chapter 15 dna and the gene- synthesis and repair

dna structure

sugar- phosphate backbone

complementary base pairs held togehter by HYDROGEN bonding

antiparallel strands 5’ 3’

where does replication start?

replication bubble forms at the origin of replication.

How is the helix opened and stabilized?

TOPOISOMERASE- untwists. like scissors, cuts and rejoins the DNA to relieve the tension
HELICASE- opens the double HELIX, breaks h bonds between the 2 dna strands

Single strand dna binding proteins- stabilize the single strands

what CATALYZES dna synthesis

DNA polymerase

dna polymerase always ads deoxyribonucleotides to the 3 prime end of a dna chain

synthesis always proceeds in

the 5’ to 3’ directions

DNA polymerase syntheszes a leading strand that moves TOWARDS replicatino fork

how is the leading strand synthesized?

dna polymerase needs a primer to start dna synthesis.

the primer is a rna strand about a dozen nucleotides long.

PRIMER is made of PRIMASE

DNA polymerase has 2 parts

1. sliding clamp that holds polymerase in place

2. dna polymerase

how is the lagging strand synthesized?

still proceeds 5-3

primase synthesizes new rna primer on the lagging strand evry time the replication fork opens

dna polymerase synthesizes short fragments of dna called okazaki fragments, which are then linked into a continous strand by ligase

telomerase

the enzyme telomerase extends telomeres (the ends of chromosomes that do not contain genes)

somatic cells do NOT have telomerase

cancer cells have too much telomerase

binds to the overhang and uses the rna that it carries as a template for dna synthesis. continues to move down the new strand adding more short dna sequences. until overhang is long enough and dna synthesis can occur

mismatch repair enzymes

correct mismatched bases after dna synthesis is complete.

nucleotide excision repair

cuts out the error and then dna polymerase dills in the gap and then ligase links the synthesized dna to the strand

deoxyribonucleotide triphosphate dntp

provides energy for the polymerization reactions in dna synthesis