Chapter 12
DNA and
RNA
Digital Illustration DNA Structure
12-1 DNA
Discuss w/ Partner:
You may use your textbook or
notes.
What
organelle is considered the
“control center” of the cell?
What structures are found in the
nucleus?
What are chromosomes composed
of?
How do genes & chromosomes
control the activity of the cell?
Answers:
 What
organelle is considered the “control
center” of the cell? The nucleus
 What structures are found in the nucleus?
Chromosomes
 What are located on chromosomes? Genes
 What are chromosomes composed of? DNA
wrapped around proteins called histones.
 How do genes & chromosomes control the
activity of the cell? By producing proteins
that regulate cellular functions or become
part of the cell structure.
I. Griffith and
Transformation
Fredrick Griffith in 1936
1.
2.
3.
In the 1920’s Griffith
was a biologist trying
to figure out how
bacteria produced
pneumonia (a deadly
disease).
2 different stains
(breeds) of
pneumonia bacteria
were isolated
(separated) from mice
The pneumonia was
grown in the lab
4. Griffith’s Hypothesis:
Material in dead bacterial
cells can genetically
change living bacterial
cells
5. Griffith’s Experiments
a)
b)
c)
Griffith set up
four individual
experiments.
Experiment 1:
Mice were
injected with the
disease-causing
strain of
bacteria.
Results: The mice
developed
pneumonia and
died.
d)
e)
Experiment
2: Mice were
injected with
the harmless
strain of
bacteria.
Results: These
mice didn’t
get sick
f)
g)
Experiment 3:
Griffith heated
the diseasecausing bacteria.
He then injected
the heat-killed
bacteria into the
mice.
Results: The mice
survived
h)
i)
Experiment 4: Griffith
mixed his heat-killed,
disease-causing
bacteria with live,
harmless bacteria and
injected the mixture
into the mice.
Results: The mice
developed
pneumonia and died.
Steps of Griffith’s transformation experiment which help scientist discover DNA as genetic material
6.
Conclusion:
the heat-killed
bacteria
passed their
diseasecausing ability
to the
harmless strain
7.
Transformation-
One strain of
bacteria is
changed by a
gene or genes
from another
strain of
bacteria.
Discuss w/ Partner:
Look at fig. 12-1 on pg. 288
What
was Griffith trying to learn when
he set up this experiment?
How did Griffith show that the diseasecausing bacteria were killed by heat?
What result was Griffith expecting
when he injected the mixture of live
harmless bacteria & heat-killed
disease bacteria?
Answers:
 What
was Griffith trying to learn when he
set up this experiment? How bacteria
caused pneumonia.
 How did Griffith show that the diseasecausing bacteria were killed by heat? He
tried to grow them in a petri dish. If bacteria
grew, then he knew that he had not killed
them.
 What result was Griffith expecting when he
injected the mixture of live harmless
bacteria & heat-killed disease bacteria? He
expected the mice to live.
8.
After his experiment Griffith
hypothesized that:

something must have information
that could change harmless
bacteria into disease-causing
ones
Discuss w/ Partner:
Who
remembers getting
immunizations ? Why do
people get immunizations?
How do they work?
Ethical Question:
Some
people believe autism is
caused by preservatives in the
vaccine. As a result some
people have chosen not to
immunize their children. What is
your opinion?
II. Avery and DNA
Oswald Avery in 1937
1.
2.
In the 1940’s a scientist named
Avery repeated Griffith’s
experiment
The research question: what
molecule in the heat-killed
bacteria was the most important
part of transformation. That
molecule is probably a part of the
gene that allows transformation to
take place.
3 . The Experiment:
a)
Avery & other
scientists made a
bacterial extract
(juice) from the
heat-killed
bacteria and
used enzymes to
destroy all of the
molecules in the
extract
b)
The enzymes destroyed
Proteins
Lipids
Carbohydrates

