3. History of DNA
Early on, protein thought as the cell’s hereditary material
because it was more complex than DNA
Fred Griffith worked with virulent S and nonvirulent R strain Pneumoccocus bacteria.
He found that R strain could become virulent when it took in DNA from heat-killed S
strain
Study suggested that DNA was probably the genetic material
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4. History of DNA Radioactive 32P was injected into bacteria!
Chromosomes are made of both DNA and protein
Experiments on bacteriophage viruses by Hershey
& Chase proved that DNA was the cell’s genetic
material
Discovery of DNA Structure
Erwin Chargaff showed the amounts of the four bases on
DNA ( A,T,C,G)
In a body or somatic cell:
A = 30.3%
T = 30.3%
G = 19.5%
C = 19.9%
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5. Chargaff’s Rule
Adenine must pair with Thymine
Guanine must pair with Cytosine
The bases form weak hydrogen bonds
T A G C
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6. James Watson (L) and Francis
Crick (R), and the model they built of the
structure of DNA
AS Biology. Gnetic control of protein structure
7. Helix
Most DNA has a right-
hand twist with 10 base
pairs in a complete turn
Left twisted DNA is
called Z-DNA or
southpaw DNA
Hot spots occur where
right and left twisted
DNA meet producing
mutations
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8. The structure of DNA and RNA
Genetic material of living organisms is either
DNA or RNA.
DNA – Deoxyribonucleic acid
RNA – Ribonucleic acid
Genes are lengths of DNA that code for
particular proteins.
9. DNA and RNA are polynucleotides
Both DNA and RNA are polynucleotides.
They are made up of smaller molecules called
nucleotides.
Nucleotide
DNA is made of two polynucleotide strands:
Nucleotide Nucleotide Nucleotide Nucleotide Nucleotide
Nucleotide Nucleotide Nucleotide Nucleotide Nucleotide
RNA is made of a single polynucleotide strand:
Nucleotide Nucleotide Nucleotide Nucleotide Nucleotide
AS Biology. Gnetic control of protein structure
10. Structure of a nucleotide
2.A Phosphate group
Phosphate groups P
are important
because they link
the sugar on one
nucleotide onto the S
phosphate of the
next nucleotide to
make a
polynucleotide.
AS Biology. Gnetic control of protein structure
11. Structure of a nucleotide
3. A Nitrogenous base
In DNA the four bases are: P
Thymine N-base
Adenine
Cytosine
Guanine
In RNA the four bases are: S
Uracil
Adenine
Cytosine
Guanine
AS Biology. Gnetic control of protein structure
12. Sugar phosphate bonds (backbone of DNA)
Nucleotides are
connected to each
other via the
phosphate on one
nucleotide and the
sugar on the next
nucleotide
A Polynucleotide
13. Nitrogenous bases – Two types
Pyrimidines Purines
Have single ring Have double
rings of Carbon
and Nitrogen
Thymine - T atom
Cytosine - C
Uracil - U Adenine - A
Guanine - G
Base-Pairings: Purines only pair with Pyrimidines
AS Biology. Gnetic control of protein structure
14. Adenine
AS Biology. Gnetic control of protein structure
15. Guanine
AS Biology. Gnetic control of protein structure
16. Cytosine
AS Biology. Gnetic control of protein structure
17. Base pairing
Sides made of a pentose
sugar deoxyribose
bonded to phosphate
(PO4) groups by
phosphodiester bonds
Center made of nitrogen
bases bonded together by
weak hydrogen bonds
Adenine links with
Thymine or Uracil by 2
hydrogen bonds
Cytosine links with
Guanine by 3
hydrogen bonds
AS Biology. Gnetic control of protein structure
22. Again:
Adenine always base pairs with Thymine (or Uracil if
RNA)
Cytosine always base pairs with Guanine.
This is because there is exactly enough room for one
purine and one pyrimidine base between the two
polynucleotide strands of DNA.
Complementary Base Pairing
23. Nature of the Genetic Material
1. It must contain, in a stable form, information
encoding the organism’s structure, function,
development and reproduction
2. It must replicate accurately so progeny cells have
the same genetic makeup
3. It must be capable of some variation (mutation) to
permit evolution
AS Biology. Gnetic control of protein structure
24. DNA
5 O 3
3 O
P 5 P
5 O
1 G C 3
2
4 4
2 1
3 5
P O
P
5
T A 3
O
O
5
P 3 P
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25. Antiparallel Strands
One strand of DNA
goes from 5’ to 3’
(sugars)
The other strand is
opposite in
direction going 3’
to 5’ (sugars)
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26. Question:
What would be the
complementary DNA
strand for the following
DNA sequence?
DNA 5’-CGTATG-3’
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