BYS219L Genetics and Evolution Ch' 7 Introduction to DNA

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BYS219L Genetics and EvolutionCh. 7
Introduction to DNA

• 8/25/09
• Greg Skibinski
• greg.skibinski_at_gmail.com

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History of DNA

• 1868, Friedrich Miescher (Swiss)
• Found an unknown substance
• Not a protein or lipid
• Contained much phosphorus, no sulfur
• Found only in nucleus
• Structure/Function was unknown

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Watson Crick (1953)

• University of Cambridge
• Sought to solve the structure of DNA
• We have found the secret of life

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What was already known in 1953?

• Genes (hereditary factors)
• Genes controlled protein structure
• Genes were on chromosomes
• Chromosomes consisted of DNAProtein

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How do we know that DNA was the genetic material?

• Three main experiments
• Frederick Griffith
• Avery McLeod
• Hershey Chase

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Frederick Griffiths experiments

• Experiments in bacteria
• Transformation of types of bacteria
• R cells could be transformed into S cells and
  become virulent
• This experiment indicated that bacteria could be
  transformed via some material

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Figure 7-2
Transforming R cells into S cells
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Avery McLeods experiments

• Proteins Destroyed using proteinase enzymes
• Lipids Destroyed with lipase enzymes
• RNA destroyed with RNAse enzyme
• DNA destroyed with DNAse enzymes

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Figure 7-3
DNA is the transforming agent
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Hershey Chase experiment (1952)

• phage T2 infects bacteria (injects DNA)
• Radiolabel the viral protein, or the DNA, follow
  the radioactivity.
• DNA has phosphorus, but no sulfur
• Proteins have sulfur, but no phoshorus
• They actually didnt know what was being injected
  into the bacteria (supposedly a genetic material)

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Figure 7-4
The phage genetic material is DNA
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What was known about DNAs chemistry?

• Nucleotides
• Nucleotide has 3 components
• phosphate
• deoxyribose
• bases (C, T, G, and A)
• Nucleotide phosphate sugar base
• Nucleoside sugarbase

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Figure 7-5
Structures of the four DNA nucleotides
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Nucleotide (Guanine)
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Chargaffs Rules
1. Total pyrimidines (TC) total purines
(AG) 2. total T total A, Total G total C A
T does not always equal G C
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Watson Crick Model (1953)
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Chapter 7
Computer model of DNA
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Figure 7-8
The structure of DNA
Antiparallel
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Figure 7-9
Two representations of the DNA double helix
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Figure 7-10
Base pairing in DNA
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Watson-Crick model

• Suggests that sequence may determine sequence of
  amino acids in protein (genetic code)
• Accounts for mutation (base substitution)
• Suggests a mechanism for copying

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Figure 7-11
Semiconservative DNA replication
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Figure 7-12
Three alternative models for DNA replication
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Meselson-Stahl experiment (1958)

• Grow bacteria (E. coli) in medium with heavy
  nitrogen (15N) for many generations
• Transfer to normal medium with light nitrogen
  (14N), grow for 2 generations.
• Extract DNA from bacteria
• Spin at ultra-high speeds (50,000 rpm), analyze
  density differences

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Figure 7-13b
DNA is copied by semiconservative replication
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Figure 7-13c
DNA is copied by semiconservative replication
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Figure 7-13a
DNA is copied by semiconservative replication
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Replication Fork

• Watson-Crick model predicts that during
  replication, there will be a replication zipper
  or fork

www-math.mit.edu
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John Cairns experiment

• Grow bacteria with tritiated thymidine(3H,
  radioactive)
• Observe after one round of replication
• Observe during second round of replication

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Figure 7-14
A replicating bacterial chromosome
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How is DNA replicated?

• enzymatical addition of bases
• Incoming bases are paired with the template
  strand
• DNA polymerase III (pol III)

genome.gov
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DNA pol III

• Synthesizes (builds the new strand ) in the 5 to
  3 direction
• READS the template strand 3 to 5.
• Can only begin synthesis at dsDNA
  (double-stranded)

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Figure 7-15
Reaction catalyzed by DNA polymerase
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Figure 7-16
DNA replication at the growing fork
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(Video)
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Figure 7-17
Synthesizing the lagging strand
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DNA Ligase Reaction
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List of proteins involved in DNA Replication

• DNA polymerase synthesizes new strand
• Beta clamp - Keeps polymerase attached to the DNA
• Helicase unwind dsDNA at the replication fork
• DNA Ligase Join adjacent DNA strands end-to-end
• Topoisomerase unwind DNA relieve tension
• SSB keep ssDNA single-stranded

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Figure 7-18
Proteins at work at the replication fork