Transcript Discovering DNA: Structure and Replication

Discovering DNA: Structure
and Replication
A collaborative effort!
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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.
Good things happen when you’re
not looking…….
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Griffith’s Experiment
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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.
Griffith’s Experiment
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Mice injected with the
disease causing bacteria
died.
Mice injected with the
harmless bacteria lived.
Mice injected with the heatkilled disease causing
bacteria also lived.
Griffith then mixed the live
harmless bacteria with
heat-killed disease causing
bacteria.
Mice injected with this
combination died.
http://www.mun.ca/biology/scarr/Fg10_03smc.gif
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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
The live harmless bacteria had received some
factor from the heat-killed bacteria, making
them lethal.
Avery modifies Griffith’s experiment.
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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
Hershey and Chase
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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.
http://www.accessexcellence.org/RC/VL/GG/images/HERSHEY.gif
Chargaff’s Rule
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Chargaff determined that in any sample of
DNA:
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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.
http://www.educarm.es/templates/portal/adminis
tradorFicheros/webquest/herencia/chargaff.jpg
Rosalind Franklin
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Franklin used x-ray
diffraction to create
pictures of DNA’s
molecular structure.
Watson got an
unapproved sneak
peak at her images
and used them with
other evidence to
determine DNA’s
helical structure.
http://www.safarikscience.org/biologyhome/7_dna/6_xray_diffraction.jpg
Watson and Crick
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Watson and Crick determined the
structure of DNA in 1953.
All the work of previous scientists
was used to determined that DNA is
shaped like a double helix, with
strands held together by the weak
hydrogen bonds formed between
the bases A-T and C-G.
http://teachers.sduhsd.k12.ca.us/lolson/im
ages/watson_crick.jpg
The Structure of DNA
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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.
DNA Replication
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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
DNA Replication
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DNA Replication occurs in four basic steps:
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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
semi-conservative
because each side of
the parent strand
serves as a template
for the 2 new DNA
strands.
http://www.dnareplication.info/images/dnareplication.jpg
Base Pairing Practice
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For each example below, please give the correct
complementary strand of DNA.
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TTGCTAG
AACGATC
TAGCGCT
ATCGCGA
ACCGTCA
TGGCAGT