1. Pre-messenger RNA processing a. What are the major steps of pre-mRNA processi

Question

1.      Pre-messenger RNA processing

a.      What are the major steps of pre-mRNA processing?

b.      How is a cap added to the 5’-end of mRNAs? What is that cap? Can you explain some functions of the cap?

c.       What processing happens at the 3’-end of eukaryotic mRNAs? What enzyme is responsible for this? Can you explain some functions of this processing?

d.      What is RNA editing? Are you prepared to identify how a single site of editing could change the expression of an mRNA?

e.      What are the catalytic sites within an intron? How are they recognized by the spliceosome (you do not have to memorize exact interactions, just understand generally how this occurs)?

f.        What is the mechanism of catalysis carried out by the spliceosome? How is this similar to Group I and Group II introns?

g.       Are you prepared to answer questions about how splicing defects may result in non-functional protein products?

h.      What is alternative splicing and how can it increase the size of the proteome?

Explanation / Answer

1.

a. Major stpes of pre-mRNA processing:

    1. Addition of 5'-cap at 5' end of the mRNA.

    2. Addition of poly-A tail at 3' end of mRNA.

    3. Splicing of introns and jooining of exons.

b. 5'-cap is modified guanine nucleotide (7-methyl guanosine) which is added to the first nucleotide (5') of mRNA. The linkage between first nucleotide and m7G is 5' to 5' triphosphate linkage. Guanine residue is methylated after its addition pre-mRNA mediated by methyl transferases. The role of capping is to prevent degradtion of pre-mRNA by ribonucleases.

c. At 3'-end of pre-mRNA a stretch of 100-200 adenosine nucleotides are added and it is known as poly-A tail. An enzyme called poly-A polymerase catalyzes addition of nucleotides.

Functions of poly-A tail:

1. Protect from degradtion.

2. Export of mature mRNA from nucleus to cytoplasm.

3. Aids in initiation of translation by acting as a binding site.

d. RNA editing is addition adn deletion of nuceotides in RNA.

Two mechanisms:

1. Substitutional editing

2. Insertion/Deletion editing

Single site change in nuceotide changes the open reading frame and hence affects translation of protein.

For example,

RNA editing of Apo-B mRNA in intestine from CAA to UAA leads to truncation of mRNA instead of coding Gln.

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