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DNA - Deoxyribose nucleic acid
• Composed of nucleotides
• store and transmit genetic
information
• replicate
• undergo changes (mutate)
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DNA (Deoxyribonucleic Acid)
A. Found in almost all living cells –in the nucleus
of eukaryotes (3 Feet/Cell)
B. 2 primary functions
1. Control protein (enzyme) production-These
enzymes then control chemical reactions in cells.
2. Duplicate itself for new cells that are created
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Nucleotide
• Deoxyribose sugar
• Phosphate group
• Nitrogenous base
– A, T, C, G
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Nitrogenous Bases
• 4 Bases
– Adenine (A), Thymine (T),
Cytosine (C), Guanine (G)
• Purine – Double ring
– A and G
• Pyrimidine – single ring
– T and C
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Frederick Griffith: Transformation
1928
• Discovered that bacteria could give other
bacteria heritable traits, even after they were
dead.
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Avery, McCarty & MacLeod:
Griffiths Refined (1944)
• Refined Griffith's Experiment
• Exposed R-strain Streptococcus to purified Sstrain protein, and purified S-strain DNA
• Only the bacteria exposed to the S-strain DNA
were transformed
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Hershey & Chase:
The "Blender" Experiment (1952)
• Worked with bacteriophages
• Conclusively demonstrated that DNA was the
molecule of heredity
• Tagged phage DNA and protein with radioactive atoms
and tracked the transmission of that radioactivity to
infected bacteria
• Nobel Prize: Hershey (1969)
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Chargaff’s Rule
• Amount of guanine equals amount of cytosine
– Amount of adenine equals thyamine
• Chargaff’s rule:
C=G
A=T
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Watson & Crick,
Franklin & Wilkins (1953)
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• Two competing teams to determine
the structure of DNA
• Watson and Crick used X-ray
diffraction data developed by Rosalind
Franklin to develop their "double
helix" model of DNA
• Nobel Prize: Watson, Crick & Wilkins
(1962)
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DNA is a Double Helix
James Watson and Francis Crick
**Similar to ladder that is twisted**
• Sugar and phosphate form the
backbone
• Bases lie between the backbone
– Nucleotides
• A, G, T, C
– Held together by H-bonds
between the bases
• A-T – 2 H bonds
• G-C – 3 H bonds
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Orientation of DNA
• The nucleotides form a chain
• Phosphate end is the 5' end
– The opposite end (sugar) is the 3'
end
• Q: Since DNA is complementary,
what end matches with the 5’
end?
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• Bases on one strand are
covalently bonded to each other
("phosphodiester bonds")
• Bases on opposite strands are
hydrogen bonded to each other
("base pairs").
• Adenine = Thymine (2 H bonds)
• Cytosine = Guanine (3 H bonds)
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Forms of DNA
1. Chromatin –Partially unwound
(Normal Situations)
2. Chromosome – tightly wound DNA
(Cell division)
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Chromosome Structure
•DNA wraps around
histones
•histones form nucleosome
Chromatin
Fibers
Histones
Nucleosomes
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Plasmids
• small, circular, double-stranded DNA molecule
– Different from chromosomal DNA
• naturally exist in bacterial cells
• Often provide bacteria with genetic advantages
– antibiotic resistance
• Hundreds to thousands of base pairs
• each daughter cell receives a copy of plasmid
• Bacteria can also transfer plasmids to one
another through conjugation
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