16.2 dna replication
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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 - PowerPoint PPT PresentationTRANSCRIPT
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16.2 DNA Replication
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DNA in Prokaryotes and Eukaryotes
• Prokaryotes:– ring of chromosome– holds nearly all of
the cell’s genetic material
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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
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DNA Replication Overview
• http://www.dnatube.com/video/365/DNA-Replication
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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
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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)
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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)
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Eukaryote DNA Replication• Begins in hundreds
of locations along the chromosome– Origins of replication
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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
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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
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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
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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)
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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
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The Finished Product• Each DNA molecule
has one original strand and one new strand
• Molecules are identical
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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
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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
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16.3 A chromosome consists of a DNA molecule packed
together with proteins
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Chromosomes
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Chromosome Structure• DNA in bacteria -
nucleoid• Chromosomes
contain both DNA and protein to form chromatin
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Chromosome Structure• Chromatin is DNA coiled
around histones (protein)
• Heterochromatin – present in interphase, slightly condensed into clumps
• Euchromatin – uncondensed chromatin (“true chromatin”)