Adv Biology 1-2

DNA Replication  DNA replication occurs during S phase of both mitosis and meiosis. Replication begins at specific sites in the DNA strand called origins of replication. ○ Each strand has many origins.

DNA Replication cont’d  Helicase binds to the origin of replication. Unwinds and separates the two stands. Single strand binding proteins prevent the strands from closing up too soon.

DNA Replication cont’d  DNA polymerase binds to each of the strands. Reads both strands simultaneously in the 3’  5’ direction. Guides incoming nucleotides to pair with the original strand. ○ Adenine pairs with thymine, guanine pairs with cytosine.

DNA Replication cont’d  The new strand grows in the 5’  3’ direction. DNA polymerase binds the incoming nucleotides together.

DNA Replication cont’d  Leading strand-the new, complementary strand made continuously in the 3’  5’ direction.  Lagging strand-the new, complementary strand made discontinuously. The lagging strand is made in chunks (Okazaki fragments) that will eventually fused into a single strand.

DNA Replication cont’d  Replication ends when DNA polymerase finishes reading the DNA strand. DNA polymerase releases the DNA. Single strand binding proteins let go. Helicase binds the strands together and rewinds the helix. Semiconservative replication-each new DNA consists of one original and one new strand.

Proofreading  DNA polymerase proofreads the DNA as it works. If an incoming nucleotide is paired incorrectly, DNA polymerase stops, and removes the wrong nucleotide. ○ The correct one is inserted and replication resumes.

Enzymes  Enzymes repair any damage the DNA sustains. Excision repair-a segment of damaged DNA is cut out and the gap is filled in with nucleotides that pair with the undamaged DNA.