Lecture 13

Question 1

What direction is DNA always synthesized?

Answer 1

5’ –> 3’

Question 2

How many chromosomes are found in humans?

Answer 2

23

Question 3

Why are there multiple origins of DNA replication?

Answer 3

Because if it wasn’t happening in multiple zones then it would take too long.

Question 4

Draw a replication bubble.

Answer 4

Should include :
- 2 parent strands
- replication fork
- leading strand
- lagging strands
- RNA primers
- okazaki fragments
- correct directions

Question 5

What does DNA pol III do?

Answer 5

• needs OH group for Ph group to attach
• makes DNA in 5’ –> 3’ direction
• adds complementary base pairs to parental template strands

Question 6

What do SSBP (single stranded binding proteins) do?

Answer 6

• prevent degradation of the single strands of DNA
• prevent complimentary bases of parental strands from snapping back together

Question 7

How does DNA replication initiation work for the leading strand?

Answer 7

• helicase pulls strands apart
• primase adds RNA primer
• DNA pol III adds DNA nucleotides one at a time
• when DNA pol III runs into SSBP, knocks them off

Question 8

How does DNA replication initiation work for the lagging strand?

Answer 8

• primase adds RNA primer
• DNA pol III ‘jumps’ adding complimentary base pairs in 5’-> 3’ direction

Question 9

What is the purpose of DNA pol I?

Answer 9

• DNA has no RNA in it, so must get rid of RNA primers
• removes RNA primers with RNase H activity
• fills the gaps made with nucleotides

Question 10

What is the purpose of DNA ligase?

Answer 10

• joins the okazaki fragments together forming phosphodiester bonds
• joins replication bubbles together

Question 11

What is the purpose of topisomerase?

Answer 11

To cut and stick DNA back together during replication to release tension.

Question 12

How are repairs made to DNA during replication?

Answer 12

• normally highly accurate, so isn’t done often
• DNA pol III has a proof-reading mechanism, detecting if there is a damaged or incorrectly placed base
• if there is a wrong base pair, it will remove it and replace it with the correct base
• this is called exonuclease activity

Question 13

How are repairs made after DNA replication?

Answer 13

• can be a variety of causes
• damaged/incorrect base removed + flanking region
• DNA pol III makes new DNA/inserts new nucleotides
• DNA ligase joins new DNA with existing DNA

Question 14

Why is it important that DNA is corrected?

Answer 14

• error becomes a part of the DNA template, and will then be replicated
• leads to permanent DNA change
• causes mutation