17. DNA REPLICATION AND REPAIR

Question 1

1. What is the benefit of having 2 strands of DNA that are complementary to one another?

Answer 1

• each strand acts as a template for building a new
  strand
• this happens during DNA Replication

Question 2

2. What is the Semiconservative Model of Replication?

Answer 2

WHEN A DOUBLE HELIX REPLICATES:
- each daughter molecules will have 1 old strand
- this is derived and conserved from the parent
molecule
- each daughter will also have 1 newly synthesised
strand

Question 3

3. What happens after the parent molecule unwinds?

Answer 3

• 2 new daughter strands are built
• these are built according to the base-pairing rules

Question 4

4. What are two adjectives that can describe DNA replication?

Answer 4

• fast
• highly accurate

Question 5

5. Which two other components are involved in DNA Replication and Repair?

Answer 5

1. More than a dozen enzymes
2. Proteins

Question 6

6. Where does DNA Replication begin?

Answer 6

• it begins at special sites
• these are called the Origins of Replication

Question 7

7. What happens when the DNA strands begin to separate?

Answer 7

• they open up a Replication Bubble

Question 8

8. What can be said about the contents of a Eukaryotic Chromosome?

Answer 8

• it may have hundreds or even thousands of Origins of
  Replication

Question 9

9. What is Bidirectional Replication?

Answer 9

• it is the replication that proceeds in both directions
  from each origin
• it does this until the entire molecule is copied

Question 10

10. Does this Diagram of the Origins of Replication of E.Coli make sense?

Answer 10

• yes

Question 11

11. Does this Diagram of the Origins of Replication of Eukaryotic Organisms make sense?

Answer 11

• yes

Question 12

12. Name the 4 contributing factors to the Initiation of Replication?

Answer 12

1. Replication Fork
2. Helicases
3. Single-Strand Binding Protein
4. Topoisomerase

Question 13

13. What is the Replication Fork?

Answer 13

• this is a Y-Shaped Region
• this is found at the end of each replicating bubble
• this is where the new DNA strands are elongating

Question 14

14. What are Helicases?

Answer 14

• they are enzymes
• they untwist the double helix at the replication forks
• this causes the unwinding of the DNA double stranded
  helix

Question 15

15. What is the Single-strand Binding protein?

Answer 15

• this binds and stabilises the single-stranded DNA
• it does this until the DNA can be used as a template

Question 16

16. What is Topoisomerase?

Answer 16

• this corrects the overwinding that happens
• this happens ahead of the Replication forks
• this happens by breaking, swivelling and rejoining DNA
  strands

Question 17

17. What are the 2 limitations of DNA Polymerase?

Answer 17

1. THEY CAN ONLY ADD NUCLEOTIDES TO A PRE-
   EXISTING NUCLEOTIDE CHAIN
   
   • they cannot add nucleotides from scratch
   • they cannot initiate synthesis of a polynucleotide
     chain without a pre-existing nucleotide chain
2. THEY CAN ONLY ADD NUCLEOTIDES IN THE 5’-3’
   DIRECTION
   
   • they can never add nucleotides in the 3’-5’ direction

Question 18

18. What is Primase?

Answer 18

• this is an enzyme
• it synthesises a short RNA primer from scratch
  (about 5-10 nucleotides long)
• it does this using the parental DNA as a template

Question 19

19. What occurs with the free 3’ end of the RNA Primer?

Answer 19

• it serves as the starting point for the synthesis of the
  new DNA strand by DNA Polymerase

Question 20

20. Does this Diagram make sense?

Answer 20

• yes

Question 21

21. What are DNA Polymerases?

Answer 21

• they are enzymes
• they catalyse the elongation of the new DNA
• this happens at the Replication Fork

Question 22

22. What do DNA Polymerases always require?

Answer 22

• they require a primer
• they require a DNA Template Strand

Question 23

23. What is the Elongation Rate in Bacteria and in Human Cells?

Answer 23

• 500 nucleotides per second in bacteria
• 50 nucleotides per second in human cells

Question 24

24. What are the 3 Overall stages of Synthesising a New DNA Strand?

Answer 24

1. Initiation
2. Elongation
3. Termination

Question 25

25. What is the Elongation of a New DNA Strand?

Answer 25

• this is the process in which nucleotides are added to
  a growing DNA strand
• each nucleotide comes from a Nucleotide
  Triphosphate (NTP)
  (this means that there are 3 Phosphate groups at thd
  end)

Question 26

26. What is AZT?

Answer 26

• this stands for Azido-Deoxy-Thymidine
• this blocks the replication process
• this is due to a modified 3’-OH group
• this is known as an Anti-Retroviral drug
• it blocks the S Phase of the Cell Cycle

Question 27

27. Does this diagram make sense?

Answer 27

• yes

Question 28

28. What does dATP stand for?

Answer 28

• this stands for Deoxy-Adenosine Triphosphate
• its sugar is Deoxyribose (DNA)
• it supplies Adenine to DNA

Question 29

29. What does ATP stand for?

Answer 29

• this stands for Adenosine Triphosphate
• it is a Ribonucleotide
• its sugar is Ribose (RNA)

Question 30

30. dATP is similar to ATP.
    What are their differences?

Answer 30

• there is a difference in their sugars

Question 31

31. What happens as each monomer of dATP joins the DNA strand?

Answer 31

• the dATP loses 3 phosphate groups
• these form the molecule known as Pyrophosphate

Question 32

32. Does this diagram make sense?

Answer 32

• yes