protein synthesis in prokaryotes

Question 1

what is required for transcription initiation?

Answer 1

• promoter
• circular double-stranded DNA
• RNA polymerase

Question 2

what type of protein is RNA polymerase?

Answer 2

• it is a multimeric protein, whereby it has many subunits

Question 3

what does the RNA polymerase consist of?

Answer 3

• core enzyme
• signma factor subunit

Question 4

what is an RNA polymerase holoenzyme?

Answer 4

• it consists of a core RNA polymerase enzyme,
• & a sigma factor subunit

Question 5

what is an RNA polymerase holoenzyme required for?

Answer 5

• transcription initiation

Question 6

what is the RNA polymerase core enzyme required for?

Answer 6

• transcription elongation

Question 7

what are the steps required for transcription intiation?

Answer 7

1. the sigma factor subunit binds to the core RNA polymerase enzyme to form the RNA polymerase holoenzyme
2. the sigma factor subunit in the holoenzyme recognises & binds to the promoter at the -35 region
3. allowing the holoenzyme to unwind & separate the DNA strands at the -10 region
4. after transcription initiation, the sigma factor is released, & the elongation process is catalysed by the core enzyme

Question 8

what are the steps involved in prokaryotic transcription termination?

Answer 8

1. the core enzyme transcribes the terminator sequence, forming a hairpin loop on the mRNA
2. the formation of the hairpin loop causes the RNA polymerase core enzyme to stall at the adenine-uracil region
3. destabilising the interaction between the DNA template strand & the mRNA, allowing the mRNA strand to dissociate from the RNA polymerase, terminating transcription

Question 9

what contributes to the stability of the hairpin loop?

Answer 9

• the hairpin loop is rich in cytosine & guanine

Question 10

does the polycistronic mRNA formed via transcription have a 5’ cao?

Answer 10

• NO, it does not contain a 5’ cap

Question 11

how is translation initiated in prokaryotes?

Answer 11

1. the initiator aminoacyl tRNA complex binds to the free 30S subunit
2. the 30S subunit-tRNA (fMet) complex then binds to the Shine Dalgarno Sequence on the mRNA
3. the 50S subunit joins with the 30S subunit-tRNA (fMet) complex to form the 70S ribosome
4. the tRNA (fMet) is positioned in the P site of the ribosome, where the anticodon of the tRNA (fMet) will base-pair with the start codon on the mRNA, initiating translation

Question 12

how is the initiator aminoacyl tRNA complex in prokaryotes different to the one in eukaryotes?

Answer 12

• in eukaryotes, the initiator aminoacyl-tRNA complex is methionine,
• whereas in prokaryotes, the initiator aminoacyl-tRNA complex is N-formylmethionine

Question 13

what consensus sequence does the Shina Dalgarno Sequence have?

Answer 13

• 5’ AGGAGG 3’

Question 14

what is the difference between prokaryotes & eukaryotes in terms of concurrence of transcription & translation?

Answer 14

• in prokaryotes, translation & degradation of mRNA begins before translation is complete
• however, in eukaryotes, translation must finish occurring before degradation of the mRNA can occur

Question 15

why can prokaryotes let transcription, translation, & mRNA degradation occur simultaneously on the same mRNA molecule?

Answer 15

• because mRNA molecules are synthesised, translated & degraded in the 5’ to 3’ direction

Question 16

is the site of polypeptide synthesis & the site of mRNA synthesis separate in prokaryotes?

Answer 16

• NO, they are not separated, as prokaryotes do not have a nuclear envelope

Question 17

does post-transcription modification occur in prokaryotes?

Answer 17

• NO

Question 18

does splicing occur in prokaryotes, & why?

Answer 18

• NO, as there are no introns present on the mRNA molecule

Question 19

what is the significance of tightly coupled transcription & translation in prokaryotes?

Answer 19

• to allow the prokaryotes to respond rapidly to environmental changes around them

Question 20

do prokaryotic mRNA have a long half-life?

Answer 20

• NO, they have a relatively short half-life,
• as they are rapidly degraded by RNases soon after they are synthesised