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DNA and Heredity
DNA and Heredity
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DNA is found in the cell’s __nucleus_______.
In the nucleus, we find the DNA in thin threads
called_chromatin______.
These thicken into larger rod-like structures
called__chromosomes_______.
Humans have __23 pairs___ of chromosomes.
Some
species have __more___, others _less___.
DNA: The Genetic Material
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The instructions for inherited traits are
called_genes_______. A gene is a small
segment of deoxyribonucleic acid, or DNA, that
is located in a chromosome.
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DNA is the primary material that causes
_inheritable__characteristics in related groups
of organisms.
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DNA is a simple molecule, composed of only
four different subunits.
DNA: The Genetic Material
As early as 1900, scientists suspected
_units of heredity____ were on
chromosomes.
 Most scientists thought _proteins____
not _nucleic __acids___ would control
heredity.
 Since chromosomes are _50/50____
DNA and protein, this was not
unreasonable.
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Searching for the Genetic Material
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Three major _experiments__ led to the
conclusion that DNA is the genetic material in
cells. These experiments were performed by
Griffith, Avery, and Hershey and Chase.
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_Griffith_ worked with two related strains of
bacteria which cause _pneumonia_in mice.
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Griffith discovered that when harmless__live
bacteria were mixed with heat-killed diseasecausing bacteria and then injected into mice, the
mice __died_.
Griffith’s Discovery of Transformation
Searching for the Genetic
Material, continued
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These results led Griffith to
discover_Transformation_. Transformation is
a change in genotype that is caused when cells
take up foreign genetic material.
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Griffith’s experiments led to the conclusion that
_hereditary__material could be transferred
between cells.
Searching for the Genetic
Material, continued
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Avery wanted to determine whether the
transforming agent in Griffith’s experiments was
protein, RNA, or DNA.
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Avery used enzymes to destroy each of these
molecules in heat-killed bacteria.
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Avery’s experiments led to the conclusion that
DNA is responsible for transformation in
bacteria.
Avery’s Experiment
Searching for the Genetic
Material, continued
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Hershey and Chase studied bacteriophages.
Bacteriophages are viruses that infect bacterial
cells and cause the cells to produce viruses.
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By using radioactive isotopes, Hershey and
Chase showed that DNA, not protein, is the
genetic material in viruses.
Hershey and Chase
Bacteriophages are made of only
__proteins_ and __nucleic_ __acids_.
 Hershey and Chase labeled the proteins
with radioactive _sulfur______ and the
nucleic acids with radioactive
_phosphorous__.
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Bacteriophage
Hershey and Chase
Hershey and Chase
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Because the radioactive proteins were
found in the solution and the radioactive
DNA was found inside the bacteria, this
was proof that DNA controlled heredity.
The Shape of DNA
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A DNA molecule is shaped like a spiral
staircase and is composed of two parallel
strands of linked subunits.
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The spiral shape of DNA is known as a double
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Each strand of DNA is made up of linked
subunits called nucleotides.
helix.
The Shape of DNA, continued
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A nucleotide is made up of three parts: a
phosphate group, a five-carbon sugar
group, and a nitrogen-containing base.
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The phosphate groups and the sugar molecules
of nucleotides link together to form a
“backbone” for the DNA strand.
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The five-carbon sugar in DNA is called
deoxyribose, from which DNA gets its full
name, deoxyribonucleic acid.
DNA
nucleotide
DOUBLE
HELIX
Nitrogen
base
Sugar
phosphate
The Information in DNA
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The information in DNA is contained in the
order of the bases, while the base-pairing
structure allows the information to be copied.
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In DNA, each nucleotide has the same sugar
group and phosphate group, but each nucleotide
can have one of four nitrogenous bases.
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The four kinds of bases are adenine (A),
guanine (G), thymine (T), and cytosine
(C).
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Bases A and G have a double-ring structure and
are classified as purines.
The Information in DNA,
continued
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Bases T and C have a single-ring structure and are
classified as pyrimidines.
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A purine on one strand of a DNA molecule is always
paired with a pyrimidine on the other strand. Specifically,
adenine always pairs with thymine, and guanine
always pairs with cytosine.
