mRNA processing in eukaryotes

Messenger RNA Processing:-

A newly synthesized eukaryotic mRNA undergoes several modifications before it leaves the

nucleus(Fig.). The first is known as capping. Very early in transcription the 5' -terminal

triphosphate group is modified by the addition of a guanosine via a 5' -5'-phosphodiester

link. The guanosine is subsequently methylated to form the 7-methyl guanosine cap. The

3' ends of nearly all eukaryotic mRNAs are modified by the addition of a long stretch of

adenosine residues, the poly-A tail (Fig.). A sequence AAUAAA is found in most

eukaryotic mRNAs about 20 bases from where the poly-A tail is added and is probably a

signal for the enzyme poly-A polymerase to bind and to begin the polyadenylation process.

The length of the poly-A tail varies, it can be as long as 250 nucleotides. Unlike DNA,

RNA is an unstable molecule, and the capping of eukaryotic mRNAs at their 5' ends and

the addition of a poly-A tail to their 3' end increases the lifetime of mRNA 

molecules  by protecting them from digestion by nucleases.

Many eukaryotic protein-coding genes are split into exon and intron sequences. Both

the exons and introns are transcribed into mRNA. The introns have to be removed and the

exons joined together by a process known as RNA splicing before the mRNA can be used

to make protein. Removal of introns takes place within the nucleus. Splicing is complex

and not yet fully understood. It has, however, certain rules. Within an mRNA the first

two bases following an exon are always GU and the last two bases of the intron are AG.

.Several small nuclear RNAs (snRNAs) are involved in splicing. These are complexes with

a number of proteins to form a structure known as the spliceosome. One of the snRNAs is

complementary in sequence to either end of the intron sequence. It is thought that binding

of this snRNA to the intron, by complementary base pairing, brings the two exon sequences

together, which causes the intron to loop out (Fig.). The proteins in the spliceosome

remove the intron and join the exons together. Splicing is the final modification made to

the mRNA in the nucleus. The mRNA is now transported to the cytoplasm for protein

synthesis.

As well as removing introns, splicing can sometimes remove exons in a process called

alternative splicing. This allows the same gene to give rise to different proteins at different

times or in different cells. For example, alternative splicing of the gene for the molecular

motor dynein produces motors that transport different types of cargo.

Fig. mRNA processing in eukaryotes.