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”)
