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DNA: the Code of Life
Basic Requirements For
Genetic Material
1) Carry genetic information from cell
to cell, generation to generation
LOTS of info!!!
2) Copy itself with fidelity
3) Mutate occasionally and carry the
mutation along with fidelity
4) Have a way to decode stored info
and translate it into action in cells.
Discovery Of Structure
• 1868: Miescher first isolated
deoxyribonucleic acid, or DNA,
from cell nuclei
Discovery Of Structure
• 1951: Linus Pauling deduced the
structure of proteins
Discovery Of Structure
• 1953: Watson and Crick
put together a model
of DNA
Discovery Of Function
• 1928: Griffith was working with S
(virulent) and R (nonvirulent)
strains of a pneumonia-causing
bacterium
• Discovered "transforming factor"
Griffith’s Experiment
• LIVE lethal bacteria
injected
• LIVE non-lethal bacteria
injected
• DEAD lethal bacteria
injected
• MIXTURE of LIVE
nonlethal & DEAD lethal
bacteria injected
– Blood showed LIVE lethal
bacteria inside
Griffith’s
Experiment
• CONCLUSIONS?
Discovery Of Function
• By 1920’s, chromosomes structure
was known to be proteins & DNA
PROBLEM: which carried
the genetic material?
• Repeating nucleotides or
• Repeating polypeptides?????
Discovery Of Function
• 1944: Avery showed that the
“transforming factor" was nucleic
acid, not protein
Confirmation of DNA Function
• Used in experiments by Hershey and
Chase to prove which of these (DNA)
was the heredity
material.
Confirmation of DNA Function
• Viruses called bacteriophages use
bacterial cells for reproduction.
• Consist of only a protein coat and a
nucleic acid core
Hershey & Chase Experiment
• “Labeled” viruses with either
– Radioactive PHOSPHORUS (found
ONLY in DNA) or
– Radioactive SULFUR (found ONLY in
proteins)
Hershey & Chase Experiment
• “Labeled” P virus
into bacteria
– Bacteria =
RADIOACTIVE
– Empty viral coats
= NOT
• “Labeled” S virus
into bacteria
– Empty viral coats
= RADIOACTIVE
– Bacteria = NOT
Hershey &
Chase
Experiment
CONCLUSION?
Molecular Structure of DNA
• DNA is composed of 4 kinds of
nucleotides
• Each nucleotide consists of:
– one 5-carbon sugar (deoxyribose)
– one phosphate group, and
– one of 4 bases
• adenine (A), guanine (G), thymine
(T), cytosine (C).
Molecular Structure of DNA
• sugar (deoxyribose)
• phosphate group
P
Molecular Structure of RNA
• sugar (ribose)
• phosphate group
P
Purines - 2 Rings
• Adenine
• Guanine
Pyrimidines - 1 Ring
• Thyamine
• Cytocine
Purines vs Pyrimidines
• Chargaff showed that
– % of A always =
% of T
– % of G always =
% of C
• Purines always with pyrimidines
• Conclusions about their
relationship within DNA???
Purines vs Pyrimidines
DNA Structure
• Rosalind Franklin and Maurice Wilkins
used X-ray diffraction techniques to
produce images of DNA molecules.
X-Ray Crytallography
DNA Structure
1) DNA = two strands
of nucleotides
twisted into a
double helix
2) Helixes run antiparallel to each
other (in opposite
directions)
– Watson’s
discovery
DNA
Structure
DNA Characteristics
3) Base pairs
are formed
by
hydrogen
bonding
of
A with T,
and
G with C
DNA Characteristics
4) Sequence of
bases in a
nucleotide strand
is different from
species to
species
DNA Characteristics
5) The length of a strand of DNA (number
of base pairs) is
different from
species to species.
6) More complex
organisms
generally have
greater #’s of
base pairs
DNA - the unwinding
DNA Replication
• Two strands of DNA
unwind and expose
their bases
– DNA polymerase
opens strands
– Begins at the 5’ end
DNA Replication
• Unattached
nucleotides pair
with exposed bases
– DNA ligases assist
in re-bonding
– DNA “swivelase”
promotes
re-winding into a
double helix
Replication Bubbles
• DNA Polymerases attach at many sites
along the original molecule
DNA Replication
• Results in DNA
molecules that
consist of one "old"
strand and one
"new" strand (semiconservative).
Historical Discoveries
• Meselson & Stahl - invented density
gradient centrifugation and used this to
prove how DNA is
replicated
Meselson-Stahl Experiment
DNA Repair
• DNA
polymerases &
DNA ligases
engage in repair
when they
"read" the
complementary
sequence on
the other strand
and restore it
Organization Of DNA
On Chromosomes
• Each eukaryotic chromosome contains
one long
DNA
molecule
(46 in
humans)
DNA of eukaryotes is highly
organized to prevent tangling
• Some histones (a protein) act as spools
to wind the DNA into units called
nucleosomes
Organization Of DNA
On Chromosomes
• Another histone stabilizes the
arrangement and allows the beaded
chain to form looped regions.
Designed
by
Anne F. Maben
These images are for
viewing only and may not
be published in
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