DNA Replication and Repair Chapter 13.3 AP Biology Fall 2010

Replication Enzymes Signaling molecules activate replication enzymes Helicases unzip the weak hydrogen bonds, causing the DNA molecule to unwind DNA polymerases attach free nucleotides to the growing strand in a 5’  3’ direction DNA ligases seal new short stretches of nucleotides into one continuous strand Complementary strands now wind up with parent strand template

How is DNA Duplicated Semiconservative Replication Enzymes break hydrogen bonds between the two nucleotide strands First, the two strands of DNA unwind and expose their bases Then, attached nucleotides unbound in cytoplasm are paired with exposed bases Thus, replication results in DNA molecules that consist of one “old” strand and one “new” strand Designated “semiconservative replication”

DNA Replication Leading strand Synthesizes in 5’ to 3’ direction One continuous strand Okazaki strand Synthesizes in 5’ to 3’ direction Fragments Both have to copy the DNA template from its 3’ to 5’ end

Fixing Mismatches and Breaks DNA proofreading mechanisms fix most errors in replication and strand breaks DNA polymerases “proofread” the new bases for mismatched pairs, which are replaced with the correct bases If DNA polymerases cannot correct mistakes, replication is arrested Mismatches that slip past proofreaders are only one type of DNA damage Repair enzymes, like glycosylases, work to fix damaged sites on DNA that slip by the proofreaders, or occur from radiation or chemicals excising the damage or mismatch and replacing it with a suitable base