1.2 The Replication of DNA

Question 1

explain ‘the events happening during DNA replication’

Answer 1

1. DNA is unwound and hydrogen bonds between bases are broken to form 2 template strands.
2. DNA polymerase can only add nucleotides in one direction.
3. Nucleotides are added (by complementary base pairing) to the 3’ (deoxyribose) end of the new template strand which is forming.

lagging strand
- replicated in fragments (because of the directionality of DNA polymerase)
- fragments are joined together by ligase
- many primers are needed

leading strand
- replicated continuously
- only one primer is needed

Question 2

what are the requirements for DNA replication and why is it necessary?

Answer 2

• DNA Template = provides the code to copy
• Supply of DNA Nucleotides = to build the new strand of DNA
• Supply of Energy (ATP) = to provide the enzymes with energy to: unwind the DNA, unzip the DNA, join up the backbone, pair up the new nucleotides and rewind the DNA helix
• Enzymes (DNA Polymerase and Ligase) = to pair up free nucleotides and join up the Okazaki fragments
• Primers = short strands of nucleotides which bind to the 3’ end of the template DNA strand allowing DNA polymerase to add DNA nucleotides

Question 3

why is DNA said to be semi-conservative?

Answer 3

as the new molecules of DNA produced each contain one of the original strands and one newly synthesised strand

Question 4

what are Okazaki fragments?

Answer 4

fragments that form during discontinuous replication of the lagging strand

Question 5

describe ‘the polymerase chain reaction (PCR)’

Answer 5

• it is a technique used for the amplification of DNA within a test tube
• it amplifies DNA using complementary primers for specific target sequences
• repeated cycles of heating and cooling amplify the target region of DNA
• can help to solve crimes , settle paternity suits and diagnose genetic disorders

In Stage 1 (T = 95°C), DNA strands are separated so hydrogen bonds are broken.
In Stage 2 (T = 55°C), the primers bind to target sequences.
In Stage 3 (T = 72°C), heat-tolerant DNA polymerase replicates region of DNA.

Question 6

describe ‘gel electrophoresis’

Answer 6

after PCR, scientists might use gel electrophoresis to separate fragments of DNA and produce a DNA profile