DNA replication (S phase) “A Perfect Copy” Before a cell can divide, the DNA must be copied so a complete set of chromosomes is available for for both new daughter cells.

When looking at a double helix structure it is easy to see how simple it would be to copy it

Parent strand Daughter strand Daughter strand

Growth Replication fork DNA polymerase Replication fork DNA replication woks in many different “bubbles” at the same time - new nucleotides added as move towards forks Okazaki fragment Leading strand Lagging strand Single-strand binding proteins (SSB)

End results – two exact copies Each new copy has one parent strand and one daughter strand strand

enzymes Helicase – unwinds and unzips - breaks weak hydrogen bonds to separate strands - forms replication fork (“bubble”) Polymerase - attaches and joins complementary nucleotides to parent strand - also proofreads - must add in 5’ 3’ direction - only works when RNA primer is added to template first - keeps each single strand of DNA in replication fork from twisting so polymerase can function Ligase - joins nucleic acids of Okazaki fragments Topoisomerase - cuts backbone on DNA to reduce excessive twisting due to replication fork - repairs cut when complete Single-strand binding proteins -SSBP

Continuous replication Discontinuous replication

What are Okazaki fragments? Why do they form?

Chapter 12

DNA replication animations YouTube - DNA Replication Animation 2P0&feature=related 2P0&feature=related HTY HTY YouTube - DNA Replication Process