A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Semi-Conservative Replication of DNA — mark scheme explained
The short answer
The semi-conservative replication of DNA is a fundamental process that ensures genetic continuity between generations of cells. This mechanism guarantees that each new cell receives an exact copy of the genetic material, maintaining the integrity and stability of the genome.
The question
Explain the role of DNA helicase and DNA polymerase in semi-conservative DNA replication. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the roles of each enzyme.
- S2
DNA helicase unwinds the double helix by breaking hydrogen bonds between complementary bases, separating the two strands.
- S3
DNA polymerase catalyzes the condensation reaction that joins adjacent nucleotides, forming new DNA strands in a 5' to 3' direction.
- S4
Step 2: Describe how these enzymes work together.
- S5
As DNA helicase separates the strands, free nucleotides are attracted to the exposed bases through complementary base pairing. DNA polymerase then extends the new strand by adding nucleotides one at a time.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the roles of each enzyme.
- S2
DNA helicase unwinds the double helix by breaking hydrogen bonds between complementary bases, separating the two strands.
- S3
DNA polymerase catalyzes the condensation reaction that joins adjacent nucleotides, forming new DNA strands in a 5' to 3' direction.
- S4
Step 2: Describe how these enzymes work together.
- S5
As DNA helicase separates the strands, free nucleotides are attracted to the exposed bases through complementary base pairing. DNA polymerase then extends the new strand by adding nucleotides one at a time.
Final answer: DNA helicase unwinds the double helix by breaking hydrogen bonds between complementary bases, separating the two strands. DNA polymerase catalyzes the condensation reaction that joins adjacent nucleotides, forming new DNA strands in a 5' to 3' direction. As DNA helicase separates the strands, free nucleotides are attracted to the exposed bases through complementary base pairing. DNA polymerase then extends the new strand by adding nucleotides one at a time.
Common mistakes
- Confusing the roles of DNA helicase and DNA polymerase. — Remember that DNA helicase unwinds the double helix by breaking hydrogen bonds, while DNA polymerase catalyzes the condensation reaction to join adjacent nucleotides.
- Forgetting the directionality of DNA synthesis by DNA polymerase. — Always specify that DNA polymerase extends new strands in a 5' to 3' direction.
- Misunderstanding the concept of semi-conservativeness. — Practice explaining that semi-conservative replication ensures each new cell gets one original and one new strand, maintaining the integrity of the genome.
- Incorrectly describing the base pairing rules. — Always remember that adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C).
- Failing to explain the importance of experimental evidence for the semi-conservative model. — Practice explaining that density gradient centrifugation showed DNA molecules with intermediate density in the first generation after transfer, supporting the idea of one original and one new strand.
- Confusing the roles of different enzymes involved in DNA replication. — Review the specific roles of each enzyme: DNA helicase unwinds the double helix, and DNA polymerase joins adjacent nucleotides.
Where the marks go
- Full worked solution (all marking points)4 marks