Powerpoint to accompany
Genetics: From Genes to Genomes
Third Edition
Hartwell ● Hood ● Goldberg ● Reynolds ● Silver ● Veres
Chapter
6
Prepared by Malcolm D. Schug
University of North Carolina Greensboro
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6-1
DNA
How the Molecule of Heredity Carries,
Replicates, and Recombines Information
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6-2
Outline of Chapter 6
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How investigators pinpointed DNA as the genetic
material
The elegant Watson-Crick model of DNA
structure
How DNA structure provides for the storage of
genetic information
How DNA structure gives rise to the
semiconservative model of molecular replication
How DNA structure promotes the recombination
of genetic information
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6-3
Chemical characterization localizes
DNA in the chromosomes.
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1869 – Friedrich Meischer extracted a
weakly acidic, phosphorous rich material
from nuclei of human white blood cells
which he named nuclein.
DNA – deoxyribonucleic acid
Deoxyribose – a sugar; acidic
 Four subunits belonging to class of compounds
called nucleotides linked together by
phosphodiester bonds
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
Chromosomes are composed of DNA.
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6-4
The
chemical
composition
of DNA
Fig. 6.2
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6-5
Are genes composed of DNA or
protein?
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DNA
Only four different subunits make up DNA.
 Chromosomes contain less DNA than protein
by weight.
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Protein
20 different subunits – greater potential variety
of combinations
 Chromosomes contain more protein than DNA
by weight.
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6-6
Bacterial transformation implicates
DNA as the substance of genes.
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1928 – Frederick Griffith – experiments
with smooth (S), virulent strain
Streptococcus pneumoniae, and rough (R),
nonvirulent strain
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Bacterial transformation demonstrates transfer
of genetic material.
1944 – Oswald Avery, Colin MacLeod, and
Maclyn McCarty determined that DNA is
the transformation material.
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Griffith experiment
Fig. 6.3aCopyright © The McGraw-Hill Companies, Inc.
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Griffith experiment
Fig. 6.3 b
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6-9
Avery, MacLeod, McCarty
Experiment
Fig. 6.4 a
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6-10
Hershey and Chase experiments
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1952 – Alfred Hershey and Martha Chase
provide convincing evidence that DNA is
genetic material.
Waring blender experiment using T2
bacteriophage and bacteria
Radioactive labels 32P for DNA and 35S for
protein
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6-11
Avery, MacLeod, McCarty
experiment
Fig. 6.4 c
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6-12
Hershey and Chase Waring blender
experiment
Fig. 6.5 a,b
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6-13
Hershey and Chase Waring blender
experiment
Fig. 6.5 c
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6-14
The Watson-Crick Model: DNA is a
double helix.
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1951 – James Watson learns about x-ray
diffraction pattern projected by DNA
Knowledge of the chemical structure of
nucleotides (deoxyribose sugar, phosphate, and
nitrogenous base)
Erwin Chargaff’s experiments demonstrate that
ratios of A and T are 1:1, and G and C are 1:1.
1953 – James Watson and Francis Crick propose
their double helix model of DNA structure.
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6-15
X-ray diffraction patterns produced by DNA
fibers – Rosalind Franklin and Maurice
Wilkins
Fig. 6.6
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6-16
DNA’s chemical constituents
3. Four nitrogenous
bases
Purines
1. Deoxyribose sugar.
Fig. 6.7a
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6-17
DNA’s chemical constituents
Assembly into a nucleotide
Fig. 6.7b
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6-18
DNA’s
chemical
constituents
Fig. 6.7c
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6-19
Chargaff’s ratios
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6-20
Complementary base pairing by
formation of hydrogen bonds
explain Chargaff’s ratios.
Fig. 6.8
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6-21
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Fig. 6.9
DNA is double helix
Strands are antiparallel
with a sugar-phosphate
backbone on outside and
pairs of bases in the
middle.
Two strands wrap around
each other every 30
Angstroms, once every 10
base pairs.
Two chains are held
together by hydrogen
bonds between A-T and GC base pairs.
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6-22
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Structurally, purines (A and G) pair
best with pyrimadines (T and C).
Thus, A pairs with T and G pairs
with C, also explaining Chargaff’s
ratios.
Fig. 6.9 d
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6-23
Double helix may assume alternative
forms.
Fig. 6.10
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6-24

Some DNA molecules are circular
instead of linear.
1.
 2.
 3.
 4.
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Some viruses carry single-stranded
DNA.
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Prokaryotes
Mitochondria
Chloroplasts
Viruses
1. bacteriophages
Some viruses carry RNA.

1. e.g., AIDS
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6-25
Four requirements for DNA
to be genetic material
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Must carry information
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Must replicate
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DNA replication
Must allow for information to change
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Cracking the genetic code
Mutation
Must govern the expression of the phenotype

Gene function
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6-26
DNA stores information in the sequence of its bases.
•Much of DNA’s sequence-specific information is accessible only when the double
helix is unwound.
•Proteins read the DNA sequence of nucleotides as the DNA helix unwinds.
Proteins can either bind to a DNA sequence, or initiate the copying of it.
•Human genome is believed to be 250 million nucleotides long. Four possible
nucleotides. Thus 4250,000,000 possible sequences in the human genome.
•An average single coding gene sequence might be about 10,000 bases long.
Thus, 410,000 possibilities for an average gene.
•Some genetic information is accessible even in intact, double-stranded DNA
molecules.
•Some proteins recognize the base sequence of DNA without unwinding it.
•One example is a restriction enzyme.
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6-27
Some viruses use RNA as the
repository of genetic information.
Fig. 6.13
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6-28
DNA replication: Copying genetic information for
transmission to the next generation

