Why must DNA be replicated before cell division?

So that each daughter cell receives the same genetic information, which ensures genetic continuity.

What does DNA helicase do?

It unwinds the double helix and breaks the hydrogen bonds between complementary bases.

What is the role of the template strands?

Each original strand determines the base sequence of the new strand, by complementary base pairing.

How are new nucleotides positioned on the template?

Free nucleotides hydrogen bond to the exposed bases by complementary base pairing (A with T, C with G).

What does DNA polymerase do?

It catalyses condensation reactions that join adjacent nucleotides, forming phosphodiester bonds.

What was the conservative model?

The parent molecule stays intact and a completely new copy is made.

What was the dispersive model?

Each strand of the daughter molecules is a mixture of old and new DNA.

What did Meselson and Stahl grow their bacteria in?

First a medium containing ¹⁵N, then a medium containing ¹⁴N.

Which model did the one-division result rule out?

The conservative model, as no DNA was all ¹⁵N or all ¹⁴N.

Which model did the two-division result rule out?

The dispersive model, as there was DNA of two different densities.

State one limitation of the Meselson-Stahl evidence.

It is indirect, as it measures density, and it was gathered from bacteria only.

DNA replicationAQA A-Level Biology: Flashcards

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What does semi-conservative replication mean?

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What does semi-conservative replication mean?: Each new DNA molecule contains one original strand and one newly made strand.
Why must DNA be replicated before cell division?: So that each daughter cell receives the same genetic information, which ensures genetic continuity.
What does DNA helicase do?: It unwinds the double helix and breaks the hydrogen bonds between complementary bases.
What is the role of the template strands?: Each original strand determines the base sequence of the new strand, by complementary base pairing.
How are new nucleotides positioned on the template?: Free nucleotides hydrogen bond to the exposed bases by complementary base pairing (A with T, C with G).
What does DNA polymerase do?: It catalyses condensation reactions that join adjacent nucleotides, forming phosphodiester bonds.
What is the new strand for the template 5-base sequence ACGTT?: TGCAA.
What was the conservative model?: The parent molecule stays intact and a completely new copy is made.
What was the dispersive model?: Each strand of the daughter molecules is a mixture of old and new DNA.
What did Meselson and Stahl grow their bacteria in?: First a medium containing ¹⁵N, then a medium containing ¹⁴N.
Which model did the one-division result rule out?: The conservative model, as no DNA was all ¹⁵N or all ¹⁴N.
Which model did the two-division result rule out?: The dispersive model, as there was DNA of two different densities.
State one limitation of the Meselson-Stahl evidence.: It is indirect, as it measures density, and it was gathered from bacteria only.

Exam questions on DNA replication

Before a cell divides, its DNA is replicated. Two enzymes are involved. Enzyme X separates the two polynucleotide strands of the double helix. Enzyme Y catalyses the joining of nucleotides on each separated strand.
Explain why enzyme X must act before enzyme Y can join nucleotides on the template strands.2 marks
E. coli bacteria were grown for many generations in a medium containing the heavy isotope ¹⁵N, so that all their DNA contained ¹⁵N. They were then transferred to a medium containing only the lighter isotope ¹⁴N and allowed to divide. DNA was extracted and separated by density using centrifugation. After one division all the DNA had a single density, between that of ¹⁵N DNA and ¹⁴N DNA. After two divisions half the DNA had this intermediate density and half had the density of ¹⁴N DNA.
Explain how the result after two divisions supports the semi-conservative model but not the dispersive model.2 marks
One strand of a DNA molecule that is about to be replicated has the base sequence ATTGCCGATC.
Explain how the new strand is formed on this template strand.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).