16.2 DNA Replication

DNA in Prokaryotes and Eukaryotes

• Prokaryotes:– ring of chromosome– holds nearly all of

the cell’s genetic material

DNA in Prokaryotes and Eukaryotes

• Eukaryotes– much larger chromosomes

• 1000 times more DNA than prokaryotes– Found in cell’s nucleus– Chromosomes

• Humans: 46 chromosomes• Drosophila melanogaster: 8 chromosomes

DNA Replication Overview

• http://www.dnatube.com/video/365/DNA-Replication

DNA Replication Overview

• DNA splits into two strands• Complementary base pairs fill in (A with T,

C with G)• Left with two DNA molecules

– Semiconservative model• One original and one new strand make up a new

DNA molecule– Identical

Prokaryote DNA Replication• DNA replication begins

at a single point and continues to replicate whole circular strand

• Replication goes in both directions around the DNA (begins with replication fork)

Layout of the Eukaryote DNA• Two DNA strands are

antiparallel– Run in opposite

directions– 3’ (three prime) – 5’

(five prime)– 5’ (five prime) – 3’

(three prime)

Eukaryote DNA Replication• Begins in hundreds

of locations along the chromosome– Origins of replication

Initiation of DNA Replication• Begins when the DNA molecule

“unzips”– Replication fork– Replication “bubble”

• Hydrogen bonds between base pairs breaks

• Helicase• Single-strand binding proteins• Topoisomerase – relieves

pressure of DNA ahead of replication fork

Synthesis of a New DNA Strand

• Each strand serves as a template for a new strand to form

• Complimentary bases will attach

• DNA polymerase– E. coli – DNA polymerase

III and DNA polymerase I– Humans – 11 different DNA

polymerase molecules

Synthesis of a New DNA Strand• RNA primer• Nucleoside

triphosphate– As each nucleotide is

added to the new strand, 2 phosphates are lost• Hydrolysis releases

energy to drive reaction

Synthesis of a New DNA Strand• Antiparallel Elongation

– Remember 3’ – 5’ and 5’ – 3’ • Replication in the 3’ to 5’ direction ONLY

– MEANING the NEW strand of DNA will form starting with the 5’ end

• Leading strand (only 1 primer needed – moves toward the replication fork)

• Lagging strand (many primers needed – moves away from replication fork)

Important Enzymes• Helicase, single-strand binding protein, topoisomerase• Primase

– Synthesis of RNA primer• DNA polymerase III (DNA pol III)

– Add new bases to DNA strand• DNA polymerase I (DNA pol I)

– Removes and replaces RNA primer from 5’ end• DNA ligase

– Links Okazaki fragments and replaces RNA primer from 3’ end

The Finished Product• Each DNA molecule

has one original strand and one new strand

• Molecules are identical

Repair of DNA

• DNA polymerase– Proofreads and repairs damaged/mismatched

DNA• Nuclease

– Removes section of DNA that is damaged– DNA polymerase and DNA ligase replace

missing portion

Telomeres

• Found at the ends of each chromosome• Contain no genes• Sequence that can be cut short and will

not affect normal functioning• TTAGGG• Telomerase lengthens telomeres in

gametes

16.3 A chromosome consists of a DNA molecule packed

together with proteins

Chromosomes

Chromosome Structure• DNA in bacteria -

nucleoid• Chromosomes

contain both DNA and protein to form chromatin

Chromosome Structure• Chromatin is DNA coiled

around histones (protein)

• Heterochromatin – present in interphase, slightly condensed into clumps

• Euchromatin – uncondensed chromatin (“true chromatin”)