Lecture 6

Question 1

What differs more, number of genes or genome size?

Answer 1

Gnome size differs more -some genomes contain large amounts of ‘extra DNA’

Question 2

What is the extra DNA?

Answer 2

Within genes: introns

Between genes: repetitive DNA

Question 3

How is RNA processed at the 5’ end of eukaryotes?

Answer 3

There is an addition of a 7-methyl G cap onto the 5’ end.
The G cap is a chemically modified molecule of GTP (it has an added methyl base)
It is attached by 3 phosphates to the RNA.
It protects the end of the RNA, if removed it becomes unstable and is degraded.

Question 4

How is RNA processed at the 3’ end?

Answer 4

3’ end of pre-mRNA is the sequence AAUAA, this acts as a signal for an enzyme to cut the pre-mRNA -cleaved.
After the cleavage another enzyme adds 100-300 adenine bases (poly A tail)

Question 5

What are introns?

Answer 5

Sequence within the transcribed region of a gene that is removed during RNA processing -the non-coding base sequences.

Question 6

What is the removal of introns called?

Answer 6

RNA splicing.

Question 7

What is intron excision catalysed by?

Answer 7

Spliceosome

Question 8

What is a spliceosome made up of?

Answer 8

snRNPs -contain snRNAs and protein complexes

Question 9

Hoe does snRNP bind?

Answer 9

Boundaries between introns and axons are consensus sequences
RNA in one of the snRNPs has a stretch of bases complimentary to the consensus sequence at the 5’ exon-intron boundary -binds to the pre-mRNA by complementary base pairing

Another snRNP binds to the pre-mRNA near the 3’ intron-exon boundary

Question 10

Hows does splicing occur?

Answer 10

snRNPs bind to the primary transcript -one at 5’ splice site, one at 3’ splice site
Interactions between snRNPs form spliceosome
Spliceosome cuts 5’ end of intron, this forms a closed loop, a lariot.

The free 3’ OH group at the end of the cut exon reacts with the 5’ phosphate of the other exon.
Spliceosome cuts the 3’ end of the intron and joins the axons
The excised intron is degraded in the nucleus

Question 11

What is alternative splicing?

Answer 11

when different exons are used in different kinds of cells -a single gene can code for multiple proteins
Particular exons of a gene may be included within or excluded from the final processed mRNA

Question 12

What is an example of a gene that shows alternative splicing ?

Answer 12

alpha-tropomyosin gene -helps to regulate muscle contraction

Question 13

How many types of tropomyosin mRNAs are there?

Answer 13

8