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DNA
Information and Heredity,
Cellular Basis of Life
Chapter 12(M)
Genes are made of DNA
The following scientists proved that
DNA is the genetic material
 Fredrick Griffith (1928)
 Oswald Avery ( 1944)
 Hershey and Chase (1952)
Fredrick Griffith (1928)
The discovery of the genetic role of
DNA began with research by Frederick
Griffith.
He studied Streptococcus pneumoniae,
a bacterium that causes pneumonia in
mammals.
One strain, the R strain, was harmless.
The other strain, the S strain, was
pathogenic.
 When
Griffith mixed heat-killed S
strain with live R strain bacteria and
injected this into a mouse it died.
 He recovered the pathogenic strain
from the dead mouse’s blood.
 Some harmless bacteria had been
“transformed” into the deadly strain
 For the next 14 years scientists
tried to identify the transforming
substance.
Transforming Substance?

Could be:
DNA
Proteins
Because scientists already knew
chromosomes consist of these
substances. So the debate started.
Oswald Avery ( 1944)
 Treated
Protein
Griffiths mixture with
digesting enzymes  remove
all proteins
DNA digesting enzymes remove all
DNA
Avery contd.

Is Protein the transforming factor?
 treated
Griffith’s mixture of heat treated
deadly strain and live harmless strains with
protein-destroying enzymes grew the
strains
 The bacterial colonies were still
transformed

Concluded that protein could not be the
transforming factor
Avery contd.
 Is
DNA the transforming factor?
treated
the mixture with DNAdestroying enzymes grew the strains
The bacterial colonies failed to
transform
 Concluded
that DNA is the genetic
material of the cell
Scientists were still skeptical 
proteins made of 20 AAs, DNA only 4
bases
Hershey and Chase (1952)

Used viruses to prove that DNA is the
genetic material.
Viruses consist of a DNA (sometimes
RNA) enclosed by a protective coat of
protein.
To replicate, a virus infects a host cell
and takes over the cell’s metabolic
machinery.
Viruses that specifically attack bacteria
are called bacteriophages or just phages.
Conclusion
Phage DNA entered the bacterial cell,
proteins did not
 DNA carries the genetic information.

The Role of DNA
Storing Information The genetic
material stores information needed by
every living cell
 Copying Information before a cell
divides this info must be copied
 Transmitting Information Each
daughter cell must receive a complete
copy of all the information

Structure of DNA
Ch 12.2
Building Blocks Of DNA
Nucleotides
 A ring-shaped sugar called
deoxyribose
 A phosphate group (a
phosphorus atom
surrounded by four
oxygen atoms)
 A nitrogenous base
("nitrogen-containing") : a
single or double ring of
carbon and nitrogen atoms
with functional groups
Nitrogenous Bases
The four nucleotides in DNA differ only
in their nitrogenous bases
 Bases:

 Thymine
(T)  single ring
 Cytosine (C) single ring
 Adenine (A) double ring
 Guanine (G) double ring
Bases
Structure of DNA
 Early 1950s R. Franklin
DNA  helix with2 strands
with the phosphate linked
to the sugar, diameter is
2nm, each turn has 10
bases
 Watson & Crick (1953)
Using Franklins work built a
model of DNA
Each strand is complementary
to the other
A pairs with T, G with C
Base Pairing
 Chargaff’s
Adenine
Rule
forms a base pair with
Thymine
Guanine forms a base pair with
Cytosine
Amounts are about the same
The Double Helix
Figure 16.5 The double helix
DNA Strands

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Two strands double helix
Compared to a ladder
Sides Sugar phosphate
backbone
Rungs pairs of nitrogenous
bases
Order of bases genetic code
Difference in order gives
individuality to each living
organism
Base pairing is the key that
allows DNA to be copied
A pairs with T, G with C
Replication of
DNA
Ch 12.3
 Complete
Replication
set of genetic
instructions passes from one
generation to the next
 The DNA molecule must be copied
Base
pairing allows DNA to be copied
DNA 2 strands, one is used as a
template
 Replication
is copied
Process by which DNA
Mechanism of Replication
 Takes
place in the nucleus
 DNA untwists & both strands are
replicated almost simultaneously
 50 bases /sec are added
 The parent DNA strands serve as a
template for making a new strand
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DNA polymerase unzips the 2
strands
Each strand is used as a template
Free nucleotides present in the
nucleus, pair with the exposed
bases
A with T, G with C
As bases pair DNA Ligase an
enzyme links the phosphate of
each nucleotide to the sugar of the
previous one
Pairing & bonding continue till 2
new strands are formed.
 Enzyme
works from 5’3’
 Synthesis is anti parallel 5’3’ and
3’5’
 As bases pair DNA Ligase an
enzyme links the phosphate of each
nucleotide to the sugar of the
previous one
 Pairing & bonding continue till 2 new
strands are formed.
DNA Replication is Semi
conservative