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The Genetic Material-DNA
- and it’s Role
• Where is the genetic
material stored?
• Before 1930, this was
not clear…
• Hammerling in
Germany studied the
small green algae,
Acetabularia to find out
Acetabularia
Cap
Stalk
Foot
Acetabularia
• Cut off cap….new one
grows
• Cut off foot…no new
growth
• Hammerling hypothesized
the information for growth
& development resides in
the foot.
• AND… the nucleus
resides in the foot
Acetabularia…
Transplantation Experiments
• In replacing caps and
feet between species,
Hammerling found
that the nucleuscontaining foot was
the determining
factor.
But there were problems with
believing that the nucleus is the
site for genetic material
• It contains primarily DNA and there’s
just not enough variability in DNA to
explain all of life!!!
• So many still believed proteins must
contain the genetic code…
Hershey-Chase Experiment
1952
• Worked with viruses
that attacked
bacteria…called
Bacteriophage
• Composed of only
protein coat & DNA
• Look like bizarre
spacecraft...
DNA
Protein
Coat
Bacteriophage
• Life cycle includes attacking
bacteria cell and injecting its
genetic material
inside…producing
thousands of new viruses
• Text pg. 201
• If virus has only protein and
DNA, one is the candidate
genetic material
• How to determine which?
Bacteria cell
DNA
Protein
Coat
Hershey-Chase Experiment
1952
In 1952, Alfred Hershey and Martha Chase conducted a
series of experiments to determine whether protein or
DNA was the hereditary material.
By labeling DNA and protein with different radioisotopes,
they would be able to determine which chemical (DNA or
protein) was getting into the bacteria. And such material
must be the hereditary material.
Since DNA contains Phosphorous (P) but no Sulfur (S), they
tagged the DNA with radioactive Phosphorous-32.
Conversely, protein lacks P but does have S, thus it could
be tagged with radioactive Sulfur-35.
Hershey-Chase Experiment
• Alfred Hershey & Martha Chase labeled
the Bacteriophage protein with radioactive
Sulphur (35S) and the DNA with radioactive
Phosphorous (32P)
• Later…The newly formed viruses inside
the cell had only 32P label.
Therefore: DNA is the Genetic Material
Structure of DNA: A Review…
• Made of Nucleotide units:
– Ribose sugar (5C sugar)
– Phosphorous group (-PO4)
– Nitrogen-containing base (A,G,C,T)
• Nucleotides are linked together by covalent
(phosphodiester) bonds
Text pg. 205
DNA Structure
• Two anti-parallel nucleotide strands
held together by H-bonds
• And twisted together to form a helical
structure
Text pg. 204
How DNA Replicates
3’
5’
3’
G
T
AT
C G
A
T
A
C
A .. T
G...C
T ..A
5’
DNA Replication
• An enzyme termed DNA
Polymerase synthesizes new
DNA from original in a 5’ to 3’
direction (the end with a free –
OH group)
• DNA replication results in two
new complementary strands of
DNA….
• One of each is from original
and one new…..semiconservative replication
5’
3’
New nucleotides can
Only add to this end
5’
3’
DNA Polymerase
AT
C G
A
T
T
3’
G
T C
A
A
T
C
G
5’
A
C
5’
3’
DNA replication involves a great many building blocks, enzymes and a
great deal of ATP energy. DNA replication in humans occurs at a rate of
50 nucleotides per second and ~500/second in prokaryotes.
Nucleotides have to be assembled and available in the nucleus, along with
energy to make bonds between nucleotides. DNA helicase enzymes
unzip the DNA helix by breaking the H-bonds between bases. Once the
polymerases have opened the DNA, an area known as the replication
bubble forks (always initiated at a certain set of nucleotides, the origin of
replication).
New nucleotides are placed in the fork and link to the corresponding parental
nucleotide already there (A with T, C with G). Prokaryotes open a single
replication fork, while eukaryotes have multiple forks.
Since the DNA strands are antiparallel, and replication proceeds in the 5' to 3'
direction on EACH strand, one strand will form a continuous copy. The top strand
here….
Text pg. 212
The Lagging Strand
…while the other, lagging strand will form a series of short pieces
with gaps. These are called “Okazaki fragments” and require the use
of other enzymes to complete the process.
Text pg. 212
Semi-conservative replication
Original DNA Strand
Original DNA Strand
New DNA Strands
Proofreading
DNA must be faithfully replicated…but mistakes
occur:
• DNA polymerase (DNA pol) inserts the wrong
nucleotide base in 1/10,000 bases
– DNA pol has a proofreading capability and can correct
errors
• Mismatch repair: ‘wrong’ inserted base can be
removed
• Excision repair: DNA may be damaged by
chemicals, radiation, etc. Mechanism to cut out
and replace with correct bases
– Text pg 213
How does DNA carry the
information for life?
• Small segments of DNA (sequences of
nucleotides) code for specific proteins
• These segments of DNA are called genes
• One gene codes for one protein…(mostly!)
• Hundreds-Thousands of genes per
chromosome