Transcript Discovering_DNA

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
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.
http://www.mun.ca/biology/scarr/Fg10_03smc.gif
Griffith’s Experiment
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Based on his results, Griffith hypothesized
that when the harmless and heat-killed
bacteria where 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
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 &
specified that genetic
material is DNA and NOT
protein.
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
http://www.safarikscience.org/biologyhome/7_dna/6_xray_diffraction.jpg
Watson and Crick
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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 CG.
http://teachers.sduhsd.k12.ca.us/lolson/im
ages/watson_crick.jpg
What is DNA?
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DNA = Deoxyribose
Nucleic Acid
Code of Life – codes for
proteins essential to
the health of the
organism.
Backbone made of
alternating Phosphorus
and Sugars
Made of matching
Nitrogen bases
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Adenine to Tyrosine
Cytosine to Guanine
What is the structure of DNA?
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DNA is composed of
Nitrogen bases, (ATGC)
anchored to a
deoxyribose sugar along
a Phosphate backbone.
Nucleotides = 5 carbon
sugar, phosphate and a
Nitrogen base.
Each nitrogen base is
bonded by weak
Hydrogen Bonds.
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Those bonds are weak for
DNA to replicate or
transcribe.
The Structure of DNA
Sugar /P
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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.
backbone
Base pair
Sugar /P
backbone
A
Hydrogen bonds
C
A
T
G
P
S
A
T
G
C
G
nucleotide
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
GCTATGT
CGATACA
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
semiconservative
because each side
of the parent strand
serves as a
template for the 2
new DNA strands.
http://www.dnareplication.info/images/dnareplication.jpg