DNA Replication and Repair

Chapter 13.3

AP BiologyFall 2010

Replication EnzymesSignaling molecules activate replication

enzymesHelicases unzip the weak hydrogen bonds,

causing the DNA molecule to unwindDNA 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 DuplicatedSemiconservative 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” strandDesignated “semiconservative replication”

DNA ReplicationLeading strand

Synthesizes in 5’ to 3’ directionOne continuous strand

Okazaki strandSynthesizes in 5’ to 3’ directionFragments

Both have to copy the DNA template from its 3’ to 5’ end

Fixing Mismatches and BreaksDNA proofreading mechanisms fix most errors in

replication and strand breaksDNA 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