Discovery of DNA
• 1850’s Gregor Mendel discovered
hereditary “factors”, but did not
know about genes or chromosomes
or DNA.
• 1928…Frederick Griffith showed
genes were responsible for heredity
• DNA from dead bacteria was picked
up by live bacteria (transformation)
and killed mice
1928…Griffiths
Discovery of DNA
• 1944…Oswald Avery showed DNA was
responsible for heredity.
– Repeated experiment, but destroyed
molecules trying to find what it was that
caused the transformation…what
caused the dead bacteria to kill the mice
– Destroyed proteins, lipids, carbohydrates,
RNA and still transformation occurred,
so must be DNA
– Did again and destroyed DNA, no
transformation occurred
Discovery of DNA
• 1952…Alfred Hershey
and Martha Chase
backed up Avery’s
experiments
• Used bacteriophages
viruses that attack
bacteria
• Has a protein coat and
DNA core
1952 Hershey and Chase
• Used sulfur and phosphorus …
proteins have sulfur, DNA has
phosphorus
• Found phosphorus so must be DNA
Discovery of DNA
• 1949 Edwin Chargaff noticed that
A&T and G&C appear almost
equally in DNA, no matter what the
organism!
• Early 1950’s…Rosalind Franklin took
first X-ray of DNA molecule
• 1953 James Watson and
Francis Crick announced
double helix shape
(based on RF’s Xray).
DNA Function in cell
DNA represents a code
That code will be copied (transcription)
The copy will leave the nucleus
The copy will be decoded to make a
protein (translation)
• The protein will run some function in
the cell or body
• That’s how DNA in the nucleus is the
‘control center’ of the cell
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DNA Structure
• Deoxyribonucleic Acid
• Makes up chromosomes
• Chromosomes have small areas called
genes
• Genes are a section of DNA that is a
code for a protein that does some
function in your body (more about
this in next unit)
DNA Structure
DNA Structure
• Double helix like a twisted ladder made of
subunits called nucleotides. Has three
parts:
– deoxyribose (sugar)
– phosphate group
– nitrogen base
• 4 different bases:
adenine
guanine
cytosine
thymine
• Bases match each other (complementary), A
matches with T, G matches with C
• Order of bases called the base sequence
DNA Replication
• DNA Copies itself during the S phase of the
cell cycle, so daughter cells have complete
copy of all genes.
• DNA Helicase (an enzyme) “unzips” the DNA
molecule by breaking the hydrogen (weak)
bonds between nitrogen bases.
• Area where “unzips” is called replication fork.
• DNA polymerase (another enzyme) matches
base pairs with their complement (A with
T, G with C…Chargaff’s Rule) on both
strands at once. The base pairs are
already in the nucleus.
DNA Replication
DNA Replication
• Because one strand is upside down
(antiparallel), one strand is copied as
one whole piece (leading strand) and
one is copied in chunks (Okazaki
fragments) and pieced together
(lagging strand) by DNA ligase
• DNA polymerase continues until whole
strand is copied. Since two new
strands have 1 original and 1 new,
it’s called semi-conservative
replication.
Prokaryotic DNA
• In Prokaryotes (bacteria),
DNA is a ring and
replication starts at one
point and goes around
both ways until it’s done.
• Thus there will be only two
replication forks.
• In eukaryotes, replication will start in
many places, so there are multiple
replication forks.
Mistakes
• Mistakes in copying DNA can be (and
often are) made, but are usually
caught by DNA polymerase and
fixed.
• If not fixed, called mutation, and the
base sequence changes.
• Sometimes this is good and leads to a
new adaptation (evolution)
• Sometimes its bad (cancer, other genetic
disorders)
Mistakes
• There are also mutations to chromosomes,
but we’ll talk about all of this in the
next unit.