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Deoxyribonucleic Acid
 The Molecular Basis
of Inheritance
Who are these guys?
AP Biology
Who were the scientists?
How did their work contribute to
the discovery or understanding of
DNA?
AP Biology
Griffith’s Transformation
Experiment - 1928
 Bacteria could get traits from other
bacteria “transforming” their traits.
AP Biology
Avery, McCarty and MacLeod 1944
 Refined Griffith’s
experiment
 Proved
transforming
agent was nucleic
acid.
AP Biology
Hershey and Chase – “Blender
Experiment “ 1952
 Further proved that DNA, not protein, is the hereditary
material
Bacteriophages!
Viruses that infect
bacteria
AP Biology
 Maurice Wilkins and Rosalind Franklin

Were using a technique called X-ray
crystallography to study molecular
structure
 Rosalind Franklin

Produced a picture of the DNA molecule
using this technique
Why was this an
important
discovery?
AP Biology
Figure 16.6 a, b
(a) Rosalind Franklin
(b) Franklin’s X-ray diffraction
Photograph of DNA
Watson and Crick – 1953
 Discovered the structure of DNA
 Nobel prize in 1962 (with Wilkins)
 Deduced that DNA was a double helix

AP Biology
Through observations of the X-ray
crystallographic images of DNA from
Rosalind Franklin
Erwin Chargaff – 1950’s
 All organisms have the same bases just
in different amounts.
 In any DNA:
 Base pairing is highly conserved
through evolution
AP Biology
What is DNA?
 Primary source of genetic information
 RNA can be used in some cases
 Eukaryotic cells – multiple, linear
chromosomes in nucleus
 Mitochondria/chloroplast have circular
chromosome
 Prokaryotic cells – 1 circular
chromosomes in cytosol
Plasmids = separate extra-piece of circular
DNA (only a few genes)
AP Biology
DNA Structure
 Monomers = nucleotides
 Nucleotide structure:
Phosphate
 Sugar (deoxyribose)
 Nitrogen base

 Adenine, guanine, thymine, cytosine
AP Biology
DNA Structure
 Antiparallel - Nucleotides on
one side run 5’ to 3’ and the
opposing side runs from 3’ to
5’
 Complementary strands –
nucleotides of one side are
bonded with their
complement not identical.
5
3
3
5
EX: Reading strand from 5’ to 3’ left top down is the
same as right bottom up.
AP Biology
Bonding in DNA
5
hydrogen
bonds
3
covalent
bonds
3
5
….strong or weak bonds?
AP
Biology
How
do the bonds fit the mechanism for copying DNA?
Nitrogen Bases and Pairing in DNA –
 Purines
adenine (A)
 guanine (G)

 Pyrimidines
thymine (T)
 cytosine (C)

 Base Pairing

A:T
 2 Hydrogen bonds

C:G
 3 Hydrogen bonds
AP Biology
Base Pairing – Chargaff’s Rule
 Amount of T = Amount of A
 Amount of G = Amount of C
Ex: Approximately how much thymine would be
found in the wheat DNA?
The amount of G’s and C’s is approx. 45% so, the amount of A’s
and T’s should be close to 55%. Thus, 55-28 = 27%
AP Biology
Semi-Conservative Replication
 Replication of DNA
Base pairing allows each
side of original strand
serves as a template for a
new strand
 New strand is 1/2 parent
template & 1/2 new DNA

AP Biology
Prokaryotic DNA Replication
 Replication moves in two directions.
 Always occurs 5’ to 3’ only.
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Eukaryotic DNA Replication
 Multiple origin sites
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DNA Replication: The Process
 DNA is unwound by Helicase enzyme



Breaks the hydrogen bonds between the
nitrogen bases of each strand
Creates the replication forks
SSB’s stabilize replication fork
single-stranded binding proteins
AP Biology
replication fork
Making New Strands
 Free, unbound nucleotides are in the nucleus
 RNA Primase puts on RNA Primer to start new strand

Designates start region of replication
 DNA Polymerase III – enzyme that bonds new
complementary nucleotides to old strand.
 makes new 5’ to 3’strands
 can only add nucleotides to 3’ end of parent strand
 Leading Strand:
 Follows replication fork (as helicase unwinds)
 One continuous strand
 Lagging Strand:
 Opposite direction of fork
 Lays down multiple strands (fragments)
AP Biology
Making Strands….
AP Biology
Replication in a Bubble..
3
5
5
3
DNA polymerase III
leading strand
5
3
3
5
3
5
5
5
3
lagging strand
3
5
3
5
lagging strand
5
5
leading strand
3
growing
replication fork
leading strand
3
lagging strand
5 5
AP Biology
growing
replication fork 5
5
5
3
Lagging Strands = Okazaki fragments!
 DNA polymerase I replaces RNA primers with
DNA nucleotides
 DNA ligase attaches the DNA pieces together
AP Biology
Replication fork
Animation
DNA
polymerase
5’
3’
DNA ligase
lagging strand
RNA
primase
Okazaki
fragments
5’
3’
5’
SSB
3’
helicase
DNA
polymerase
5’
3’
leading strand
direction of replication
AP Biology
SSB = single-stranded binding proteins
Editing & Proofreading DNA
 Specific Enzymes
that can:

Cuts and removes
abnormal bases

proofreads

repairs mismatched
 Mutations are a
permanent change in
DNA
AP Biology
Telomeres
 Repeating, non-coding

sequences at the end of
chromosomes
Shorten after each cell
division
 shorter telomeres =
aging cells
 Telomerase – doesn’t
let telomeres shorten

AP Biology
works in stem cells
and cancer cells