DNA Replication

1. A cell's entire DNA is copied, or replicated
2. Occurs during the synthesis (S) phase during eukaryotic cell cycle
3. both strands of double helix can be template for reproduction of new strand
4. Helicase and Polymerase
• DNA helicase (enzyme) breaks hydrogen bonds holding two strands together and forms a replication fork
• results in two branching strands of DNA backbone with exposed bases
• exposed bases let DNA be read" by DNA polymerase (enzyme) 
• builds into complementary DNA strand
• 5'-->3'
• new strands are "built" in opposite directions
• leading strand: constructs in 5'-->3' direction
• made in continuous manner
• lagging strand: constructs in 3'-5' direction
• lagging strand synthesized in short segments called Okazaki fragments
• on lagging strand, primase (enzyme) builds short RNA primer
• DNA polymerase can then use free 3' OH group on RNA primer to make DNA in 5'-->3' direction
• RNA fragments are taken out, and replaced with DNA nucleotides
• DNA ligase (enzyme) attaches DNA nucleotides together, finishing the lagging strand
• there are many "points of origins" on a DNA strand
• DNA replication repeats until every point meets
• each resulting DNA molecule is identical to the original DNA molecule