"The pre-mRNA is modified in three ways:
- -Segments of the pre-mRNA are spliced and removed from the pre-mRNA and the remaining pieces joined together. The spliced and removed pieces are called introns and the pieces that remain and are joined together are called exons.
- -A methyl cap is added to the 5' end of the RNA strand.
- -A poly-A tail is added to the 3' end of the RNA strand."
I'm just a bit confused as to why the pre-mRNA needs to be modified in the first place, aka why segments of the pre-mRNA are spliced and removed etc. (from the first bullet point). I also don't quite understand the other two bullet points either. Would someone be able to explain this in more depth?
Pre-mRNA needs to be modified:
1. to remove segments of mRNA that are "non-coding" - in a region of DNA, not all of it is necessary for that particular protein to be made and thus areas of it (called introns) are removed. This is called splicing. Splicing basically allows one sequence of DNA/RNA to code for various proteins depending on how it has been cut up.
e.g. if pre-mRNA has the sequence ABCDEFG
splicing may result in:
ABDFG creating protein type 1
ABCEG creating protein type 2
ACDEF creating protein type 3
This can be really helpful considering that we needs heaps and heaps of different kinds of proteins for all the different cells in the body but we only have so much DNA (about 2 metres in length apparently)
2. the mRNA has a poly-A (i.e. lots of Adenines) added to its tail end (3' end) and has a methyl (technically 7-methylguanosine - but not needed for vce) cap added to the "head" end of the strand (5' end). This capping and tailing allows the mRNA to be more stable (i.e. prevents enzymes from degrading the RNA strand) and thus assists it during its "transportation" from the nucleus to the cytosol. (imagine if enzymes were able to degrade the RNA as it moves to the cytosol, no protein would ever be successfully made) The cap and tail also allows translation to be more efficient (how... i'm not too sure sorry
)
Hope that helps