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Chapter 9
DNA – The Genetic Material
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EARLY DISCOVERIES…
Fred Griffith – trying to develop a
vaccine for pneumonia
…..TRANSFORMATION
Oswald Avery….
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Discovered that the substance
absorbed by the harmless bacteria
was DNA…
DNA is the Transformation factor!
Hershey – Chase Experiment
studies bacteriophages…
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Discovering DNA:
Erwin Chargaff found
that the amount of
Adenine always equals the
amount of Thymine !....AND
that Guanine amounts
always equaled Cytosine.
Rosaline Franklin and
Maurice Wilkins –X-Ray
diffraction shows DNA is a
coiled helix
Francis Crick and James Watson
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Discovered the 3-dimensional
structure of DNA……. Showing
base pairing in a double helical
staircase…..Nobel prize 1950
Structure of DNA
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I. DNA: (Deoxyribonucleic Acid)
Building blocks called…
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Nucleotides: (3 parts):
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1.Deoxyribose (simple sugar)
2.Phosphate Group
3.Nitrogen Base(A, C, T or G)
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ADENINE
CYTOSINE
THYMINE
GUANINE
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BASE PAIRING
PURINES
– ADENINE AND GUANINE
PYRIMIDINES
- THYMINE AND CYTOSINE
ADENINE – HYDROGEN BOND – THYMINE
GUANINE – HYDROGEN BOND - CYTOSINE
THE SIDES OF THE DNA MOLECULE
PHOSPHATE
DEOXYRIBOSE SUGAR
PHOSPHATE
DEOXYRIBOSE SUGAR
PHOSPHATE
DNA =
2 strands of nucleotides
joined by hydrogen
bonds between the
bases forms a
“DOUBLE HELIX”
or
twisted ladder
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A. DNA Replication:
Each organism has its
unique nucleotide
sequence in its DNA -->from parents
DNA must be copied
for cell division
(Replication)
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STEPS IN
DNA REPLICATION:
Enzymes called DNA
Helicases break bonds
between bases - DNA
“unzips”
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Free nucleotides in pair
up with exposed bases
guided by DNA
Polymerases.
2 new strands formed
and rewind
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DNA)
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Free nucleotides
…….How proteins
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are made !!
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II. PROTEIN SYNTHESIS:
20 Different amino acids
3 Nitrogen bases code for 1 amino acid = CODON
“Triplet code” - 64 different codons (3 stop codons) --->
20 amino acids
Order of bases in DNA determines type and order of amino
acids in protein
Code is universal among all organisms = common ancestor
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A. TRANSCRIPTION: Getting DNA to
make a m RNA ….which carries the
sequence for a particular polypeptide
chain!
DNA IS IN THE NUCLEUS
Transcription takes place in the nucleus!
RNA structure differs from DNA:
Single strand of nucleotides
Ribose - simple sugar
Bases - uracil instead of thymine U -- A
A, U, C, G
STEPS OF TRANSCRIPTION:
1. DNA makes complimentary copy of mRNA (using
U in place of T)
2. mRNA carries code out of nucleus into cytoplasm
where ribosomes are located!
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B. TRANSLATION: Interpreting the
code from the mRNA - on the
ribosomes.
tRNA (transfer) carries amino acids and
bring them to ribosomes to be
assembled into proteins
The order of amino acids determines
the type of polypeptide assembled.
(protein)
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tRNA has anticodon (3 bases that code for
a specific amino acid)
tRNA matches its anticodon with the mRNA
codon and drops off its amino acid to bond
with the next amino acid
Continues until a “stop codon” is reached