Question

1.      Pre-messenger RNA 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?

Answer 1

ANS 1.

RNA editing is a process where the sequence of the messenger RNA (mRNA) is altered after it is transcribed from the DNA, by this sequence of DNA is not used to encode the product protein.

RNA editing can be confined to a single base or may affect the entire mRNA. For example in the mammalian intestine cells, the codon 2153 is modified in the messenger RNA where the C codon CAA is changed to U to make the sequence codon UAA and causes the termination. The change of C to U takes place in messenger RNA and gene does not show any change in its base sequence. Mainly RNA editing proceeds from 3’end to 5’end.

ANS 2.

Introns have coding sequences which may be originated as independent elements that codes for a function which are involved in the ability to splice out of RNA or to migrate between the DNA molecules. The self-splicing is catalyzed by the intron itself and the excised intron retains this catalytic ability. Base pairing between the consensus sequences generates the secondary structures in the introns and theses structures generate active sites which catalyzes the splicing.

The nuclear splicing requires ATP and generates lariat (a rope like structure with running knot at the end) during splicing. Splicing is mediated by spliceosomes composed of five snRNPs and the splicing is based on only short consensus sequences which are located in the introns.

ANS3.

Processing of messenger RNA precursor is mediated by spliceosomes, which are assembly of 4-5 types of ribonuceloprotein. RNA splicing starts with splicesome assembly. The splicing process is catalyzed by the U1, U2,U3, U4,U5 and U6 snRNAPs and by the other splicing factors. snRNP U1 recognizes and binds to GU consensus sequence to the left side of the intron and U2 binds to the specific UACUAAC sequence upstream to the right side of intron. U5 recognizes AG sequence of the right junction and binds to it. The snRNP containing U4 and U6 combines with the developing complex and completes the splicesome assembly.

Catalysis of splicesome is different from the type I and II introns: Intron I is noncoding and its excision joins the first exon with the second exon. And Intron II is coding but does not have the translation initiation site.

ANS 4.

Splicing defect arises by the mutation in the gene and it is called as splicing site mutation in this type insertion, deletion or change in the number of nucleotides at the specific site of splicing during the processing of precursor mRNA into mature mRNA. Splice defect consensus sequences recognizes the exon which are located at the terminus of introns. Deletion of the splicing site with one or more introns in mature mRNA leads to the production of abnormal proteins.

ANS 5.

The generation of different mature mRNA s from a particular type of gene transcript occurs by varying the use of 5’-3’ splice sites is called Alternative Splicing. Alternative splicing works mainly in four ways:

1. By using different promoters
2. Using different poly A site
3. Retaining certain introns
4. Retaining or removing certain exons

With the help of these steps cell type specific factors are responsible for activating or repressing the processing sites where they happens to bind.