Nucleic Acids 5

Question 1

What is the coding strand?

Answer 1

The sense strand
The DNA strand which has the same base sequence as the RNA being synthesised (except T for U)

Question 2

What is the non-coding strand ?

Answer 2

Antisense strand
The template strand for RNA synthesis, whose sequence is complementary to the RNA molecule

Question 3

What does RNA polymerase require for activity ?

Answer 3

1. All 4 nucleotide triphosphates
2. A double stranded template DNA molecule that includes a promoter sequence
   A primer is not required

Question 4

Describe the stages of transcription

Answer 4

1. Initiation:
   RNA polymerase binds to promoter
   DNA strand partially unwinds
   RNA synthesis begins
2. Elongation:
   RNA polymerase moves along the DNA molecule synthesising an RNA copy
3. Termination:
   RNA polymerase dissociates from DNA releasing the new RNA molecule

Question 5

How does transcription occur in E.coli ?

Answer 5

1. Initiation- RNA polymerase binds to promoter which is identified using the -35 hexamer and the pribnow box- consensus sequences
   DNA unwinds
   RNA synthesis begins

Question 6

What forms of RNA polymerase are there in E.coli ?

Answer 6

1. Holoenzyme- a hexamer with the subunit structure Alpha2BetaBeta’Omegasigma. It carries out initiation but not elongation
2. Core enzyme is a pen tamer with the subunit structure Alpha2BetaBeta’omega. It carries out elongation but not initiation.

Question 7

How does transcription in eukaryotes differ from that in bacteria ?

Answer 7

1. Eukaryotes have separate RNA polymerases for mRNA, rRNA and tRNA
2. Eukaryotes make 1° transcription (aka pre-mRNA) that is processed in nucleus to form mRNA
3. Different promotors

Question 8

What types of RNA polymerase do eukaryotes have ?

Answer 8

1. RNA polymerase I- synthesises rRNA
2. RNA polymerase II- synthesises mRNA
3. RNA polymerase III- synthesises tRNA

Question 9

Describe RNA processing in eukaryotes

Answer 9

RNA polymerase II produces a 1° transcript which then form mRNA in the nucleus
1. Capping- addition of methylated G to the 5’ end of the 1° transcript. The cap stabilises the mRNA molecule.
2. Cleavage and polyadenylation- the 3’ end of the 1° transcript is cleaved and a “poly A tail” of approx. 250 Adenine residues is added. This stabilises the mRNA.
3. RNA splicing- most eukaryotic genes are split into exons and introns. The exons are spliced together and the introns are removed to generate mRNA.

Question 10

What do promotors recognised by RNA polymerase II consist of?

Answer 10

A core promotor that may include a TATA box, and one or more proximal promotor elements