The Molecular Basis of
Inheritance
CAMPBELL AND REECE
CHAPTER 16
The Search for Genetic Material
 once Morgan proved genes are in
chromosomes big debate started:
 Is the genetic material in chromosomes the
DNA or the proteins?
 @ first case for proteins seemed stronger
 very heterogenous
 great specificty
Evidence that DNA can Transform
Bacteria
 1928 Griffith studied Streptococcus
pneumoniae
Transformation
 term coined by Griffith
 change in genotype & phenotype due to the
assimilation of external DNA by a cell
 Avery spent the next 14 years identifying the
“transforming agent”
Avery’s Experiment
Avery
 & his colleagues announced DNA was the
transforming agent
 many were skeptical
Was bacterial DNA anything like eukaryotic
DNA?
 Nothing much known about DNA
 Most scientists held to belief that proteins had to
be transforming agent

Bacteriophages
 are viruses that infect bacteria
 “phages” for short
 Virus made of a protein coat covering genetic
material

to produce more viruses it must invade a cell
& take over the cell’s metabolic machinery
Hershey & Chase Experiment
Additional Evidence that DNA is Genetic
Material
 Chargraff
already knew DNA made up of:
 Deoxyribose
 Phosphate group
 Nitrogenous Base (A, G, C, T)
 analyzed DNA from # of species
 1950: base composition of DNA varies between
species
 made DNA more credible

Chargraff’s Rule
 no matter what the source of DNA tested:
What is the structure of DNA?
 early 1950’s:
scientists convinced
DNA carried genetic
info
 focus now on DNA’s
structure
 knew arrangement of
DNA’s covalent bonds
Watson & Crick
 Cambridge, England
 2 young, unknown scientists
 same lab: Franklin & Wilkins doing x-ray
crystallography on protein structure
X-Ray Crystallography of DNA
 Rosalind Franklin
had purified some
DNA and showed
results to Watkins
who was familiar
with pattern made by
a helical structure
Watson & Crick
 began building models that satisfied:
known chemical properties of DNA
 nitrogenous bases relatively hydrophobic
 phosphate groups carry (-) charge
 Chagraff’s rules
 helical structure
 how could this structure pass on genetic
information?

DNA Structure
 antiparallel:
arrangement of
sugar-phosphate
backbones in a DNA
double helix
 means 1 strand runs
5’  3’ going “up” *
the other runs 5’  3’
going “down”
DNA Structure
 because of size differences in dbl ringed
purines vs. single ringed pyrimidines Watson
& Crick knew could not have a purine linked
with itself or the other purine
 also knew that adenine & thymine could form
H bonds (2) with each other & cytosine &
guanine could for 3 H bonds
Watson & Crick
 their 1 page paper published in Nature in
April 1953
 Watson, Crick, and Wilkins received Nobel
Prize in 1962 (Franklin died in 1958)
DNA Replication
 Watson and Crick’s 2nd paper stated their




hypothesis on how DNA replicates:
DNA model is pair of complimentary
templates
prior to replication H bonds broken & chains
separate & unwind
each chain then acts as template for
formation onto itself of a new complimentary
chain
allows for exact duplication
DNA Replication
Watson & Crick’s Semiconservative Model
of DNA Replication
 predicts when a dbl helix replicates, each of
the 2 daughter molecules will have 1 old
strand and 1 new strand
 Conservative Model: 1 new daughter
molecule with 2 new strands & the original
molecule
 Dispersive Model: all 4 strands of DNA after
replication have mixture of old & new parts
3 Models of DNA Replication
DNA Replication
 begins @ particular sites called:
 Origins of Replication
 short stretches of a specific sequence of
nucleotides
 many bacterial loops of DNA have single
origin
 proteins that initiate DNA replication
recognize the sequence / attach to the DNA /
separate the 2 strands by breaking H bonds
creating “bubbles”
Prokaryotic Replication of DNA
Eukaryotic DNA Replication
Replication Bubble
Replication Forks
 @ each end of the replication bubble
 Y-shaped region where DNA is unwinding
 proteins that participate in the unwinding:
1. helicases

