Messenger RNA (m-RNA)-
- The term messenger RNA was coined by Francois Jacob (French biologist) and Jacques Monad (French biochemist). Messenger RNA is a linear molecule formed inside the nucleus by the process of transcription from DNA.
- The sequence of mRNA is complementary to the template DNA strand. The nitrogenous bases on mRNA strand are arranged in form of codons, made up of three nitrogenous bases. Messenger RNA carries genetic information from nucleus (chromosomal DNA) to cytoplasm for protein synthesis. After being transported to cytoplasm m-RNA combines with ribosomes to form polyribosomes or polysomes.
- Each polysome contains several ribosomes (normally five) which form a complex with m-RNA. Generally, each gene transcribes its own m-RNA therefore there are approximately as many types of m-RNA molecules as there are genes.
- The characteristic feature of m-RNA is its heterogeneous nature. Messenger RNA significantly differs in their size and molecular weight. This difference arises due to difference in size and number of cistrons.
Life span of m-RNA-
- Messenger RNA formed in eukaryotes is much more stable and has a longer life span as compared to the m-RNA formed in prokaryotes which survive for a very short time period probably for about one minute or even less than a minute. Messenger RNA might have short life span but to compensate it they possess high turnover number. Messenger RNA comprises about 10% of total cellular RNA. Immediately after formation it is transported from nucleus into cytoplasm where it gets deposited on ribosomes
- Messenger RNA has been classified as monocistronic and polycistronic. Monocistronic m-RNA is a type of m-RNA which codes for only one protein whereas polycistronic m-RNA can synthesis more than one type of protein from same m-RNA molecules.
- This is because monocistronic m-RNA contains codons of single cistron whereas polycistronic m-RNA contains codons for more than one cistron. Monocistronic m-RNA is characteristic feature of eukaryotes while prokaryotes contain polycistronic m-RNA.
Differance between monocistronic m-rRNA and polycistronic m- RNA
Components of structure of m-RNA-
1. Messenger RNA molecule comprises of several segments namely 5ˈ cap, non-coding region, initiation codon, coding region, termination codon, another non coding region and 3ˈpoly(A). Messenger RNA strand begins with a cap region present at 5ˈend of m-RNA strand. Presence of cap has been found to be associated with binding of m-RNA molecules to ribosomes.
2. The next segment present in m-RNA sequence is a non coding region. The length of this region is about 10-100 nucleotides. As its name suggests it is non coding region i.e., it does not codes for any protein.
3. Non-coding region is followed by initiation codon (AUG). Initiation codon marks the beginning of protein synthesis. Next segment in m-RNA strand is coding region which is translated to form a protein.
4. A termination codon is present at end of each coding segment. When this termination codon is reached the process of protein synthesis is terminated as these codons do not code for any amino acids. Termination codon is also called stop codon. There are three termination codons, UAA, UAG and UGA.
5. Coding region is followed by another non coding segment. This segment is about 50-150 nucleotides long and contains a specific sequence AAUAAA.
6. The last segment in m-RNA strand is poly (A) sequence i.e., large number of AAA..... (About 200-250) nucleotides are present. The addition of poly (A) sequence in m-RNA occurs inside nucleus before; m-RNA is transported from nucleus to cytoplasm.
Outline of RNA processing and structure of m-RNA
