DNA Replication (College Board AP® Biology): Study Guide

DNA replication

• DNA replication occurs during S phase of the cell cycle; this means that there is enough genetic information for transmission between generations

• The process of DNA replication ensures that DNA is copied accurately, ensuring continuity of hereditary information

• DNA replication is a semiconservative process; this means that in each replicated DNA molecule half of the content comes from the original piece of DNA, and half is new

  • One strand of DNA serves as the template for a new strand of complementary DNA

The DNA replication process

1. Helicase enzyme unwinds the DNA strands, exposing the template strand

   • This unwinding increases supercoiling in other regions of the DNA molecule; topoisomerase enzyme relaxes this supercoiling in front

     of the replication fork

2. RNA primers attach to the template strand

   • These are short, single-stranded pieces of DNA that form a region to which DNA polymerase can bind

3. DNA polymerase synthesizes new strands of DNA by joining the deoxyribose sugar and phosphate groups of adjacent nucleotides and creating a new sugar-phosphate backbone

   • This occurs continuously on the leading strand and discontinuously on the lagging strand

   • The reason for this is that DNA polymerase can only add new nucleotides to the 3' end of a DNA molecule; new molecules are therefore always built in a 5' to 3' direction

4. Ligase enzyme joins the fragments of DNA on the lagging strand to complete the new DNA molecules