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Molecular Basis of Inheritance
Chapter 16
Deciphering DNA
The Search for Genetic Material
 Known
 Genes on chromosomes
 Chromosomes made of DNA and protein
 Unknown
 Which chromosomal component was the genetic material
 Protein
 Heterogeneous class of macromolecules with specific functions
 Case stronger initially
 Nucleic acids
 Physical and chemical properties too uniform for amount of variation
 Experimentation gradually changed perceptions
 DNA’s role clarified by studying bacteria and their viruses
Frederick Griffith
o Streptococcus
pneumoniae model
o S encapsulated and
virulent; R nonencapsulated and nonvirulent
o Heat killed S cells
mixed with R cells
created S cells
o Concluded that S
cells have a chemical
component that can
transform other cells
Oswald Avery
 Identified the transforming substance from Griffith’s work as
DNA
 Focused on DNA, RNA, and protein
 Extract components from pathogenic bacteria
 Each individually inactivated and tested for transformation ability
 Degradation of DNA only substance to prevent
 Not uniformly supported
 Proteins better candidates
 Doubted bacterial DNA similar to that of complex organisms
 Little still known about DNA
Alfred Hershey and Martha Chase
o Tracked protein and
DNA of E. coli phage
T2
o Bacteriophage
is a virus that
infects bacteria
o Radioactive isotopes
to label cells
o Determined that
DNA entered
bacteria and directed
virus reproduction
not protein
Existing Knowledge of DNA
 Polymer of nucleotides with 3
components
 Pentose sugar (deoxyribose) and a
phosphate group
 Purines = two rings
 Adenine (A)
 Guanine (G)
 Pyrimidines = one ring
 Thymine (T)
 Cytosine (C)
Erwin Chargaff
 The amount of A, T, G, and C in the DNA vary from species to
species
 Evidence of molecular diversity to increase DNA credibility
 Chargaff ’s rules
 In each species, the amount of A = T while the amount of C = G
 Importance unknown until discovery of double helix
Organism
A
T
C
G
Human
30.3%
30.3%
19.9%
19.5%
Chicken
28.8%
29.2%
20.5%
21.5%
Grasshopper
29.3%
29.3%
20.5%
20.7%
Sea Urchin
32.8%
32.1%
17.7%
17.3%
Wheat
27.3%
27.1%
22.7%
22.8%
Yeast
31.3%
32.9%
18.7%
17.1%
E. coli
24.7%
23.6%
26.0%
25.7%
Rosalind Franklin
 X-ray diffraction image of
DNA
 DNA is helical in structure
 Uniform in width and spacing
between bases
 Suggested that there were 2
strands = double helix
 Concluded that sugar-
phosphate backbones were on
the outside
 Evidence was groundwork for
Watson and Crick
James Watson and Francis Crick
 Double helix with anti-parallel strands
 Sugar-phosphate backbone on outside
 Paired nitrogenous bases on inside
 Complimentary hydrogen binding of a purine and a pyrimidine
 A with T form 2 bonds, G with C form 3 bonds
 Consistent with Chargoff and Franklin
 Awarded the Nobel Prize
DNA Double-Helix Structure
DNA Replication
DNA Replication
 Each strand of original DNA serves as a template
 Nucleotides match to template according to base pairing rules
 1 ‘parent’ DNA strand produces 2 new ‘daughter’ strands
DNA Replication Models
 A) Two parent strands eventually
come back together
 B) Watson and Crick: each
daughter strand with 1 old parent
strand
 C) All four strands have a mixture
of new and old DNA
 Matthew Meselson and Franklin
Stahl’s work confirmed the semiconservative model
Replication Efficiency
 E. coli with 4.6 million nucleotide pairs replicates in less than
an hour
 Humans with 6 billion pairs a few hours, with only about 1
error every 10 billion nucleotides
 Enzymes and proteins are responsible
 Better understood in prokaryotes than eukaryotes
 Process is fundamentally similar
Origins of Replication
 Short specific
nucleotides sequences
 Prokaryotes with 1,
eukaryotes with
multiple
 Proteins recognize
and attach
 Separates strands and
opens them up to
form a replication
bubble
 Proceeds in both
directions until fully
copied
Overall DNA Replication
COMPONENTS (Table 16.1)
• Helicase
• Single-strand binding protein
• Topoisomerase
• Primer and primase
• DNA pol III and I
• Leading and lagging strands
• Okazaki fragments
• DNA ligase
KEY POINTS:
• DNA pol binds to 3’ end
• Strands grow 5’
3’ only
Proofreading and Repairing DNA
 DNA polymerases also proofread each
nucleotide
 Incorrect pairs are removed
 Mismatch pairs result from those that
evaded the polymerases
 Alternate enzymes remove and replace
 Nucleases cut out damaged DNA
 Polymerases and ligases fill gap with
nucleotides
 Skin cell repair from UV light damage