DNA Replication IB Biology HL 1 Mrs. Peters Spring 2014

Why does DNA replicate?  To create an exact copy for cell division, takes place during the S phase of Interphase  Used in Mitosis and Meiosis

3.4.3 Semi-Conservative Replication  Replication is semi- conservative Each parent strand of DNA is combined with a new daughter strand Based on Meselson- Stahl experiment using 15 N and 14 N bacteria

7.2.1 & DNA Replication Replication takes place in the 5’-3’ direction. Due to two different strands of DNA, it has a leading strand and a lagging strand. Replication takes place on many locations along the chromosome at one time.

7.2.1 DNA Replication Steps 1. Helicase (enzyme) unwinds the parental double helix at origin of replication, causing a bubble in the DNA strand

7.2.2 DNA Replication Steps 2. Single-strand binding proteins stabilize the unwound parental DNA 3. The leading strand is synthesized continuously in the 5’- 3’ direction by DNA polymerase III (enzyme)

7.2.2 DNA Replication Steps 4. The lagging strand is synthesized in discontinuous sections which are joined together. Primase (enzyme) synthesizes a short RNA primer, which is extended by DNA polymerase III to form an Okazaki fragment  RNA Primer is a short section of RNA nucleotides DNA polymerase III attaches first DNA nucleotide to RNA primer, then continues to add DNA nucleotides

7.2.2 DNA Replication Steps 5. DNA polymerase I (enzyme) replaces the RNA primer with DNA nucleotides 6. DNA ligase (enzyme) joins the Okazaki fragment to the growing strand

7.2.3 DNA Replication  Replication Fork is located at the end of the replication bubble, this is where new strands are elongated.  Happens at numerous points on a chromosome at one time

DNA Replication  Enzymes proofread DNA during replication to check for errors and repair them Mismatch Repair: mismatched nucleotides are repaired by enzymes to fix the incorrectly paired nucleotides. Nucleotide Excision Repair: damaged segments can be cut out by nuclease and the resulting gap is filled in with properly paired nucleotides (DNA polymerase and ligase)

DNA Replication  Nucleotide excision repair process Nuclease (enzyme) cuts damaged DNA in two places, damaged section removed Repair synthesized by DNA polymerase III filling in missing nucleotides DNA ligase seals the new and old DNA to complete strand

DNA Replication  Eukaryotic DNA has special nucleotide sequences at the ends called telomeres Telomeres do not contain genes, instead contain multiple repetitions of one short nucleotide sequence TTAGGG in humans