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DNA
History
Structure
Replication
History of DNA

Scientists thought protein was the
heredity material

Several Scientists disproved this and
proved it was DNA:
Griffith

Worked with virulent S and nonvirulent R strain of
pneumococcus bacteria
 Made the harmless R strain transform into S strain
when it took in DNA from the killed S strain.
Hershey & Chase

Worked with
radioactively traced
bacteriophages
(viruses that attack
bacteria).

Further proof that
DNA was the cell’s
genetic material.
Radioactive
32P
was injected into bacteria!
Chargaff

Developed the base
pairing rule by
comparing the
amounts of A, T, C,
G in the cell.
 Nucleotide pairing:
Nucleotide Bases in Somatic Cells
% Present
35.00%
30.3 30.3
30.00%
25.00%
19.5 19.9
20.00%
A
T
15.00%
G
– Adenine  Thymine
10.00%
C
– Guanine  Cytosine
5.00%
0.00%
Franklin

Took x-ray
photographs
of DNA

Determined
double-helix
structure
Watson & Crick

Watson and Crick
used Franklin’s x-rays,
Chargraff’s rule and
developed their model
of the DNA molecule
(for which they won a
Nobel Prize).
Structure of DNA
Shape of DNA

Double stranded,
twisted helix, called
a double helix

Shape similar to a
twisted ladder
Nucleotide Monomers
 DNA
Nucleotide is the basic unit
(monomer)
 Made of:
Sugar – Deoxyribose
 Phosphate group (PO4)
 Base – A T C G (Adenine, Thymine,
Cytosine, Guanine)

DNA Nucleotide
(glue picture provided in NB)
Phosphate
Group
O
O=P-O
O
5
CH2
O
N
C1
C4
Sugar
(deoxyribose)
C3
C2
Nitrogenous base
(A, G, C, or T)
Polymer Structure

Sides made of phosphate group and
deoxyribose sugar

Center (rungs) made of nitrogen bases
bonded by hydrogen bonds (A = T
and C = G)
Direction of Nucleotides

Each carbon in the sugar is given a
number 1’ – 5’

Sides are antiparallel – one side goes 5’
to 3’ and the other 3’ to 5’

This determines the direction that it is
“read” by enzymes
DNA Structure
(glue picture provided in NB)
5
P
O
3
3
5
O 5
O
C
G
1
4
P
5
P
2
3
O
T
3
2
1
4
5
O
A
3
O
3
P
5
P
P
Location of DNA

DNA is a large
(macro) molecule,
and stays in the
nucleus
DNA Replication
S Phase of Cell Cycle

DNA must be
copied before the
cell can divide
 Occurs during the
synthesis (S) part of
cell cycle (before
mitosis).
S
phase
G1
interphase
Mitosis
-prophase
-metaphase
-anaphase
-telophase
G2
Helicase enzyme

DNA unwinds and unzips with help of DNA
helicases
 These enzymes break the hydrogen bonds
between base pairs.
 This point is called the replication fork.
5’ Parental DNA Molecule
3’
3’
Replication
Fork
5’
Polymerase Enzyme

DNA polymerase moves in new nucleotides
 Can only add to 3’ end of nucleotide
 Follows the base-pairing rule
 Two identical DNA strands are formed
5’
Nucleotide
DNA Polymerase
Direction of Replication
RNA
Primer
3’
5’
Proofreading enzymes

Other enzymes
“proofread” the
replicated strand
looking for errors
(mutations).
 Incorrect
nucleotides are
removed and
replaced