DNA Replication Replication is semiconservative one strand is

DNA Replication • Replication is semi-conservative (one strand is old, one strand new)

Origins of Replication • Where DNA Replication starts • Differs between organisms – Prokaryotes = 1 origin of replication – Eukaryotes = many different origins of replication

General Process of DNA Replication • Step 1: Initiator Proteins bind to origins of replication to begin replication – Attracts other enzymes involved in replication process • Step 2: – DNA Helicase separates the DNA helix

General Process • Step 3: Primase builds a primer at the 5’ end of the new DNA strand Primase 5’ 3’ 5’ RNA Primer DNA Polymerase III

General Process • Step 4: DNA Polymerase III builds the new strand of DNA in a 5’ to 3’ direction

-Nitrogenous bases are located in the cytoplasm -They enter the nucleus through the pores in the nuclear membrane

Problem… • How can both strands of DNA be replicated in a 5’- 3’ direction at the same time if they are antiparallel? • Answer: leading and lagging strands

Leading and Lagging Strands • Leading strand is synthesized continuously in the direction of replication (goes in the same direction as helicase) • Lagging strand is synthesized in short fragments the opposite direction of replication (opposite direction as helicase)

DNA Ligase • DNA Ligase seals Okazaki fragments together – Forms covalent bonds between nucleotides to create a continuous strand of DNA

Thymine is added accidentally. DNA polymerase cuts the damaged DNA. The proper base is added. Ligase joins the two spots.