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 the formation of the new complementary strands by DNA polymerase


  • Unwinds the DNA and separates the two polynucleotide strands by breaking the hydrogen bonds between complementary base pairs
  • The two separated polynucleotide strands act as templates for the synthesis of new polynucleotide strands

DNA Polymerase

  • Synthesises new strands from the two parental template strands
  • Free deoxynucleoside triphosphates (nucleotides with three phosphate groups) are aligned opposite their complementary base partner and are covalently bonded together by DNA polymerase to form a complementary nucleotide chain
  • The energy for this reaction comes from the cleavage of the two extra phosphate groups

3.4.2  Explain the significance of complementary base pairing in the conservation of the base sequence of DNA

Each of the nitrogenous bases can only pair with its complementary partner (A=T ; G=C)

Consequently, when DNA is replicated by the combined action of helicase and DNA polymerase:

  • The new strands formed will be identical to the original strands separated from the template
  • The two DNA molecules formed will be identical to the original molecule

DNA Replication is a Semi-Conservative Process

3.4.3  State that DNA replication is semi-conservative

DNA replication is a semi-conservative process because when a new double-stranded DNA molecule is formed:

  • One strand will be from the original molecule
  • One strand will be newly synthesised