Helicase unwinds and separates the parental DNA strands at the replication fork. DNA polymerase then uses each parental strand as a template to synthesize a new complementary strand by adding nucleotides according to base pairing rules, thus precisely conserving the DNA sequence. This semi-conservative process produces two identical daughter DNA molecules each with one original parental strand and one newly synthesized strand.

1. Topic 3.4 - DNA Replication
3.4.1. Explain DNA replication in terms of unwinding of the double helix and separation of the
strands by helicase, followed by formation of the new complementary strands by DNA polymerase.
A. Unwinding double helix
• helicase controls unwinding of coiled DNA
B. Separation of strands by helicase
• helicase separates complementary strands of DNA, producing a
replication fork
• single strand binding proteins keep the two strands of DNA separate
C. Formation of complementary strands by DNA polymerase
• DNA polymerase uses a single parent strand of DNA as a template, adding free
nucleotides from solution to the parent/template strand according to the complementary
base pairing rules (A=T, G=C)
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2. 3.4.2. Explain the significance of complementary base pairing in the conservation of the base
sequence of DNA.
• DNA polymerase uses a single parent strand of DNA as a template, adding free
nucleotides from solution to the parent/template strand according to the complementary
base pairing rules (A=T, G=C)
• The 2 parent strands in the double helix of DNA can be represented as A
and A’
• When the 2 strands separate, each acts as a template for DNA synthesis
• Using strand A as a template, DNA polymerase, using complementary base pairing rules,
adds a newly synthesized A’ strand, thus producing A - A’ double-stranded DNA identical
to the parent DNA
• Using strand A’ as a template, DNA polymerase, using complementary base pairing rules,
adds a newly synthesized A strand, thus producing A - A’ double-stranded DNA identical
to the parent DNA
• Therefore, 2 DNA molecules, each A - A’ are produced
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3. 3.4.3. State that DNA replication is semi-conservative.
• Semi-conservative = when the double helix replicates, each of the two daughter molecules will
have
1. One old strand, derived from the parent molecule
2. And one newly made strand, from free nucleotides in solution, using the parent strands as
templates, and base pairing rules (A=T, G=C) to match newly synthesized strands complementary
to parental strands
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