TRANSCRIPTION DNA-mRNA

Question 1

What happens in transcription and translation

Answer 1

Trc - make messenger RNA copy of the information in a gene that codes for a protein

Trl - conversion of RNA message into protein at ribosomes

Question 2

What extra steps are present in gene expression in Eukaryotes that arent present in Prokaryotes?

Answer 2

RNA processing (splicing step) and RNA transport (made in nucleus so must travel to ribosome)

Question 3

Differences in transcription and translation in Pro vs Euk

Answer 3

-Both happen close together in Pro as nucleoid is in cytoplasm
-Transcription doesnt have to be complete for translation to start in prokaryote - cant happen in eukaryotes due to compartmentalisation

Question 4

What is a gene composed of?

Answer 4

Specific base sequences organised in a way that allows DNA to be transcribed
-Contains a promotor, terminator and other regulatory sequences

Question 5

What is the regulatory sequence?

Answer 5

Site for the binding of regulatory proteins
-Role of regulatory proteins is to influence the rate of transcription

Question 6

What attracts RNA polymerase?

Answer 6

-10 (pribnow box) and -35 bases before transcription start site there is a common or consensus sequence
These are known as the core promotor elements

Question 7

What makes a promoter a good promoter?

Answer 7

The more closely the promoter resembles the consensus sequence the stronger the promotor effect and the better the promoter is.

Question 8

What is the enzyme that catalyses transcription?

Answer 8

RNA polymerase

Question 9

What are the 5 subunits of the core enzyme of bacterial RNA polymerase and what do they do?

Answer 9

-alpha 2- 2 alpha subunits assemble the enzyme and bind regulatory factors (influence level of trc)
-ß - has the polymerase activity (catalyses the synthesis of RNA) which includes chain initiation and elongation
-ß’ - binds to DNA (nonspecifically)
-w - restores denatured RNA polymerase to its functional form in vitro, has protective function to the ß’ subunit and promotes enzyme assembly

Question 10

What is the additional factor involved in RNA polymerase holoenzyme and what is its role?

Answer 10

-o
Role:
–Reduce affinity of enzyme for non-specific DNA
–Increase affinity for promoter sites - directs core enzyme to where it should go

Question 11

When does the sigma (o) factor dissociate from the core enzyme?

Answer 11

once a promoter has been bound - then goes to find another core enzyme

Question 12

What does RNA polymerase 1 synthesise?

Answer 12

A pre-rRna 45S which matures into 28S, 18S and 5.8S rRNAs which will form the major RNA sections of the ribosome

Question 13

What RNA polymerases do eukaryotes have?

Answer 13

RNA polymerase 1, 2 and 3
-diff types characterized by type of RNA they synthesise

Question 14

What does RNA polymerase II synthesis?

Answer 14

Synthesises precursors of mRNAs - most studied type - a range of transcription factors are required for its binding promoters

Question 15

What does RNA polymerase III synthesise?

Answer 15

Synthesizes tRNAs, 5S rRNA and other small RNAs found in the nucleus and cytosol

Question 16

How is the initiation of transcription achieved in eukaryotes?

Answer 16

-Transcriptiom initiation is more complex in euk than in pro
-RNA polymerase does not directly recognise the core promoter sequences
-Transcription factors mediate the binding of RNA polymerase forming a transcription initiation complex

Question 17

What happens in the next stage (stage 2) of transcription - Elongation?

Answer 17

-The polymerase unwinds the template DNA ahead of it and rewinds the DNA behind it, maintaining an unwound region of about 17 bps in region of transcription
-The template strand of DNA is copied to RNA using complimentary base pairing and insertion of appropriate ribonucleotides

-Multiple RNA polymerase can transcribe a single DNA molecule simultaneously
-RNA polymerase can proofread

Question 18

What are the 2 processes that can take place in bacteria during termination?

Answer 18

-Rho-dependent
-Rho-independent

both lead to release of mRNA transcript

Question 19

How does termination take place in eukaryotes?

Answer 19

Less understood but occurs in conjunction with polyadenylation of the transcripts

Question 20

What happens in Rho-dependent termination?

Answer 20

-Rho protein binds the single stranded RNA as it exits from polymerase enzyme complex, it can move along the mRNA strand and uses a helicase activity to release mRNA from the enzyme complex.

Question 21

What happens in Rho-independent termination?

Answer 21

-Terminated due to specific sequence in terminator DNA
-Terminator DNA contains invert repeat which causes complimentary pairing within transcript RNA forming hairpin structure
-PolyT in template strands leads to polyU /polyA structure - combined with hairpin leads to instability and termination

Question 22

What is the template strand? What is it also referred to as?

Answer 22

-Serves as DNA template for transcription
-Addition of nucleotides occurs in the 5’ to 3’ direction by RNA polymerase
-Directionality of template strand should be 3’ to 5’

-Also referred to as antisense strand, non-coding strand or neg strand, transcribed strand

Question 23

What is the coding strand? What direction does it run?

Answer 23

-Runs in the 5’ to 3’ direction
-Contains the same nucleotide sequence in the mRNA strand.
-Also referred to as sense strand, non-template strand or pos strand