Synthesis of Messenger RNA - mRNA

Synthesis of Messenger RNA - mRNA
 Messenger RNA is transcribed on a DNA strand through the enzymatic action of RNA polymerase. Synthesis begins at the 5' end and proceeds to the 3' end. RNA polymerase attaches to DNA at the initiator site and opens up a short segment of the DNA double helix.

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. The RNA polymerase moves along the DNA template, and the growing mRNA strand is peeled off till a termination site is reached.

Hydrogen bonding then again takes place between the complementary bases of the two DNA strands. Synthesis of mRNA is similar to replication of DNA except for the following differences: (1) ribose nucleotides are used instead of deoxyribose nucleotides; (2) adenine pairs with uracil instead of thymine, and (3) only one strand of DNA transcribes the mRNA strand.

The transcribed strand undergoes processing before it becomes mRNA. During processing elements that are no longer of any use are removed and elements thus make the RNA functionally more effective are added. Prokaryotic mRNAs normally undergo very little processing. In bacterial cells there is a very short time interval between transcription and translation.
A large proportion of the mRNA may be translated before transcription is completed. In some cases degradation of mRNA may begin even before completion of transcription. Methylation and polyadenylation which occur to a considerable extent in eukaryotes are not prevalent in bacterial cells. Nevertheless a certain amount of processing does occur.

In the T7 phage five contiguous genes transcribe 'early RNA' which is cut during transcription to five individual mRNAs by RNase III. In eukaryotes the transcribed RNA molecules are larger than mature mRNA by 20% in the slime mould Dictyostelium and about X 4 to X 5 in mammalian cells. The large RNA molecules containing pre-mRNA transcripts are known as heterogeneous nuclear RNAs (hnRNA).