Lecture 7 - How is DNA Copied and How Are Genetic Changes Generated and Inherited?

Question 1

What are the 3 different models of relication?

Answer 1

• conservative
• semiconservative
• dispersive

Question 2

What is the conservative model of replication?

Answer 2

parental strands stay together during replication

Question 3

What is the semi conservative model of replication?

Answer 3

parental strands separate and act as a template

Question 4

What is the dispersive model of replication?

Answer 4

broken strands - some old some new

Question 5

What is the process of the experiment to prove the semiconservative model of replication was in place?

Answer 5

• bacteria are cultured in a medium with 15N (heavy isotope)
• bacteria transferred to medium with 14N (lighter isotope)
• DNA sample centrifuged after one replication will haven band around halfway down the tube
• DNA sample centrifuged after second replication will have too bands - more dense half way and less dense at the top

Question 6

What are the 3 main stages of DNA replication?

Answer 6

1) initiation
2) elongation
3) termination

Question 7

How many origins of replication are there in each bacterial chromosome?

Answer 7

one

Question 8

Describe the process of initiation in E. coli

Answer 8

• Dna-ATPase bins to the origin
• DnaC loads DnaB onto the single strand
• primase binds, releasing DnaC and activating DnaB

Question 9

What is Dna_ATPase?

Answer 9

ATPase -> hyrolyses ATP

Question 10

What is DnaC?

Answer 10

inhibits DnaB from unwinding the DNA

Question 11

What is DnaB?

Answer 11

helicase enzyme in bacteria - opens the replication fork

Question 12

What is primase?

Answer 12

synthesises short RNA primers

Question 13

What is the role of the single stranded binding proteins (SSB)?

Answer 13

protects the single stranded DNA until it has become double stranded again

Question 14

What is the difference between eukaryotes and prokaryotes replication?

Answer 14

prokaryotes only have one origin of replication, whereas eukaryotes have more than one as they have linear chromosomes.

Question 15

Describe the process of elongation in E.coli

Answer 15

• polymerase extends from 3’ end of growing chain and RNA primer
• primase can initiate new strand

Question 16

Describe elongation in the leading strand

Answer 16

• extension is continuous
• primase makes RNA primer then leaves
• DNA polymerase III extends from 3’ end of the primer
• following helicase

Question 17

Describe elongation in the lagging strand

Answer 17

• primase and polymerase III replicate 5’ to 3’, even though replication fork is moving 3’ to 5’
• pol III will only replicate until it reaches the next primer
• pol I removes primer as it has exonuclease activity
• DNA ligase joins fragments

Question 18

What does the exonuclease activity of pol I mean?

Answer 18

proofreading function from 3’ to 5’

Question 19

What happens in termination of replication in E.coli?

Answer 19

• replication forks meet
• DNA ligase joins fragments