dna structure & replication ap biology. what is a nucleotide?
TRANSCRIPT
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DNA Structure & Replication
AP Biology
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What is a Nucleotide?
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DNA exhibits complementary base pairing
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The two strands of DNA are anti-parallel
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The Mystery of DNA Replication Revealed by its Structure
• Complementary base pairing reveals a possible copying mechanism for genetic material—Watson & Crick
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DNA Replication: Simple View
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The Basic Principle: Base Pairing to a Template Strand
• Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication
• In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Animation: DNA Replication Overview
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• Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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DNA Replication: A Closer Look
• The copying of DNA is remarkable in its speed and accuracy
• More than a dozen enzymes and other proteins participate in DNA replication
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Fig. 16-12b
0.25 µm
Origin of replication Double-stranded DNA molecule
Parental (template) strandDaughter (new) strand
Bubble Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
Overview of DNA Replication
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Step 1• Replication begins at special sites called origins
of replication, where the two DNA strands are separated by proteins, opening up a replication “bubble”
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Step 2• Replication proceeds in both directions
from each origin, until the entire molecule is copied.
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Detailed VIEW of DNA Replication
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Enzymes & Proteins involved in DNA Replication
• Helicases: Untwist the double helix at the replication forks
• Single-strand binding protein: Binds to and stabilizes single-stranded DNA until it can be used as a template
• Topoisomerase: Corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
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Elongating a New DNA Strand
• DNA polymerase catalyzes the elongation of DNA at the replication fork
• DNA polymerase adds nucleotides to the growing end of the DNA strand
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Antiparallel Elongation• DNA polymerases add nucleotides only to the
free 3end of a growing strand; therefore, a new DNA strand can elongate only in the 5 to 3direction
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• The “Leading Strand” is the strand of new DNA formed towards the replication fork.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Animation: Leading Strand
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Lagging Strand
• The “Lagging Strand” is formed 5’ to 3’, in the direction away from the replication fork.
• The lagging strand does not elongate continuously, instead it is formed in a series of fragments called “Okazaki fragments”
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Lagging Strand & Okazaki
Fragments• DNA ligase joins
Okazaki fragments
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DNA Replication Summary
• DNA Replication is a semi-conservative process
• Replication differs in the production of the leading and lagging strands and occurs bi-directionally
• http://www.mcb.harvard.edu/losick/images/trombonefinald.swf• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free
/0072437316/120076/micro04.swf::DNA%20Replication%20Fork
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DNA Proofreading & Repair
• DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides
• In mismatch repair of DNA, repair enzymes correct errors in base pairing
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DNA Proofreading & Repair
• In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA
• Damage shown is a thymine dimer from UV Radiation
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Replicating the Ends of DNA Molecules
• Linear (eukaryotic) DNA can’t be completely replicated by DNA polymerase b/c it only adds to the 3’ end of the molecule
• Result: DNA is shortened with each replication
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Telomeres To the Rescue
• Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called telomeres
• Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules
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Telomerase in Germ Cells
• Telomerase catalyzes the lengthening of telomeres in germ cells
• Telomeres in cancer cells are very short• Some cancer cells have active
telomerase…