Download - Discovering the Structure of DNA
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Discovering the Structure
of DNA
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What is DNA?
• DNA = deoxyribonucleic acid• Stores, transmits and copy all information• Located in the cell’s nucleus
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What do you know about DNA?
• Codes for proteins• nucleic acid• Monomer is a nucleotide• The three parts of a nucleotide:
– 1. Phosphate group– 2. Sugar (deoxyribose)– 3. Nitrogen base
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Nitrogen bases• nitrogen two types:
purine- 2 carbon rings pyrimidine. -1 carbon
ring
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More about nitrogen bases• DNA has 4 nitrogen
bases:– Thymine (T)– Adenine (A)– Cytosine (C)– Guanine (G)
• A G - purines• C T - pyrimidines.
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A collaborative effort!
• Early 1900s– known: information is passed from cell to cell. – Unknown: what carried the information?
• scientists thought a protein • others that it was a nucleic acid.• Three major experiments helped shows
nucleic acid carried cell information:– Griffith– Avery– Hershey-Chase
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Frederick Griffith
• studied pneumonia bacteria
• 1928• isolated two strains of
pneumonia• injected them into
mice
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Griffith got lucky?– Something transferred
from heat-killed bacteria
– What happened?
• Griffith’s conclusions:• Transformation = process
by which one strain of bacteria changes the gene(s) of another bacteria
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Avery• DNA was transforming bacteria • 1943• Uses pneumonia ( just like
Griffith)• Showed gene is made of DNA• Scientists were slow to accept
the results
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Hershey and Chase
• bacteriophages to see if information is carried on proteins or DNA
• 1950
• Further supported Avery’s experiment that genetic material is DNA
http://www.accessexcellence.org/RC/VL/GG/images/HERSHEY.gif
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Discovery of the structure of DNA
• Many scientists contributed to determining the structure of DNA– Erwin Chargaff– Rosalind Franklin– James Watson &
Francis Crick
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Erwin Chargaff • Base pairing rule• 1950
• percentages of bases in DNA
• 10% A = 10% T• 40% C = 40% G
• This is Chargaff’s Rule!
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Rosalind Franklin• x-ray photography to try
to find DNA structure• 1952• Her “Photo 51” revealed
DNA’s structure• Died of cancer in 1958
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Watson and Crick• Credited with finding
the structure of DNA)
• 1953
• Watson got a sneak peak at Franklin’s x-ray photos and used them with other evidence
• Did not give credit to Franklin
http://teachers.sduhsd.k12.ca.us/lolson/images/watson_crickjpg
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DNA structure
• Twisted ladder • made of nucleotides• Sides- phosphate/sugar • Rung- nitrogen bases
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What bonds with what?
– A bonds with T– G bonds with C – Bonds
sides- covalent- strong rungs - hydrogen - weak
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Your turn...the structure of DNA• On the
diagram:– Circle and
label a nucleotide.
– Label the sugar and phosphate molecules.
– Label the bases
– Label a base pair.
– Label the sugar-phosphate backbones.
– Label the hydrogen bonds.
nucleotide
P
S
A
C
A
A
T
T
CG
G
Base pairSugar /P
backbone
G
Sugar /P
backbone
Hydrogen bonds