Describe semi-conservative DNA replication
In S phase, each parental DNA strand acts as a template; DNA polymerase builds new strands and DNA ligase joins fragments on the lagging strand.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does semi-conservative replication produce?
What you will learn
- Explain why replication is semi-conservative
- Describe the roles of DNA polymerase and DNA ligase
- Compare leading- and lagging-strand synthesis using the 5′ to 3′ direction
Each daughter DNA molecule conserves one parental strand
The two parental DNA strands separate and each acts as a template for a new complementary strand. Each daughter DNA molecule contains one original strand and one newly made strand, so replication is semi-conservative.
Replication occurs during S phase before mitosis. Complementary base pairing helps preserve the sequence information in the new strands.
DNA polymerase builds only 5′ to 3′
DNA polymerase adds complementary DNA nucleotides to a growing strand in the 5′ to 3′ direction. Because the parental strands are antiparallel, synthesis is continuous on the leading strand and discontinuous in short sections on the lagging strand.
DNA ligase joins the short sections on the lagging strand by sealing the sugar–phosphate backbone. The syllabus limits named replication enzymes here to DNA polymerase and DNA ligase; other enzyme names are not expected.
Pause and explain
What is the role of DNA ligase in lagging-strand synthesis?
Worked example
A daughter DNA molecule contains one parental strand and one new strand. Explain why this is semi-conservative replication.
Show the worked solution
- Each parental strand templates a new complementary strand.
- Each daughter molecule retains one original strand.
Answer Each parental strand acts as a template for a new complementary strand. Each daughter DNA molecule conserves one of the original parental strands and contains one newly synthesised strand, so replication is semi-conservative.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outline semi-conservative DNA replication.
Give me a hint
Include parental templates and the composition of each daughter molecule.
Compare my reasoning
- The parental strands separate and each acts as a template.
- A new complementary strand is made beside each original strand.
The two parental strands separate and each acts as a template for a new complementary strand. Each daughter DNA molecule contains one parental strand and one newly synthesised strand.
Look for these in your work
- Template role stated
- One-old/one-new composition stated
Explain why lagging-strand DNA is made in short sections and name the enzyme that joins them.
Give me a hint
Relate antiparallel templates to the 5′ to 3′ rule.
Compare my reasoning
- DNA polymerase adds nucleotides only 5′ to 3′.
- DNA ligase joins the short sections.
Because the DNA strands are antiparallel and DNA polymerase synthesises only 5′ to 3′, the lagging strand is made discontinuously in short sections. DNA ligase joins those sections.
Look for these in your work
- Direction rule linked to discontinuity
- Ligase role correct
A replication diagram labels polymerase as adding DNA nucleotides 3′ to 5′ and ligase as choosing complementary bases. Correct both labels.
Give me a hint
Separate nucleotide addition from joining fragments.
Compare my reasoning
- Polymerase adds nucleotides 5′ to 3′.
- Ligase joins short sections of the lagging strand.
DNA polymerase adds nucleotides to the growing strand in the 5′ to 3′ direction and uses complementary base pairing. DNA ligase joins the short lagging-strand sections.
Look for these in your work
- Polymerase direction corrected
- Ligase role corrected
Common mistakes
- Saying each daughter DNA molecule contains two old or two new strands
- Claiming DNA polymerase adds nucleotides in either direction
- Assigning DNA ligase the role of selecting complementary bases
What can you explain now?
What does semi-conservative replication produce?
Compare with the explanation
Each DNA molecule has one old and one new strand
Each daughter molecule conserves one parental strand.
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Make it stick
Why is DNA replication semi-conservative, and how do polymerase and ligase contribute to leading and lagging strands?
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