Transcript Discovering DNA: Structure and Replication

Discovering DNA: Structure
and Replication
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A collaborative effort!
• The discovery of DNA resulted from the
combination of contributions from several
scientists.
• Each conducted experiments that provided
different pieces of information needed to
solve the puzzle of the role and structure
of DNA.
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Good things happen when you’re not
looking…….
• Griffith’s Experiment
– 1928 – Frederick Griffith conducted experiments
with mice to study the effects of pneumonia
bacteria.
– Griffith isolated two strains of pneumonia bacteria –
one caused the lung disease (pneumonia) and the
other did not.
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Griffith’s Experiment
•
Mice injected with the
disease causing bacteria
died.
•
Mice injected with the
harmless bacteria lived.
•
Mice injected with the
heat-killed disease
causing bacteria also
lived.
•
Griffith then mixed the
live harmless bacteria
with heat-killed disease
causing bacteria.
•
Mice injected with this
combination died.
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http://www.mun.ca/biology/scarr/Fg10_03smc.gif
• Based on his results, Griffith hypothesized
that when the harmless and heat-killed
bacteria were mixed, some factor was
exchanged between them, making the live
harmless bacteria deadly.
• Transformation – process in which one
strain of bacteria is changed by the
gene(s) of another bacteria
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Avery modifies Griffith’s experiment.
• In 1943, Avery, with fellow
scientists, conducted an
experiment similar to Griffith’s,
except they used enzymes to
selectively destroy molecules
one at a time.
• When they injected harmless
bacteria with only lipids, carbs,
or proteins: transformation did
not occur.
• When they used the nucleic
acids (DNA): transformation did
occur, the bacteria became
lethal.
• This helped to determine that
DNA stores and transmits
genetic information.
http://www.synapses.co.uk/genetics/avery1.gif
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Hershey and Chase
• Hershey and Chase (1950)
conducted experiments with
bacteriophages (viruses that
attack bacteria) to determine if
genetic information is carried on
proteins or DNA.
• They used radioactive elements
to ‘mark’ DNA and protein.
• Only the radioactively-labeled
DNA was found in bacteria cells.
• These findings further supported
the conclusions of Avery’s
experiment & specified that
genetic material is DNA and
NOT protein.
http://www.accessexcellence.org/RC/VL/GG/images/HERSHEY.gif
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Chargaff’s Rule
• Chargaff determined that in any sample of
DNA:
– The # of adenines (A) = the # of thymines (T)
– The # of cytosines (C) = the # of guanines (G)
• Thus in DNA, the bases A and T pair
together, and C and G pair together.
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http://www.educarm.es/templates/portal/adminis
tradorFicheros/webquest/herencia/chargaff.jpg
Rosalind Franklin and Watson & Crick
• Franklin used x-ray
diffraction to create
pictures of DNA’s
molecular
structure.
• In 1956, Rosalind
became ill with
cancer and died
within two years of
her diagnosis.
• James Watson and Frances Crick
determined the structure of DNA in
1953 using their data and the work of
previous scientists.
• Watson got a sneak peak at Franklin’s
X-ray images and used them with other
evidence to determine DNA’s structure.
• The structure of DNA was determined
to be shaped like a double helix, with
strands held together by the weak
hydrogen bonds formed between the
bases A-T and C-G.
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The Structure of DNA
• On the diagram:
• Circle and label a
nucleotide.
• Label the sugar
and phosphate
molecules.
• Label the bases
that are not
already labeled
• Label a base
pair.
• Label the sugarphosphate
backbones.
• Label the
hydrogen bonds.
________
________
Base pair
________
________
G
T
A
G
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_________
DNA Replication
• DNA must be replicated (copied) in order to insure
that during cell division, each daughter cell receives a
complete copy.
• DNA replication occurs in the nucleus during S phase
of the cell cycle, before chromatin (DNA wrapped
around proteins) condenses into chromosomes.
http://ghr.nlm.nih.gov/handbook/illustrations/chro
mosomestructure.jpg
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DNA Replication
• DNA Replication occurs in four basic steps:
– Step 1 – Helicase unzips the strand of DNA by breaking
the weak hydrogen bonds between base pairs.
– Step 2 – DNA polymerase inserts the appropriate bases.
– Step 3 – A new sugar-phosphate backbone is built.
– Step 4 – The sequence is proofread by DNA polymerase.
DNA replication is
semiconservative
because each side
of the parent strand
serves as a
template for the 2
new DNA strands.
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http://www.dnareplication.info/images/dnareplication.jpg