Question

a. Even after transcription, in eukaryotes there are several steps required for an mRNA to be exported from the nucleus (so it can be translated into a protein). What are these steps?

Answer 1

In Eukaryotes, transcription occurs in nucleus while translation occurs in cytoplasm. The formation primary transcript from DNA to pre-mRNA is known as transcription. Pre-mRNA undergoes for various modifications to convert into mRNA. This is known as processing of pre-mRNA. It involves mainly four steps:

a) 5' capping : mostly 7-methylguanosine residue at 5' end of nascent mRNA is joined via a 5'-5' triphosphate bond. It provide protection to mRNA from degradation and help in transport of mRNA from nucleus to cytoplasm.

b) 3' cleavge/ polyadylation: : A series of about 250 adenosines at 5' of mRNA i.e. poly-A tail. These adenosines are added to the transcript by a template-independent RNA polymerase called as Poly-A polymerase .

C) Splicing: In this step, introns which are non-coding sequence are removed from the transcript and exons are joined together. Exons are the coding sequences.

D) RNA editing : RNA editing is the changing of nucleotide sequence of RNA so that a mature mRNA differs from that encoded by genomeing sequence. It can be done by two types: site specific base modification and insertion-deletion editing.

After these modifications , pre-mRNA is changed to mRNA which can be transfer from nucleus to cytoplasm for translation.