
It is the most abundant RNA (70-80% of total) which has 3-4 types. Some of its types (23S, 28S) are the longest of all RNAs. As the name indicates, rRNA is a constituent of ribosomes.
Here it lies coiled in between and over the protein molecules. Depending upon their sedimentation coefficient, RNAs of eukaryotes are of four types— 28S, 18S, 5.8S and 5S.
Procaryotic ribosomes have three types of RNAs— 23S, 16S and 5S. 28S, 5.8S and 5S (23S and 5S in prokaryotes) occur in larger subunit of ribosome while 18S (16 S in prokaryotes) is found in smaller subunit of ribosome. rRNA is transcribed in the form of a longer chain of 45S in eukaryotes and 30S in prokaryotes.
In eukaryotic transcript the arrangement in 5′ → 3′ direction is 18S — 5.8S — 28S. Several methylations occur prior to removal of spacer RNA. Removal of spacer RNA breaks the transcript into 2-3 parts. 5S is often transcribed separately.
Functions:
(i) rRNAs bind protein molecules and give rise to ribosomes,
(ii) У end of 18S rRNA (16S in prokaryotes) has nucleotides complementary to those of cap region of mRNA.
(iii) 5S rRNA and surrounding protein complex provide binding site for tRNA.
rRNAs get associated with specific proteins to form ribosome subunits. 50S subunit of prokaryotic ribosome contains 23S rRNA, 5S rRNA and some 32 protein molecules. 30S subunit of prokaryotic ribosome has 16S rRNA and about 21 protein molecules.
60S subunit of eukaryotic ribosome contains 28S rRNA, 5S rRNA, 5.8S rRNA and about 50 protein molecules. 40S subunit of eukaryotic ribosome consists of 18S rRNA and some 33 protein molecules.
Type # 2. Transfer RNA (tRNA):
It is also called soluble or sRNA. There are over 100 types of tRNAs. Transfer RNA constitutes about 15% of the total RNA. tRNA is the smallest RNA with 70-85 nucleotides and sedimentation coefficient of 4S. The nitrogen bases of several of its nucleotides get modified e.g., pseudouridine (ψ), dihydrouridine (DHU), inosine (I).
This causes coiling of the otherwise single-stranded tRNA into L-shaped form (three dimensional, Klug, 1974) or clover-like form (two dimensional, Holley, 1965). About half of the nucleotides are base paired to produce paired stems. Five regions are unpaired or single stranded— AA-binding site, T ψ С loop, DHU loop, extra arm and anticodon loop.
(i) Anticodon:
It is made up of three nitrogen bases for recognising and attaching to the codon of mRNA.
(ii) AA-Binding Site:
It lies at the 3′ end opposite the anticodon and has CCA— OH group. This CCA group is added after the transcription (5′ end bears G). Amino acid or AA-binding site and anticodon are the two recognition sites of tRNA.
(iii) T ψ C Loop:
It contains pseudouridine. The loop is the site for attaching to ribosomes,
(iv) DHU Loop:
The loop contains dihydrouridine. It is binding site for aminoacyl synthetase enzyme,
(v) Extra Arm:
It is a variable site arm or loop which lies between T ψ C loop and anticodon. The exact role of extra arm is not known.
Functions:
(i) tRNA is adapter molecule which is meant for transferring amino acids to ribosomes for synthesis of polypeptides. There are different tRNAs for different amino acids. Some amino acids can be picked up by 2-6 tRNAs. tRNAs carry specific amino acids at particular points during polypeptide synthesis as per cidons of mRNA.
Codons are recognised by anticodons of tRNAs. Specific amino acids are recognised by particular activating or aminoacyl synthetase enzymes,
(ii) They hold peptidyl chains over the mRNAs.
Type # 3. Messenger RNA (mRNA):
It is a long RNA which constitutes 2-5% of the total RNA content. It brings instructions from the DNA for the formation of particular type of polypeptide. The instructions are present in the base sequence of its nucleotides. Ii is called genetic code. Three adjacent nitrogen bases specify a particular amino acid.
Formation of polypeptide occurs over the ribosome. mRNA gets attached to ribosome. tRNAs are induced to bring amino acids in a particular sequence according to the sequence of codons present over mRNA. mRNA has methylated region at the 5′ terminus.
It functions as a cap for attachment with ribosome. Cap is followed by an initiation codon (AUG) either immediately or after a small noncoding region. Then there is coding region followed by termination codon (UAA, UAG or UGA). There is then a small noncoding region and poly A area at the 3’ terminus (Fig. 9.24). An mRNA may specify only a single polypeptide or a number of them.
The former is called monocistronic while the latter is known as polycistronic. Polycistronic mRNA is more common in prokaryotes. Eukaryotic mRNA is usually monocistronic.
The life time of mRNA is also variable. In some lower forms it is from a few minutes to a few hours. On the other hand the mRNAs of higher forms seem to have a long life. It is several days in case of young red blood corpuscles which continue to form haemoglobin even when nucleus has degenerated.
Functions:
(i) mRNA carries coded information for translation into polypeptide formation.
(ii) Through reverse transcription it can form compact genes which are used in genetic engineering. The phenomenon also occurs in nature and has added certain genes in the genomes,
(iii) It has a cap region for attachment to ribosome.
Type # 4. Genetic RNA:
It is found in some viruses called riboviruses. Genetic RNA may be single stranded (e.g., Tobacco Mosaic Virus or TMV) or double stranded (e.g., Rheovirus). Genetic RNA acts as a hereditary material. It may, however, not replicate directly, but form DNA in the host cell to produce RNA of its own types.