unwind double helix
2. single-strand binding proteins

bind to single strands prevents them from
rewinding
3. topoisomerases

untwisting dbl helix puts strain on ahead of
replication fork, these proteins relieve strain by
breaking, swiveling, & rejoining DNA strands
Replication Forks
Replication of DNA
 initial nucleotide chain made during DNA
synthesis is actually a strand of RNA
 this RNA chain called a primer which is made
by an enzyme called primase (last slide)
 primase starts a complementary RNA chain
from a single RNA nucleotide then adds 1 @
time
Primers
 when primer 5 – 10 nucleotides long...new
DNA strand will start from the 3’ end of the
RNA primer
DNA Polymerase
 enzyme that catalyzes the synthesis of new
DNA by adding nucleotides to a pre-existing
chain
2 major one in prokaryotes
 11 different ones in eukaryotes

most require a primer & DNA template strand
 rate: ~500 nucleotides/s in bacteria

~ 50 nucleotides/s in human cells

Source of Nucleotides
 are in form of nucleoside triphosphates
dATP
Nucleoside Triphosphates
 are chemically reactive (like ATP, except
sugar is deoxyribose, not ribose)
 as each nucleotide joins the growing end of a
DNA strand 2 of the phosphate groups are
lost as a molecule of inorganic phosphate in a
couple exergonic reaction that drives the
polymerization reaction
Polymerization Reaction
Antiparallel Elongation
 each strand of DNA has directionality (1-way
street)
 & each strand oriented in opposite directions
to each other
 DNA polymerase III can add nucleotides only
to the free 3’ end of a primer or growing DNA
strand
along 1 template DNA polymerase synthesizes
complementary strand continuously (5’  3’
direction)
 called the Leading Strand

Antiparallel Elongation
 along opposite strand because of orientation,
DNA polymerase III must work in direction
away from the replication fork
 called Lagging Strand
synthesized in short segments called:
 Okazaki Fragments
 ~ 1,000 – 2,000 nucleotides long in E. coli
 ~ 100 – 200 nucleotides long in eukaryotes

DNA Replication Complex
 easy to think of DNA polymerase as a
locomotive moving down template track but
not really how it works:
1. various proteins that participate in DNA
replication form a large complex
2. DNA replication complex doesn’t move, the
DNA template moves thru the complex
DNA Replication Complex
Proofreading & Repairing DNA
 ~ 1/10 billion base pairs in completed DNA
will be incorrect
 but right after strands replicated errors ~
100,000 times more common
 DNA polymerases “proofread” each
nucleotide against its template as soon as it is
added
 when error found, incorrect nucleotide
removed, correct 1 inserted
Proofreading & Repairing
 some errors evade DNA polymerase…other
enzymes remove & replace incorrectly paired
nucleotides
 some errors arise after replication: damage
to DNA relatively common: usually corrected
by b/4 becoming permanent mutations
 cells continuously monitor & repair damaged
DNA
Repair Enzymes
 ~100 in E. coli
 ~ 130 in humans
 most organisms use same mechanism to
repair errors or damage
involves cutting out damaged area using DNAcutting enzyme called nuclease
 gap then filled with correct nucleotides done by a
DNA polymerase & DNA ligase
 1 of these systems called nucleotide excision
repair

Nucleotide Excision Repair
Telomeres
 repetitive sequences @ ends of eukaryotic
chromosomes
shorter as we age (with each round of DNA
replication)
 so preserves the ends of linear DNA & postpones
erosion of genes
 telomerase catalyzes the lengthening of
telomeres in germ cells

Telomeres
Prokaryotic DNA
 usually loop of DNA
 some associated proteins
 loop of DNA + proteins = nucleoid
Eukaryotic Chromatin
 includes:
1. DNA
2. histones
3. other proteins
Nucleosomes
 histones bind to each other & to the DNA to
form nucleosomes: the most basic unit of
DNA packing
 histone tails extend outward from each beadlike nucleosome cone
 additional coiling &
folding  highly
condensed
chromosome as seen
in mitosis
 Euchromatin: term for the less coiled
chromatin seen in interphase cells

easily accessible for transcription
 Heterochromatin: portions of chromatin that
remains highly condensed even in interphase

mostly inaccessible to transcription