DNA Replication (OCR A Level Biology): Revision Note

Semi-conservative replication of DNA

• Before a cell divides it needs to copy its DNA to ensure that the two new daughter cells will both receive a complete set of genetic information

• The DNA is copied during a process known as semi-conservative replication

  • Semi = half

  • Conservative = the same

• At the end of the replication process each new DNA molecule consists of one strand that is conserved from the DNA molecule being copied and one strand that is newly created by the cell

Semi-conservative replication of DNA creates DNA molecules that consist of one conserved DNA strand (pink) and one new strand (blue)

The process of semi-conservative replication

• DNA replication occurs during the S phase of the cell cycle 

• The events of DNA replication are as follows:

  • The enzyme DNA helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel strands

  • Free DNA nucleotides are attracted to the exposed bases on each of the template strands

    • The free nucleotides contain extra phosphates, and are known as activated nucleotides

  • Adjacent nucleotides are then joined together by the enzyme DNA polymerase to form a new sugar-phosphate backbone

    • DNA polymerase catalyses condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides

    • The resulting bond is a phosphodiester bond

  • Hydrogen bonds form between complementary bases to form the ladder of the DNA double helix

Adjacent nucleotides are joined together by DNA polymerase to create the new complementary DNA strands

The importance of conserving one DNA strand

• Retaining one original DNA strand maximises accuracy during the DNA replication process, as each new DNA strand is created from an existing template

• Each new cell will always contain the same genetic information as its parent, meaning that genetic continuity is ensured between generations of cells

  • This is important because cells in our body are replaced regularly and we need the new cells to carry out the same roles as the parent cells

Random, spontaneous mutations

• Although the process of DNA replication is highly accurate, copying errors do occur, e.g.:

  • bases inserted into the complementary strand in the wrong order

  • an extra base being inserted

  • a base being left out

• These mistakes occur at random and are spontaneous, meaning that they do not have any particular cause

• Changes to the DNA base sequence are known as mutations