8.1 Gene Expression

Question 1

What enzyme/machinery is used in DNA rep, Transcription, translation respectively?

Answer 1

1. DNA polymerase
2. RNA polymerase
3. Ribosome

Question 2

What components are used for DNA replication, Transcription, Translation respectively?

Answer 2

1. dNTPs - deoxynucleotidetriphosphates
2. NTPs - ribonucleotidetriphosphates
3. Amino acids

Question 3

Describe the initiation stage of transcription.

Answer 3

1. Promoter Sequence recognition - TATAAA
2. Transcription initiation factor joins here
3. RNA polymerase recruited to wherever TF is

Question 4

Describe the elongation stage in transcription

Answer 4

1. DNA unwinds and opens
2. DNA coding strand (5’ to 3’) used as a template
3. RNA polymerase moves along and inserts free nucleotides by base pairing (replacing T’s with U’s)
4. Form mRNA

Question 5

How does transcription terminate?

Answer 5

1. RNA polymerase reaches the end of the gene (3’ termination sequence)
2. Stops adding nucleotides to pre-mRNA
3. Falls off template DNA and pre-mRNA is released

Question 6

What are the three types of RNA processing that turn pre-mRNA to mature mRNA? Explain each.

Answer 6

1) Capping
- methylated guanine forms 5’- 5’ linkage on the end of mRNA with 3 phosphines in between
- provides protection and plays role in translation

2) polyadenlyation or tailing
- after ‘cut’ sequence, polyApolymerase adds approx 200 adenine bases onto 3’ end of mRNA
- uses energy

3) splicing
- get rid of introns and put exons together

Question 7

Describe the ribosomes used in translation for i) prokaryotes then ii) eukaryotes

Answer 7

i) prokaryotes have 70S - 30S+50S

ii) eukaryotes have 80S - 40S+60S

Question 8

Describe the initiation stage in translation

Answer 8

1. 5’ methylated guanine cap on mRNA recognised by tRNA
2. Binds to 40S ribosome unit
3. ATP => ADP so tRNA can move over to first codon - start codon - AUG (so tRNA carries methionine)
4. 60S ribosome unit attaches

Question 9

How does the amino acid chain elongate during translation?

Answer 9

1. Large subunit has P and A sites
2. Methionine tRNA Starts in P site, next codon dependent tRNA joins at A site
3. Peptide bond forms between AA
4. Uncharged tRNA released by ribosome so P site free
5. Ribosome moves over so P site now taken up and A site free
6. N to C growth

Question 10

What is N to C growth?

Answer 10

N terminus - amine terminus
C terminus - carboxyl terminus

Growth of polypeptides through addition of AA happens in this way

Question 11

How does termination in translation occur?

Answer 11

1. Stop codon reached - UAA, UAG, UGA
2. Water released
3. AA chain released

Question 12

What is an intron?

Answer 12

DNA sequences present in genes that are transcribed but removed by splicing so do not go on to form amino acids in the resulting polypeptide chain

Question 13

What is an exon?

Answer 13

Coding piece of DNA (forms AA in polypeptide chain in translation)

Question 14

How does RNA splicing occur?

Answer 14

Specific sequences in introns that control splicing
Two cuts by ends of introns by spliceosome
Ends ligased together

Question 15

What is capping on mRNA and when does it occur?

Answer 15

Methylated guanine added to 5’ end of mRNA with three phosphates in between.
This happens as soon as transcription starts at transcription start site after TATA box