DNA replication

Question 1

Job of DNA helicase?

Answer 1

Unwinds double helix and separates it into 2 strands; replication fork –> breaks H bonds

Question 2

Job of single-stranded binding proteins?

Answer 2

Stabilise denatured DNA after strands have been separated

Question 3

Job of DNA primase?

Answer 3

Synthesises short RNA primer on DNA strand (starting point) to start replication

Question 4

What happens as strands separate?

Answer 4

Bases exposed, free nucleotides pair with complementary bases on template strand

Question 5

Job of DNA polymerase II?

Answer 5

Binds to primer and carries out elongation of new strand of DNA on leading strand (joins nucleotides)

Question 6

What is required for addition of new nucleotide to strand?

Answer 6

Free 3’-OH (initially provided by RNA primer)

Question 7

Why is DNA replication semi conservative?

Answer 7

Each new molecule contains one original, conserved strand and one newly synthesised strand

Question 8

What direction is DNA added?

Answer 8

5’ to 3’

Question 9

What are Okazaki fragments?

Answer 9

Short fragments of DNA synthesised on lagging strand

Question 10

Lagging strand: 3’ CGCATGT 5’

Answer 10

Leading strand: 5’ GCGTACA 3’

Question 11

Describe process of DNA replication

Answer 11

1. Double helix unwinds and separates into 2 strands at the replication fork with help of DNA helicase
2. Single-stranded binding proteins stabilise denatured DNA (both strands used as template)
3. DNA primase synthesises short RNA primer (starting point) to start replication
4. Free DNA nucleotides pair with complementary bases on template strand and H bonds form (bases exposed as strands separate)
5. DNA polymerase binds to RNA primer and carries out elongation of new strand of DNA (joins nucleotides)
6. New nucleotides join to adjacent ones by phosphodiester bonds
7. DNA ligase seals gaps on both strands
8. 2 identical molecules of DNA form

Question 12

What does RNA primer initially provide?

Answer 12

Free 3’-OH

Question 13

What is RNA primer made by?

Answer 13

DNA primase

Question 14

Job of DNA ligase?

Answer 14

Seals gaps between fragments on both strands

Question 15

Which enzyme is responsible for:

a) Separating two strands of DNA
b) Stabilising denatured DNA
c) Synthesising RNA primer on DNA strand
d) Joining nucleotides together (elongation of new DNA strand)
e) Sealing gaps between fragments on strands

Answer 15

a) DNA helicase
b) Single-stranded binding proteins
c) DNA primase
d) DNA polymerase
e) DNA ligase

Question 16

What are leading / lagging strands? How are they formed?

Answer 16

When DNA unzips you are left with one leading strand and one lagging strand (one strand running 5’-3’ and the other running 3’-5’).

DNA is synthesised in the 5’-3’ direction:

• The 5’-3’ strand becomes the LEADING strand
• The 3’-5’ strand becomes the LAGGING strand

Question 17

Describe the synthesis of DNA on the leading strand

Answer 17

Continuous (DNA polymerase can add DNA bases with ease in 5’-3’ direction)

Question 18

Describe synthesis of DNA on lagging strand?

Answer 18

Discontinuous –> made as short strands called OKAZAKI FRAGMENTS

Question 19

What is each short strand of DNA on the lagging strand primed with? What then replaces these RNA primers with a short row of DNA in the 5’-3’ direction?

Answer 19

New RNA primer

DNA polymerase I

Question 20

One the new DNA has been made (lagging strand), how are all the RNA primers removed?

What fills in the gaps left by these RNA primers?

Answer 20

By the enzyme exonuclease

DNA polymerase fills in the gaps and then DNA ligase seals the gaps between fragments

Question 21

What is required for DNA synthesis by DNA polymerase?

Answer 21

1. Template strand
2. Free 3’-OH (originally provided by RNA primer)
3. All 4 dNTPs