12 - Gene Expression

Question 1

What are the three stages of making a polynucleotide/polypeptide and what things are needed?

Answer 1

1. Initiation
2. Elongation
3. Termination
   - Enzyme
   - Activated substrates
   - Template

Question 2

What are the three things needed to make DNA?

Answer 2

1. DNA Polymerase
2. dNTP’s
3. Template of DNA

Question 3

What are the three things needed to make RNA?

Answer 3

1. RNA Polymerase
2. NTPs
3. Template of DNA

Question 4

What are the three things needed to make a polypeptide?

Answer 4

1. Ribosome
2. Activated amino acids
3. mRNA template

Question 5

In DNA replication, what terminates replication?

Answer 5

When the replication forks meet

Question 6

How does transcription occur?

Answer 6

1. RNA polymerase binds to a promotor region at the start of a gene with the aid of transcription factors
2. DNA double helix separated and the 3’ to 5’ acts as a template strand, whilst the 5’ to 3’ which the promoter is on, is the coding strand
3. RNA nucleotides match up with complementary bases on DNA and RNA polymerase joins them together in 5’ to 3’ direction
4. Termination sequence reached and pre-mRNA released, with same code as 5’ to 3’

Question 7

What is the promoter sequence?

Answer 7

• Sequence on DNA that is recognised by a transcription factor and marks the start of transcription
• TATAAA box upstream or at 5’ of gene

Question 8

What does upstream and downstream mean?

Answer 8

Downstream is the direction of the gene (+1)

Upstream is behind the gene (-1)

THERE IS NO ZERO

Question 9

How is pre-mRNA modified?

Answer 9

• Capping (5’)
• Tailing (3’)
• Splicing

Question 10

What is the point of capping and tailing?

Answer 10

• Prevent degradation by polynucleases
• Increase stability of mRNA

- 5’ to 5’ Cap with methylguanine

- Poly A tail at 3’ using polyA polymerase

Question 11

Why is mRNA a bit longer than it should be before the polyA tail is added?

Answer 11

• Transcription termination sequence is further down than polyadenylation sequence
• Endonucleases cleave off chain after polyadenylation sequence before polyA polymerase adds polyA tail

Question 12

What is splicing?

Answer 12

• Removing introns from pre-mRNA by splicing at specific intron sites
• Stick exons together
• If mutation in 5’ and 3’ ends of intron sites some introns may not be spliced out

Question 13

What is a polyribosome/polysome?

Answer 13

• Lots of ribosomes on a single mRNA all making individual proteins

Question 14

What are ribosomes made up of?

Answer 14

Question 15

Why is their more than one type of RNA polymerase?

Answer 15

• 3 types to make different types of RNA
• e.g polymerase II makes mRNA, I makes rRNA

Question 16

Why are there so many different types of mRNA?

Answer 16

• pre-mRNA can be spliced in lots of different ways
• 20,000 genes and different mRNA for each gene

Question 17

What are the three characteristics of the genetic code?

Answer 17

• Degenerate
• Non-overlapping
• Triplet

Question 18

What are the initiation and termination sequences for translation?

Answer 18

Start codon (Met) - AUG

Stop codon (no aa) - UAA, UAG, UGA

Question 19

How many nucleotides long is a polypeptide with 300 amino acids?

Answer 19

300 x 3 = 900

900 + 3 = 903

ADD THREE FOR THE STOP CODON AS THIS DOESNT CODE FOR AN AA

Question 20

What is the structure of tRNA?

Answer 20

• Single stranded
• Clover
• Stem loops
• Anticodon
• A at 3’ end that amino acids bind to

Question 21

What is the relevance of the anticodon?

Answer 21

• Recognises codon on mRNA
• Amino acid bound to tRNA has complementary sequence to 3’ to 5’ anticodon

Question 22

How does one tRNA molecule recognise multiple codons?

Answer 22

• Wobble position
• Contains an I at the 5’ end of the anticodon, and I can bind to C, A and U

Question 23

How are amino acids for translation activated?

Answer 23

LOTS OF ENZYMES NEEDED FOR TRANSLATION

Question 24

What are the stages of translation?

Answer 24

1. Initiation:

• Methionyl tRNA recognises 5’ cap
• Small ribosome subunit binds to cap using GTP
• When AUG reached, large ribosome subunit attaches using ATP and ready to translate

2. Elongation:

- AUG moved from A site to P site

• Ribosome reads next codon in A site and tRNA with complementary anticodon enters
• Peptide bond forms between two amino acids by peptidyl transferase
• Translocation of ribosome so first tRNA molecule leaves and another tRNA enters
• Continues

3. Termination

• Stop codon enters A site
• Release factors enter P-site and cause enzymes forming peptide bonds to add water to amino-acyl tRNA causing release of peptide and tRNA