Prokaryotic Transcription

Question 1

Is primer needed for RNA synthesis?

Answer 1

No.

Question 2

What are the major types of RNA and what are their roles?

Answer 2

Messenger RNA - encode information that is translated to proteins.
Transfer RNA - translation machinery to translate RNA to protein
Ribosomal RNA - same as tRNA

Question 3

Are all types of RNA made by the same RNA polymerase?

Answer 3

Yes, in prokaryotes.

Question 4

What are the requirements for Transcription?

Answer 4

1. DNA Template
2. Ribonuclease triphospates (ATP,GTP,UTP,CTP) - building blocks of new RNA strand
3. RNA polymerase

Question 5

How many strands of the dsDNA template are transcribed?

Answer 5

One - the template strand (antisense).

Question 6

What is the coding (sense) strand in RNA transcription.

Answer 6

Same sequence as the DNA transcript, but with uracil instead of thymine.

Question 7

What are the stages of transcription?

Answer 7

Initiation
Elongation
Termination

Question 8

How is transcription in prokaryotes initiated?

Answer 8

A promoter DNA sequence directs RNA polymerase to the correct site to begin transcription.

Question 9

What are the two common promoter sequences in prokaryotes?

Answer 9

-10 and -35 elements ( located 10 and 35 nucleotides upstream - towards the 5’ end of the start site. (Physical card 18).

Question 10

How are the promoter sites in bacteria recognised (their code)?

Answer 10

TTGAACA and TATAAT are the average sequences deduced from analysing many promoter sequences. Not the same for every promoter - just the most common.

Question 11

The efficiency or strength of a promoter sequence dictates what?

Answer 11

The frequency of transcription - e.g once every 2 seconds to once every 10 minutes.

Question 12

Strong promoters have sequences that resemble what?

Answer 12

The average consensus sequence - they are more likely to be transcribed.

Question 13

What will happen if the distance between promoter sequences is too far away or too close?

Answer 13

Will impact how often the gene is transcribed. The sigma subunit may not align with the promoter sequences if they are too far apart/close.

Question 14

Promoters are recognised by _______.

Answer 14

The RNA polymerase sigma subunit.

Question 15

What is the role of the sigma subunit?

Answer 15

Decreases the general affinity of RNA polymerase to DNA (looser grip so it can move along the sequence quickly looking for the promoter).

Question 16

What happens to the sigma subunit once transcription is initiated?

Answer 16

It is released from the core enzyme.

Question 17

What happens during the elongation phase of transcription in prokaryotes?

Answer 17

Physical card 19.

Question 18

Elongation proceeds within a _________.

Answer 18

Transcription bubble.

Question 19

RNA has a proofreading activity? When?

Answer 19

Yes. When in the presence of accessory proteins.

Question 20

Why does transcription terminate?

Answer 20

Signals within the newly synthesised RNA - NOT the DNA template.

Question 21

What are the two types of stop signals in RNA transcription?

Answer 21

Hairpin loop

Rho protein binding to newly made RNA strand.

Question 22

Explain how the hairpin loop signal works to terminate RNA transcription.

Answer 22

Formed by GC rich region with 4 or more uracil. Uracil region dissociates from DNA template strand and seeks coding strand - re-anneal with the RNA.

Question 23

Explain how the Rho binding signal works to terminate RNA transcription.

Answer 23

Uses ATP to go along newly formed RNA strand until it collides with the transcription bubble and makes the RNA strand break free from DNA template.

Question 24

How is the expression of functionally related genes controlled? Give an example.

Answer 24

As an operon. Bacteria use glucose as a main source of energy, but can use lactose when glucose is scarce. Bacteria will need to produce a set of enzymes that breakdown and transport lactose

Question 25

What is the lac operon?

Answer 25

3 lactose genes have the same promoter and so are transcribed together as one mRNA.

Question 26

What happens to the lac genes when lactose is absent?

Answer 26

Repressor binds to the operator site and blocks the expression of lac genes as there is no need to make the lac genes.

Question 27

What happens to the lac genes when lactose is not present?

Answer 27

A lactose derivative - allolactose - binds to a repressor and it detaches from the operator site - allowing lac genes to be expressed. The shape of the promoter is also changed so it can’t bind to the sequence.

Question 28

How can lac expression be regulated by glucose.

Answer 28

Physical card 20.

Question 29

What is the trp operon?

Answer 29

Bacteria can make tryptophan using enzymes encoded for by 5 genes in the trp codon. All under same promoter.

Question 30

What occurs at the trp operon when tryptophan is present?

Answer 30

Bacteria take up tryptophan and not make any so trp genes are switched off - tryptophan binds to the repressor and the two bind to the operator site to prevent transcription.

Question 31

What occurs at the trp operon when tryptophan is absent?

Answer 31

No tryptophan, repressor cant bind to the operator and so transcription of trp genes allowed.