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DNA: The Genetic Material
Chapter 12
The Genetic Material
Frederick Griffith, 1928
studied Streptococcus pneumoniae, a
pathogenic bacterium causing pneumonia
there are 2 strains of Streptococcus:
- S strain is virulent
- R strain is nonvirulent
Griffith infected mice with these strains
hoping to understand the difference
between the strains
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The Genetic Material
Griffith’s results:
- live S strain cells killed the mice
- live R strain cells did not kill the mice
- heat-killed S strain cells did not kill the
mice
- heat-killed S strain + live R strain cells
killed the mice
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The Genetic Material
Griffith’s conclusion:
- information specifying virulence passed
from the dead S strain cells into the live R
strain cells
- Griffith called the transfer of this information
transformation
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The Genetic Material
Avery, MacLeod, & McCarty, 1944
repeated Griffith’s experiment using purified
cell extracts and discovered:
- removal of all protein from the
transforming material did not destroy its
ability to transform R strain cells
- DNA-digesting enzymes destroyed all
transforming ability
- the transforming material is DNA
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The Genetic Material
Hershey & Chase, 1952
- investigated bacteriophages: viruses that
infect bacteria
- the bacteriophage was composed of only
DNA and protein
- they wanted to determine which of these
molecules is the genetic material that is
injected into the bacteria
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The Genetic Material
- Bacteriophage DNA was labeled with
radioactive phosphorus (32P)
- Bacteriophage protein was labeled with
radioactive sulfur (35S)
- radioactive molecules were tracked
- only the bacteriophage DNA (as indicated
by the 32P) entered the bacteria and was
used to produce more bacteriophage
- conclusion: DNA is the genetic material
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DNA Structure
DNA is a nucleic acid.
The building blocks of DNA are
nucleotides, each composed of:
– a 5-carbon sugar called deoxyribose
– a phosphate group (PO4)
– a nitrogenous base
• adenine, thymine, cytosine, guanine
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DNA Structure
The nucleotide structure consists of
– the nitrogenous base attached to the 1’
carbon of deoxyribose
– the phosphate group attached to the 5’
carbon of deoxyribose
– a free hydroxyl group (-OH) at the 3’
carbon of deoxyribose
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DNA Structure
Nucleotides are connected to each other to
form a long chain
phosphodiester bond: bond between
adjacent nucleotides
– formed between the phosphate group of
one nucleotide and the 3’ –OH of the
next nucleotide
The chain of nucleotides has a 5’ to 3’
orientation.
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DNA Structure
Determining the 3-dimmensional structure of
DNA involved the work of a few scientists:
– Erwin Chargaff determined that
• amount of adenine = amount of thymine
• amount of cytosine = amount of guanine
This is known as Chargaff’s Rules
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DNA Structure
Rosalind Franklin and Maurice Wilkins
– Franklin performed X-ray diffraction
studies to identify the 3-D structure
– discovered that DNA is helical
– discovered that the molecule has a
diameter of 2nm and makes a complete
turn of the helix every 3.4 nm
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DNA Structure
James Watson and Francis Crick, 1953
– deduced the structure of DNA using
evidence from Chargaff, Franklin, and
others
– proposed a double helix structure
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DNA Structure
The double helix consists of:
– 2 sugar-phosphate backbones
– nitrogenous bases toward the interior of
the molecule
– bases form hydrogen bonds with
complementary bases on the opposite
sugar-phosphate backbone
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DNA Structure
The two strands of nucleotides are
antiparallel to each other
– one is oriented 5’ to 3’, the other 3’ to 5’
The two strands wrap around each other to
create the helical shape of the molecule.
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DNA Replication
Matthew Meselson & Franklin Stahl, 1958
investigated the process of DNA replication
considered 3 possible mechanisms:
– conservative model
– semiconservative model
– dispersive model
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DNA Replication
Meselson and Stahl concluded that the
mechanism of DNA replication is the
semiconservative model.
Each strand of DNA acts as a template for
the synthesis of a new strand.
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DNA Replication
•DNA replication is the process of copying a DNA molecule. Replication is
semiconservative, with each strand of the original double helix (parental molecule)
serving as a template (mold or model) for a new strand in a daughter molecule. This
process consists of:
•Unwinding (initiation): old strands of the parent DNA molecule are unwound
as weak hydrogen bonds between the paired bases are “unzipped” and broken
by the enzyme helicase.
•Complementary base pairing (elongation): free nucleotides present in the
nucleus bind with complementary bases on unzipped portions of the two strands
of DNA; this process is catalyzed by DNA polymerase.
•Joining (elongation): complementary nucleotides bond to each other to form
new strands; each daughter DNA molecule contains an old strand and a new
strand; this process is also catalyzed by DNA polymerase.
•termination – replication is terminated differently in prokaryotes and eukaryotes
Ends in prokaryotes when origin is reached
Ends in eukaryotes when telomere is reached
telomeres – repeated DNA sequence on the ends of eukaryotic
chromosomes
•DNA replication must occur before a cell can divide; in cancer, drugs with
molecules similar to the four nucleotides are used to stop replication.
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Prokaryotic Versus Eukaryotic Replication
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•Prokaryotic Replication
•Bacteria have a single loop of DNA that must
replicate before the cell divides.
•Replication in prokaryotes may be bidirectional
from one point of origin or in only one direction.
•Replication only proceeds in one direction, from 5'
to 3'.
•Bacterial cells are able to replicate their DNA at a
rate of about 106 base pairs per minute.
•Bacterial cells can complete DNA replication in 40
minutes; eukaryotes take hours.
•Eukaryotic Replication
•Replication in eukaryotes starts at many
points of origin and spreads with many
replication bubbles—places where the DNA
strands are separating and replication is
occurring.
•Replication forks are the V-shape ends of
the replication bubbles; the sites of DNA
replication.
•Eukaryotes replicate their DNA at a slower
rate – 500 to 5,000 base pairs per minute.
•Eukaryotes take hours to complete DNA
replication.
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•Replication Errors
•A genetic mutation is a permanent change in the sequence
of bases.
•Base changes during replication are one way mutations
occur.
•A mismatched nucleotide may occur once per 100,000
base pairs, causing a pause in replication.
•Proofreading is the removal of a mismatched nucleotide;
DNA repair enzymes perform this proofreading function
and reduce the error rate to one per billion base pairs.
•Incorrect base pairs that survive the proofreading process
contribute to gene mutations.
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