c)
RNA
transformation still occurred,
therefore those molecules are
not responsible for
transformation
d)
e)
4.
The
experiment
was repeated
using enzymes
that destroy
DNA.
Transformatio
n did not
occur
Conclusion:
DNA stores &
transmits
genetic
information
Avery’s transformation experiment
III. The Hershey-Chase
Experiment
Martha Chase and Alfred Hershey in 1953
1.
Virusnonliving
particle that
is much
smaller than
a cell, that
can infect
living
organisms
S.E.M. of a T4 bacteriophage virus.
(Reproduced by permission of
Photo Researchers, Inc.)
Bacteriophage
- a type of virus
that infects
bacteria.
2.
a)
Are made of a
DNA or RNA core
surrounded by a
protein coat
S.E.M. of bacteriophages attacking a bacterium
3.
When
bacteriophages
goes inside a
bacterium:
a) The virus attaches
to the cell surface
b) The virus then
injects its DNA or
RNA into the
bacteria.
c) The genes of the
virus make the
bacterium produce
more viruses
d) Result: the
bacterium “pops”
& is destroyed &
hundreds of new
viruses burst out.
The lytic cycle considered the main method of
viral replication, because it results in the
destruction of the infected cell
4.
5.
Radioactive Markers- used to
track a substance inside an
organism.
Hershey & Chase were trying to
figure out if genes were made of
protein or DNA.
 They
wanted to figure out which
part of the virus (the protein coat or
DNA core) went inside the
bacterium
6.
Hershey & Chase
used two
radioactive
markers to find out:
a)
b)
Sulfur-35 (35S)
tracks or marks
proteins
Phosporous-32
(32P) tracks or
marks DNA.
7.
If 35S was found in the bacteria, it
would mean that the viruses’
protein had been injected into the
bacteria.
8. If 32P was found in the bacteria, then it was
the DNA that had been injected.
9.
Result: Nearly all the radioactivity in the
bacteria was from phosphorus (32P).
10.
Conclusion: The genetic material of the
bacteriophage was DNA not protein.
IV. The Components &
Structure of DNA
Basic DNA structure
1.
DNA is :
a)
b)
c)
Made of units called nucleotides
A long molecule
The nucleotides have 3 parts: deoxyribose,
a phosphate group, & a nitrogenous base
(has nitrogen in it).
2. There are four kinds of nitrogenous bases in
DNA:
• Adenine (A)
• Guanine (G)
• Cytosine (C)
• Thymine (T)
5.
6.
A biochemist named
Erwin Chargaff studied
the amount of each
base in DNA.
Conclusion: Chargaff’s
Rules:
 The
% of Guanine (G) is
always equal to the %
of Cytosine (C)
 The % of Adenine (A) is
always equal to the %
of Thymine (T)
 This is the same in all
organisms
Edwin Chargoff c. 1930
7.
8.
9.
A scientist named
Rosalind Franklin
studied DNA
X-ray diffraction- a
technique she used
to learn about the
structure of DNA in
1952
Franklin was the first
to see the structure
of DNA
Rosalind Franklin
This x-ray image lead to the discovery of the
structure of DNA
10.
11.
12.
In1953 two scientist
named Watson &
Crick studied the
structure of DNA
using x-ray
diffraction data
collected by
Rosalind Franklin
Based on Franklin’s
x-ray they built a 3D model out of wire
& cardboard
A double helix in
which 2 strands are
wound around
each other
James Watson & Francis Crick would not
have been able to build their double helix
DNA model for which they were awarded
a Nobel Prize in 1962 w/o Franklin’s x-ray
image of DNA structure
13.
They discovered that hydrogen bonds
form b/w the nitrogen bases
 This
is what holds the 2 strands together (H
bonds)
14.
15.
Base Pairing- H bonds only form b/w
adenine (A) & thymine (T); and cytosine
(C) and guanine (G)
Base Pairing explains Chargaff’s Rules
Discuss w/ Partner
 Pg.
292-293 Look at the timeline answer
the following questions:


Why did no DNA discoveries b/w the 1920’s
and 50’s?
Why did discovery speed up during the
1950’s?