Bellwork  What base pairs with adenine?  What base pairs with cytosine?

DNA REPLICATION- PG. 48

What is DNA replication?  Before a cell divides, it copies DNA  Makes an exact copy of DNA.  One complementary strand of DNA is used to make the new copy.  Original strand of DNA can never leave the nucleus—otherwise, there would be no DNA left in the cell!

Steps of Replication 1. The enzyme DNA helicase unwinds the helix by breaking hydrogen bonds that hold DNA together, and the bases become exposed. 2. The enzyme, DNA polymerase, adds nucleotides to form a new strand that is complementary to the “template” strand.

Replication—Semiconservative  Each new DNA contains one old strand and one new strand  Semi-conservative model—one strand conserved, new strand is complementary

Errors during replication--Polymerase  Occasionally, the wrong nucleotide is added to the new strand of DNA—ex: A-G is wrong!  DNA polymerase “proofreads” each new strand quickly and can fix errors—Ex: A-G is changed to A-T