Transcription in Prokaryotes

Question 1

Define the transcriptional unit

Answer 1

formed when RNA polymerase binds to the region of the gene called the promoter. Encodes the first nucleotide of RNA at the +1 site.

Question 2

What is the synthesis capacity of mRNA in a cell?

Answer 2

One third

Question 3

What percentage is mRNA at equilibrium in a cell?

Answer 3

3-5%

Question 4

What are the four steps of transcription?

Answer 4

1. Promoter recognition
2. Initiation
3. Elongation
4. Termination

Question 5

What occurs during promoter recognition?

Answer 5

RNA pol. binds to duplex DNA @ promoter and DNA is unwound

Question 6

What occurs during initiation?

Answer 6

Very short chains of mRNA are synthesized and released, as the RNA pol. tries unsuccessfully to leave the promoter.

Question 7

What occurs during Elongation?

Answer 7

RNA pol. is finally successful in leaving the promoter, and mRNA synthesis of the strand begins

Question 8

What occurs during Termination?

Answer 8

synthesis halted when a phosphodiester bond is completed on the last nucleotide. Dissociation

Question 9

How many types of RNA pol. in prokaryotes?

Answer 9

only one

Question 10

Is the value of the Delta G for nucleotide addition positive or negative?

Answer 10

Negative

Question 11

What subunits of the RNA pol. contribute to make the Core Enzyme?

Answer 11

• 2 alpha subunits (structural function)
• 1 B-B subunit (contains catalytic center)
• One “w” subunit

Question 12

The addition of the what factor causes the Core Enzyme to become the holoenzyme?

Answer 12

The sigma factor

Question 13

What is unique about the holoenzyme?

Answer 13

Unlike the Core Enzyme, the holoenzyme can initiate transcription.
It is also able to increase affinity for the promoter by 100x, while decreasing affinity for all other sequences

Question 14

What does the binding of the sigma factor to RNA pol. do?

Answer 14

It increases the affinity of RNA pol. to promoters, while decreasing affinity for non-promoter sequences.

Question 15

What were the three models presented for how RNA pol. finds the promoter? Which one is not valid?

Answer 15

1. Diffusion
2. Exchange
3. Sliding
   - sliding is invalid

Question 16

Explain how the model Diffusion for RNA pol. locating promoter works

Answer 16

RNA pol, in solution, binds to DNA in trial and error process until it finds the promoter (high affinity)

Question 17

Explain how the model Exchange for RNA pol. locating promoter works

Answer 17

• RNA pol. is already bound to DNA at random
• while still bound to one strand, it runs along an adjacent part of the strand, looking for a region of high affinity (promoter)
• when a promoter is found, it switches off the sequence it was on to the promoter