Transcription And Translation

Question 1

Where does transcription happen?

Answer 1

Nucleus

Question 2

What RNA polymerase does it use?

Answer 2

RNA polymerase II

Question 3

Describe the initiation stage

Answer 3

Initiation code recognised (tata)
Transcription factors bind at this code, upstream of gene and attract RNA polymerase to start mRNA production
RNA polymerase separates DNA strands for RNA nucleotides to bind to the template strand

Question 4

Describe elongation stage

Answer 4

RNA polymerase travels along template strand picking up BPs and copying on to complimentary RNA strand
Process continues until genetic sequence is transcribed in to mRNA

Question 5

Describe termination

Answer 5

Replication forks meet

mRNA released

Question 6

3 processes in converting immature mRNA to mature mRNA

Answer 6

Capping
Polyadenylation
Splicing

Question 7

Describe capping

Answer 7

mRNA adds a methyl-guanine cap to 5’ end

Bonded with a 5’-5’ triphosphate linkage to stabilise mRNA

Question 8

Describe polyadenylation

Answer 8

At 3’ end lots f adenine nucleotides are added (~200)

Protects against degrading

Question 9

Describe splicing

Answer 9

Pre mRNA has introns and exons. Introns removed by :

• endonucleases - breaks with in the polynucleotide can be specific / non specific
• exonucleases - degrade polynucleotides from their ends - can be either 3’ or 5’ specific

Question 10

3 different types of RNA

Answer 10

mRNA ( lots of kinds, few copies of each present, contains info on protein code, RNA polymerase II, ~2%)
rRNA ( few kinds and many of each, used to bind to mRNA and provides location for tRNA, >80% , RNA polymerase I)
tRNA ( 100s of kinds, many of each, ~15%, RNA polymerase III, tRNA uncharged without AA and charged with one = aminoacyl-tRNA )

Question 11

Where does translation happen?1

Answer 11

In the cytoplasm on the ribosomes of the rER

Question 12

Describe translation initiation?

Answer 12

At 5’ cap end of mRNA the 40s subunit with Met-tRNA binds
To start the starting codon 5’AUG must be recognised - codes for methionine
Anticodon on the tRNA is 5’CAU
60s subunit then binds for translation to start

Question 13

Describe translation elongation

Answer 13

rRNA has 2 sites for the tRNA to bind :
*p site = site for holding the peptide chain
*a site = site for accepting the tRNA
Met -tRNA occupies the p site, then another amino-acyl-tRNA enters to occupy the a site (requires GTP). Methionine forms a peptide bond with the next amino acyl-tRNA in the p site, making the tRNA at the p site become uncharged, this now leaves and ribosome moves along (translocation )

Question 14

Describe translation termination

Answer 14

Termination requires a stop codon to be read on the mRNA
Can be one of 5’UAA, 5’UAG or 5’UGA
No tRNA can bind to those, so peptide and tRNA are hydrolysed to release the protein into the cytoplasm

Question 15

Gene expression in Bacteria is different, this is because bacteria have:

Answer 15

Simple promoters
Different transcription factors 
Single RNA
Coupled transcription/translation
No past translational processing
Short lives mRNA 
Simple ribosomes
Distinctive translation initiation mechanism 
Different translation factors
No 5' capping, 3'tailing or splicing (therefore no introns)

Question 16

What is a polysome?

Answer 16

An mRNA template covered with many actively translating ribosomes

Question 17

What can mutations outside the coding region cause?

Answer 17

Mutations to the promoter regions where transcription factors bind can affect gene expression to either constitutively activate or deactivate it