Download DNA is information molecule

DNA: The Information
Molecule
How did scientists use evidence to
discover that the DNA molecule
contains information?
A Historical Perspective
Type of Molecule
Proteins
Carbohydrates
Fats
DNA
Structure
Function
Complex
A few known functions
Simple
Energy and Structural
Simple
Energy and Structural
Simple
Unknown
What molecule contains
information in cells?
Experiment 1: Griffith (1928)
•Molecules from dead virulent Streptococcus can
transform benign Streptococcus into a virulent strain.
There are two strains of Streptococcus pneumoniae.
ROUGH COLONY (R)
R strain is benign
(Lacking a protective
capsule, it is recognized
and destroyed by
host’s immune system)
SMOOTH COLONY (S)
S strain is virulent
(Protective capsule
prevents detection by
host’s immune system)
Explain each treatment:
1. What data shows the heatkilled S strain did not cause
death of the mouse in
treatment #4?
2. What data shows S strain
molecules cannot kill the
mouse alone; the
streptococcus must be
alive?
3. What data shows that the
R strain alone will not kill
the mouse?
4. What data shows that the
S strain IS virulent?
Griffith’s Conclusion
• The R strain was able to take in molecules from
the dead S strain.
• These molecules somehow gave the R strain
characteristics of the dead S strain.
– Ability to make protective coating
So, we know that there is a molecule that transfers
information…
BUT…we still haven’t proved what those
molecules are!
What molecule contains
information in cells?
• Experiment 2. (Avery et al., 1942)
– Extracted material from dead virulent
Streptococcus and treated it with enzymes to
destroy either DNA or RNA or proteins.
DETERMINING THAT DNA IS THE HEREDITARY MATERIAL
Heat-killed
S cells
1. Remove the lipids
and carbohydrates
from a solution of
heat-killed S cells.
Proteins, RNA, and
DNA remain.
Lipids
Carbohydrates
Add
Add
proteinases
proteinases
Sample should
Sample
contain should
Contain
NO PROTEIN
NO PROTEIN
Add R cells
Add
Add
ribonuclease
ribonuclease
Sample should
Sample
contain should
Contain
NO RNA
NO RNA
Add R cells
Add
Add
deoxydeoxyribonuclease
ribonuclease
Sample should
Sample
contain should
Contain
NO DNA
NO DNA
Add R cells
2. Subject the
solution to
treatments of
enzymes to destroy
either the proteins,
RNA, or DNA.
3. Add a small
portion of each
sample to a culture
S cells
S cells
No S cells containing R cells.
appear
appear
appear
Observe whether
transformation has
Transformation occurs
No transformation occurs
occurred by testing
for the presence
Conclusion: Transformation cannot occur unless __________ is present.
virulent S cells.
Therefore, ______________ must be the hereditary material.
Are Genes Composed of DNA or
Protein?
T2 Virus
Cell
Virus protein coat
Virus DNA
1. Start of infection.
Virus DNA enters host
cells. Protein coat
does not.
2. Virus DNA directs
the production of new
virus particles.
3. End of infection.
New generation of
virus particles burst
from host cell.
HOW DO VIRUSES WORK?
Virus
Virus
particle
Virus particle
particle
Host
Host
cell
genome
Host cell
cell genome
genome
Free particles
in tissue or
enviroment
4. Particles assemble
inside host, then burst
or bud to exterior.
1.
1.
Viral
genome
1. Viral
Viral genome
genome
enters
enters
host
cell.
enters host
host cell.
cell.
Protein
3. Viral mRNA are
translated and proteins
processed.
DNA
DNA
mRNA
mRNA
2.
2. Viral
Viral genome
genome
is
is replicated
replicated and
and
transcribed.
transcribed.
Hershey Chase Experiment
1952
Two treatments:
1. Radioactive
amino acids
2. Radioactive
nucleotides
Treatment 1
Radioactive
protein (35S)
Centrifuge and measure
radioactivity in pellet
and supernatant
T2 Phage
Bacterium
Radioactivity in
supernatant, but
not pellet
Radioactive phage
infects bacterial cells
Blender separates
protein coats from
bacterial surface
Treatment 2
Radioactive
DNA (32P)
Radioactivity in
pellet, but not
supernatant
Therefore, it is the viral DNA, and not protein, that
programs cells to make copies of the virus.
Are Genes Composed of DNA
or Protein?
• T2 virus experiment convinces skeptics
that DNA is the genetic material since the
material injected by the virus into host
cells is DNA, not protein.
T2 Virus
What we learned
DNA is the molecule that carries information
Information in DNA determines which
proteins can be made by an organism
DNA can be passed between organisms in
nature.
Bacteria can take in pieces of DNA from
their environment. This process is called
transformation.