DNA Replication Biology 12

Two Jokes today….because I forgot last time!

What is DNA replication? Process by which DNA copies itself Before a cell starts mitosis, it replicates its DNA

Semi-Conservative Replication: Each strand acts as a template for a new double helix. Each new double-stranded molecule is composed of one parental strand and one new strand. Dds

Important: The two strands in DNA are anti-parallel One strand is in a 5' to a 3' while the other is in the opposite direction( 3' to 5' )

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The Steps:

1. Separation of Strands Weak hydrogen bonds that hold bases together are “unzipped” by an enzyme called DNA helicase

2. Keeping the strand separated Single stranded binding proteins (SSBs) prevent the strands for sticking back together (reannealing)

Note: Replication proceeds only in the 5’ to 3’ direction (on the daughter strand)! This makes one side much simpler than the other.

3. Building Complimentary Strands Primase creates a RNA primer which allows DNA polymerase to start attaching new nucleotides DNA polymerase continues until it reaches another RNA primer

3. Building Complimentary Strands Leading strand Built continuously From parent 3’ end toward replication fork

3. Building Complimentary Strands Lagging strand Built in short Okazaki fragments between RNA primers oDiscontinuous oFrom parent 3’ end oAway from replication fork

Another picture…. Okazaki fragments

3. Building Complimentary Strands DNA ligase Joins Okazaki fragments together ▫Links all the new pieces of DNA together ▫Lagging strand only!

4. Proofreading DNA polymerase checks for proper base pairing If mistake found: ▫Removes ▫Replace with proper nucleotide

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To do : Re-read page Page 229: ▫Questions 1, 3-6, 9-15 Page 230 ▫Questions 3, 6, 10,13,14 Biology 12 is so much fun!