there's this a level question in a ten-year series book in 2008, the question asks what is the sequence in which it occurs:
1. polyadenylation
2. removal of introns
3. splicing exons
4. capping
And the options given are:
a) 1,3,2,4
b) 2,4,1,3
c) 3,1,4,2
d) 4,2,3,1
I've looked it up but it doesnt fit to anyone of the choices given. Please help!
To determine the correct sequence in which the given events occur during gene expression, let's break it down:
1. Capping: The addition of a modified guanine nucleotide (cap) at the 5' end of the pre-mRNA molecule. This occurs early in the transcription process.
2. Removal of introns: Introns are non-coding regions within the pre-mRNA molecule. Splicing involves the removal of these introns, leaving behind only the exons (coding regions).
3. Splicing exons: After the introns are removed, the exons are joined together, resulting in a continuous coding sequence.
4. Polyadenylation: The addition of a poly-A tail (a string of multiple adenine nucleotides) at the 3' end of the mRNA molecule. This occurs after the pre-mRNA is transcribed.
Now, let's try to match these events with the given options:
a) 1,3,2,4: According to this option, capping occurs first, followed by splicing exons, then the removal of introns, and finally polyadenylation. This order is incorrect since capping should occur before splicing (option is wrong).
b) 2,4,1,3: According to this option, removal of introns occurs first, then polyadenylation, followed by capping, and finally splicing exons. This order is also incorrect since capping should occur before polyadenylation (option is wrong).
c) 3,1,4,2: According to this option, splicing exons occurs first, then capping, followed by polyadenylation, and finally the removal of introns. This order is also incorrect since capping should occur before splicing (option is wrong).
d) 4,2,3,1: According to this option, polyadenylation occurs first, then the removal of introns, followed by splicing exons, and finally capping. This order is correct, as capping occurs last. Thus, option d) is the correct answer.
So, the correct sequence is: 4,2,3,1, which corresponds to option d).