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Base-pairing rules are dictated by the chemical
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The hydrogen bonds between bases keep the two
strands of DNA together.
structure of the bases.
Discovering DNA’s Structure
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Watson and Crick used information from experiments
by Chargaff, Wilkins, and Franklin to determine the
three-dimensional structure of DNA.
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Chargaff showed that the amount of adenine always
equaled the amount of thymine, and the amount of
guanine always equaled the amount of cytosine.
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Franklin and Wilkins developed X-ray diffraction
images of strands of DNA that suggested the DNA
molecule resembled a tightly coiled helix.
X-Ray diffraction
Discovering DNA’s Structure,
continued
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Watson and Crick used both Chargaff’s data and
the X-ray diffraction studies to create a
complete three-dimensional model of DNA.
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Their model showed a “spiral staircase” in which
two strands of nucleotides twisted around a
central axis.
Watson and Crick
DNA Replication
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Because DNA is made of two strands of
complementary base pairs, if the strands are
separated then each strand can serve as a
pattern to make a new complementary strand.
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The process of making a copy of DNA is called
DNA replication.
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In DNA replication, the DNA molecule
unwinds, and the two sides split. Then, new
bases are added to each side until two identical
sequences result.
DNA Replication, continued
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As the double helix unwinds, the two complementary
strands of DNA separate from each other and form Y
shapes. These Y-shaped areas are called replication
forks.
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At the replication fork, new nucleotides are added to
each side and new base pairs are formed according to
the base-pairing rules.
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Each double-stranded DNA helix is made of one new
strand of DNA and one original strand of DNA.
DNA Replication
Replication Proteins
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The replication of DNA involves many proteins that
form a machinelike complex of moving parts. Each
protein has a specific function.
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Proteins called DNA helicases unwind the DNA double
helix during DNA replication. These proteins wedge
themselves between the two strands of the double helix
and break the hydrogen bonds between the base pairs.
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Proteins called DNA polymerases catalyze the
formation of the DNA molecule by moving along each
strand and adding nucleotides that pair with each base.
Replication Proteins, continued
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DNA polymerases also have a “proofreading”
function.
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During DNA replication, mutations sometime
occur and the wrong nucleotide is added to the
new strand.
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If a mismatch occurs, the DNA polymerase can
backtrack, remove the incorrect nucleotide, and
replace it with the correct one. This will correct
9,999 out of 10,000 errors.
Prokaryotic and Eukaryotic
Replication
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All cells have chromosomes, but eukaryotes and
prokaryotes replicate their chromosomes differently.
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In prokaryotic cells, replication starts at a single site. In
eukaryotic cells, replication starts at many sites along
the chromosome.
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Prokaryotic cells usually have a single chromosome
which is a closed loop attached to the inner cell
membrane.
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Replication in prokaryotes begins at one place along the
loop. This site is called the origin of replication.
Prokaryotic and Eukaryotic
Replication, continued
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Two replication forks begin at the origin of replication
in prokaryotes.
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Replication occurs in opposite directions until the forks
meet on the opposite side of the loop.
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Eukaryotic cells often have several chromosomes which
are linear and contain both DNA and protein.
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Replication starts at many sites along the chromosome.
This process allows eukaryotic cells to replicate their
DNA faster than prokaryotes.
Prokaryotic and Eukaryotic
Replication, continued
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Two distinct replication forks form at each start
site, and replication occurs in opposite
directions.
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This process forms replication “bubbles”
along the DNA molecule.
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Replication bubbles continue to get larger as
more of the DNA is copied.
Prokaryotic v. Eukaryotic
Prokaryotic and Eukaryotic
Replication, continued
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The smallest eukaryotic chromosomes are often 10
times the size of a prokaryotic chromosome. Eukaryotic
chromosomes are so long that it would take 33 days to
replicate a typical human chromosome if there were only
one origin of replication.
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Human chromosomes are replicated in about 100
sections that are 100,000 nucleotides long, each
section with its own starting point.
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Because eukaryotic cells have multiple replication forks
working at the same time, an entire human chromosome
can be replicated in about 8 hours.