Complementary base pairing produces
semiconservative replication.
Double helix unwinds
 Each strand acts as template
 Complementary base pairing ensures that T
signals addition of A on new strand, and G
signals addition of C.
 Two daughter helices produced after
replication

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6-29
Fig. 6.14
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6-30
Experimental proof of semiconservative
replication – three possible models
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Semiconservative replication – Watson and
Crick model
Conservative replication: The parental
double helix remains intact; both strands of
the daughter double helix are newly
synthesized.
Dispersive replication: At completion, both
strands of both double helices contain both
original and newly synthesized material.
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6-31
Fig. 6.15
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6-32
Meselson-Stahl experiments confirm
semiconservative replication.
Fig. 6.16
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6-33
The mechanism of DNA replication

Arthur Kornbuerg, a nobel prize winner
and other biochemists deduced steps of
replication.
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Initiation
Proteins bind to DNA and open up double helix.
 Prepare DNA for complementary base pairing
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Elongation
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Proteins connect the correct sequences of nucleotides
into a continuous new strand of DNA.
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6-34
Fig. 6.18a
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6-35
Fig. 6.18b
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Enzymes involved in replication
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Pol III – produces new stands of complementary
DNA
Pol I – fills in gaps between newly synthesized
Okazaki segments
DNA helicase – unwinds double helix
Single-stranded binding proteins – keep helix open
Primase – creates RNA primers to initiate
synthesis
Ligase – welds together Okazaki fragments
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6-37
Replication is bidirectional.
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Replication forks move in opposite directions.
In linear chromosomes, telomeres ensure the
maintenance and accurate replication of
chromosome ends
In circular chromosomes, such as E. coli, there is
only one origin of replication.
In circular chromosomes, unwinding and
replication causes supercoiling, which may impede
replication.
Topoisomerase – enzyme that relaxes supercoils
by nicking strands
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6-38
The bidirectional replication of a
circular chromosome
Fig. 6.19a-b
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6-39
Fig. 6.19c-f
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6-40
Cells must ensure accuracy of
genetic information.
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Redunancy
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Basis for repair of errors that occur during
replication or during storage
Enzymes repair chemical damage to DNA.
Errors during replication are rare.
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6-41
Recombination reshuffles the
information content of DNA.
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During recombination, DNA molecules
break and rejoin.
Meselson and Weigle - Experimental
evidence from viral DNA and radioactive
isotopes
Coinfected E. coli with light and heavy
strains of virus after allowing time for
recombination
Separated on a CsCl density gradient
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6-42
Meselson and Weigle demonstrate
recombination occurs by breakage and
rejoining of DNA.
Fig. 6.20
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6-43
Heteroduplexes mark the spot of
recombination.
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Products of recombination are always in
exact register; not a single base pair is lost
or gained.
Two strands do not break and rejoin at the
same location; often they are hundreds of
base pairs apart.
Region between break points is called
heteroduplex.
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6-44
Heteroduplex region
Fig .6.21a-b
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6-45
In heterozygotes, mismatches within
heteroduplexes must be repaired.
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Gene conversion – a deviation from
expected 2:2 segregation of alleles due to
mismatch repair.
Studied most extensively in yeast where
tetrad analysis makes possible to follow
products of meiosis
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6-46
Insert figure 6.21c
Redrawn here
Gene
conversion in
yeast
Mismatch leads to 3:1
ratio of a:A. Ratio of B:b
and C:c which lie outside
of heteroduplex are both
2:2, as expected.
Fig. 6.21 c
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6-47
Double stranded break model of
meiotic recombination
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Homologs physically break, exchange parts, and
rejoin.
Breakage and repair create reciprocal products of
recombination.
Recombination events can occur anywhere along
the DNA molecule.
Precision in the exchange prevents mutations from
occurring during the process.
Gene conversion can give rise to unequal yield of
two different alleles. 50% of gene conversions are
associated with crossing over of adjacent
chromosomal regions, and 50% of gene
conversions are not associated with crossing over.
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6-48
Double stranded break formation
Spo11 protein breaks one chromatid on both strands.
Fig. 6.23 step 1
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6-49
Resection
5’ ends on each side of break are degraded to produce two 3’
single stranded tails.
Fig. 6.23 step 2
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6-50
First strand invasion
RecA binds 3’ tail and double helix allowing invasion and migration.
Fig. 6.23 step 3
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6-51
Formation of Holliday junctions
New DNA synthesis forms two X structures called Holliday junctions.
Fig. 6.23 step 4
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6-52
Branch migration
Both invading strands zip up and migrate while newly
created heteroduplex molecules rewind behind.
Fig. 6.23 step 5
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6-53
The Holliday intermediate
Interlocked nonsister chromatids disengage. Two resolutions
are possible.
Fig. 6.23
step 6
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6-54
Alternative resolutions
Endonuclease cuts Holliday intermediate
Fig. 6.23
step 7
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6-55
Probability of crossover occurring
Resolution of Holliday junction in same plan results in noncrossover
chromatids. Resolution in different planes results in crossover.
Insert figure 6.23 Step 8
Redrawn here
Fig. 6.23 step 8
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6-56
Essential Concepts
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DNA is the nearly universal genetic material.
The Watson-Crick model shows that DNA is a
double helix composed of two antiparallel strands
of nucleotides: each nucleotide consists of one of
four nitrogenous bases (A,T,C, or G), a
deoxyribose sugar, and a phosphate. An A pairs
with a T and a G pairs with a C.
DNA carries information in the sequence of its
bases, which may follow one another in any order.
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6-57