Question

10a. Name the three post-transcriptional processing events that take place to generate a mature mRNA from a nascent transcript?

1. ________________________________________________________

2. ________________________________________________________

3. ________________________________________________________

b. Describe a specific example of how one of these processing events can provide functional diversity for a given gene.

Answer 1

polyadenylation

In mRNA, the 3' end is modified by the addition of multiple adenine residues to produce a structure called a poly-adenylated (poly A) tail. The length of a poly-A tail is highly variable, but over the lifetime of the mRNA these adenine residues are slowly removed. Thus the length of a poly-A tail determines the half-life of mRNA: a longer tail means a longer half-life. The poly-A tail also makes it easier for us to purify eukaryotic mRNA in the lab by annealment to a complementary poly-thymine (poly-T) primer.

Capping

In mRNA, the 5' end is modified by the addition of a 7-methylguanosine cap (a methylated guanine residue). The cap is attached the 'wrong way round' forming a 5'-5' pyrophosphate bond, rather than the usual 3'-5' phosphodiester bond. This has two key roles: it enables ribosomal recognition, and thus greater efficiency of protein translation, as well as protecting the mRNA from degradation by 5'-3' exonuclease enzymes.

Splicing

One such modification is splicing. The RNA contains a mixture of introns and exons when it is first transcribed. Splicing serves to remove introns, and it occurs either by the action of a spliceosome, or by self-splicing. Alternative splicing shuffles the composition of exons in a transcript, meaning that numerous different transcripts can be made from the same gene.