1.5.2 DNA replication

Question 1

Is DNA replicated conservatively or semi-conservatively?

Answer 1

Semi-conservatively

Question 2

What does each new double helix consist of? What does this ensure?

Answer 2

one original strand
one new strand
ensures continuity of genetic information between generations

Question 3

What are the first two steps of DNA replication?

Answer 3

DNA helicase breaks hydrogen bonds between complementary bases
allows unwinding of double helix

Question 4

What are the middle steps of DNA replication?

Answer 4

free DNA nucleotides are attracted to exposed bases and form complementary base pairs through hydrogen bonding
adenine pairs with thymine and cytosine pairs with guanine
replication occurs in 5’ to 3’ direction

Question 5

What are the last two steps of DNA replication?

Answer 5

phosphodiester bonds formed by condensation reactions between adjacent free nucleotides catalysed by DNA polymerase
two new DNA molecules formed, each with one original and one new strand

Question 6

What does DNA polymerase do?

Answer 6

• proofreads newly added nucleotides, reducing the mutation rate and ensuring high fidelity replication
• catalyses the formation of new phosphodiester bonds to create the new sugar-phosphate backbones of the two new DNA strands

Question 7

Mark scheme of DNA replication

Answer 7

helix is unwound
two strands are separated
DNA helicase
by breaking hydrogen bonds between bases
new strands formed on each of the two single strands
nucleotides added to form new strands
complementary base pairing
A-T G-C
DNA polymerase forms the new complementary strands
replication is semi-conservative
each of the DNA molecules formed has one old and one new strand

Question 8

What is the role of DNA ligase?

Answer 8

joins Okazaki fragments; without it, the lagging strand remains fragmented

Question 9

Use knowledge of enzyme action to explain why DNA polymerase works in opposite directions.

Answer 9

DNA has antiparallel strands
shape of nucleotides is different/nucleotides aligned differently
enzymes have active sites with specific shape
only substrates with complementary shape can bind with active site of enzyme