AS · Practice questions · Semi-conservative replication of DNA

Unzip, pair, join.

Six original Cambridge-style questions on DNA replication, from the roles of the two enzymes to predicting density gradient bands.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Template, polymerase, ligase.

Semi-conservative: each new molecule = one old strand + one new.
DNA polymerase builds new strands 5′ to 3′ only.
Leading: continuous. Lagging: fragments joined by DNA ligase.
Both enzymes make phosphodiester bonds; hydrogen bonds form by pairing.

Recall

[2 marks]

State the roles of DNA polymerase and DNA ligase in DNA replication.

DNA polymerase joins free DNA nucleotides, paired with the template strand, into a new strand by forming phosphodiester bonds (adding to the 3′ end). ✓
DNA ligase joins the short fragments of the lagging strand by forming phosphodiester bonds between them. ✓

Explain

[5 marks]

Describe the semi-conservative replication of a DNA molecule.

Hydrogen bonds between bases break and the double helix unwinds / the two strands separate. ✓
Both strands act as templates. ✓
Free DNA nucleotides pair with exposed bases by complementary base pairing: A with T, C with G. ✓
DNA polymerase joins adjacent nucleotides with phosphodiester bonds, building the new strand 5′ to 3′. ✓
Each new DNA molecule contains one original strand and one newly made strand. ✓

Explain

[3 marks]

Explain why one new strand is made continuously but the other is made as a series of short fragments.

The template strands are antiparallel, and DNA polymerase can only add nucleotides in the 5′ to 3′ direction. ✓
On the leading strand template, 5′ to 3′ synthesis is towards the replication fork, so it can continue as the fork opens. ✓
On the lagging strand template, 5′ to 3′ synthesis is away from the fork, so new fragments must keep being started near the fork, and these are joined by DNA ligase. ✓

Data

[3 marks]

Bacteria grown for many generations in ¹⁵N are transferred to ¹⁴N. Predict the percentages of heavy, intermediate and light DNA after three generations, and state which model of replication your answer assumes.

Heavy 0%; intermediate 25%; light 75%. ✓
After one generation 2 hybrid molecules; after two, 2 hybrid + 2 light; after three, 2 hybrid + 6 light (2 of 8 = 25%). ✓
Assumes semi-conservative replication. ✓

Analysis

[2 marks]

Explain how the result after one generation in ¹⁴N rules out conservative replication.

Conservative replication would keep the parent molecule intact, giving one heavy band and one light band. ✓
Only one band of intermediate density is seen, so every molecule contains one heavy and one light strand. ✓

Explain

[2 marks]

Suggest why it is important that DNA is replicated with great accuracy before cell division.

Each daughter cell must receive an identical copy of the genetic information. ✓
Errors would change base sequences (mutations), which could change polypeptides and harm cell function, and would be passed on to all descendant cells. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.