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Messenger RNA (mRNA)

The name messenger RNA for the RNA for the RNA carrying information for protein synthesis from the DNA (genes) to the sites of proteins formation (ribosomes). It consists of only 3 to 5% of the total cellular RNA.

Size. The molecular weight of an average sized mRNA molecule is about 500,000, and its sedimentation coefficient is 8S. It should be noted however, that mRNA varies greatly in length and molecular weight. Since most proteins contain at least a hundred amino acid residues, mRNA must have at least 100 x 3 = 300 nucleotides on the basis of the triplet code.

Structure of mRNA.

Messenger RNA is always single stranded. It contains mostly the bases adenine, guanine, cytosine and uracil. There are few unusual substituted bases. Although there is a certain amount of random coiling in extracted mRNA, there is no base pairing. In fact base pairing in the mRNA strand destroys its biological activity. mRNA is transcribed on DNA (genes), its base sequence is complementary to that of the segment of DNA (genes).

The mRNA molecule has the following structural features.

1. Cap. At the 5’ end of the mRNA molecule is most eukaryote cells and animal virus molecules is found a ‘cap’.

eukaryote mrna

2. Noncoding region 1(NC1).
The cap is followed by a region of 10 to 100 nucleotides. This region is rich in A and U residues, and does not translate protein.

3. The initiation condon is AUG in both prokaryotes and eukaryotes.

4. The coding region consists of about 1,500 nucleotides on the average and translates protein.
5. Termination condon. Termination of translation on mRNA is brought about by a termination codon. In eukaryotes the termination codons are UAA, UAG or UGA.

6. Noncoding region 2 (NC2) consists of 50-150 nucleotides and does not translate protein. This region contains an AAUAAA sequences in all the examples sequenced.

7. Poly (A) sequence. At the 3’ end is a polyadenylate or poly (A) sequence, which initially consists of 200-250 nucleotide, but which becomes shorter with age.

Stability. The cell does not contain large quantities of mRNA. This is because mRNA, unlike other RNAs is constantly undergoing breakdown. It is broken down to its constituent rib nucleotides by ribonucleases.

Synthesis. Messenger RNA is transcribed on a DNA strand through the enzymatic action of RNA polymerase. There is complementary base pairing between the free bases of one of the DNA strands and ribonucleotides, resulting in the formation of an mRNA strand.

Modifications taking place in transcribed mRNA are of three types:

(1) Polyadenylation is the addition of adenylate residues at the 3’ end to form a long chain of poly (A) consisting of 180-200 nucleotides. This takes place by the action of poly(A) synthetase.

(2) Capping consists of the formation of a ‘cap’ at the ‘5’ end by condensation of a guanylate residue. Evidence indicates that at least in some cases capping is essential for the formation of an mRNA-ribosome complex.

(3) Methylation is the addition of methyl groups to a few adenylate residues, and to the two or three nucleotides at the 5’ end of mRNA.

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