Download WELCOME TO BIOLOGY 2002 - University of Indianapolis

How Do Genes Work?
Are Genes Composed of DNA or Protein?

Chromosomes, the known carriers of genes, are made of DNA
and proteins

Proteins are more variable in structure than DNA, which
was not thought to be sufficiently complex to code for all
the operations of a cell.

DNA = a polymer made of 4 different types of nucleotides
(adenine, cytosine, guanine, thymine)

Protein = a polymer made of 20 different types of amino acids
Experiments showing DNA is the genetic material
ROUGH COLONY (R)
SMOOTH COLONY (S)
There are two strains
of Streptococcus
pneumoniae.
R strain is benign
(Lacks a protective
capsule, recognized
and destroyed by
host’s immune system)
S strain is virulent
(Polysaccharide
capsule prevents
detection by
host’s immune
system)
Experiments by Griffiths, 1928
Conclusion:
Experiments by Avery, MacLeod and McCarty (1944)
Heat-killed
S cells
Add
proteinases
Add
proteinases
Sample should
Sample
should
Contain
contain
NO PROTEIN
NO PROTEIN
1. Remove lipids and
carbohydrates from a
solution of heat-killed S
cells. Proteins, RNA, and
DNA remain.
Lipids
Carbohydrates
Add
RNAse
Add
ribonuclease
Sample should
Sample
should
Contain
contain
NO RNA
NO RNA
Add
Add
DNAse
deoxy-
2. Treat solutions with
Sample
Sampleshould
should enzymes to destroy
Contain
proteins, RNA, or DNA.
contain
NODNA
DNA
NO
ribonuclease
3. Add a portion of
each sample to a
culture containing R
S cells
S cells
No S cells cells. Observe if
appear
appear
appear
transformation
Transformation occurs
No transformation occurs occurs.
Add R cells
Add R cells
Add R cells
Conclusion: Transformation cannot occur unless DNA is present.
Therefore, DNA must be the hereditary material.
Are Genes Composed of DNA or Protein?

Transformation experiments provide the first evidence that
genes are DNA.
• Griffith: material from dead virulent Streptococcus can transform
benign Streptococcus into a virulent strain.
• Avery et al.: extracted material from dead virulent Streptococcus
and treated it with enzymes to destroy either DNA or RNA
or proteins. Only DNA-destroying enzymes prevented
transformation, hence DNA is the genetic material.
Are Genes Composed of DNA or Protein?

Not everyone was convinced

Hershey and Chase (1952): T2 virus experiments convinces
skeptics that DNA is the genetic material since the material
injected by the virus into host cells is DNA, not protein.
Bacteriophages are viruses that infect bacterial cells
How do viruses work?
Virus particle
Virus
Virus particle
particle
Host cell genome
Host
Host cell
cell genome
genome
New
viruses
released
4. Particles
assembled
inside host.
1. Viral genome
1.
1. Viral
Viral genome
genome
enters
host
cell.
enters
enters host
host cell.
cell.
Protein
3. Viral particles
produced
DNA
DNA mRNA
mRNA
2. 2.Viral
genome
Viral genome
is isreplicated
and
replicated and
transcribed.
transcribed.
Experiments by Hershey and Chase (1952)

Protein contains sulfur, but not phosphorous

DNA contains phosphorous, but not sulfur

Growing viruses with radioactive sulfur will label proteins but
not DNA

Growing viruses with radioactive phosphorous will label DNA
but not proteins
Experiments by Hershey and Chase (1952)
The structure and replication of genetic material

DNA structure was determined by James Watson and Francis
Crick using data from Rosalind Franklin

In order to pass on genetic material from parent (cell) to
offspring (cell), the genetic material must be duplicated: DNA
replication
Sister chromatids
Centromere
One chromosome
(Unreplicated state)
DNA replication
One chromosome
(Replicated state)
Figure 16.0 Watson and Crick
Figure 16.4 Rosalind Franklin and her X-ray diffraction photo of DNA
P
Free DNA nucleotides are called dNTPs
Structure of dNTPs
P
P
5'
CH2
Base
O
4'
1'
2'
3'
OH
Review of DNA structure: DNA and RNA are polymers made
of nucleotide subunits
Nucleotide
Pyrimidines
NH2
O
–
O P O
5
O
N Nitrogenous
base
1
4
O
Phosphate
2
3
group
5-carbon
sugar
Ribose
OH
HO CH2 O
4
C H H 1C
3 2
H C C H
OH OH
Deoxyribose
N
N
O
H
Cytosine
(C)
NH2
N
N
H3C
N
N
NH
N
O
H
Uracil
(U)
5
HO CH2 O
OH
4
C H H 1C
3 2
H C C H
OH H
O
NH
N
–
5
O
O
H
Thymine
(T)
Purines
O
N
N
NH
N
H
H
Adenine
(A)
Guanine
(G)
NH2
5'
A single strand of nucleotides
is made when a
phosphodiester bond is
formed between the 3’ C of
one nucleotide and the 5’ C of
another
3'
DNA is double stranded with the 2 strands held together by
hydrogen bonds between complementary bases
3´ 5´
5´
DNA is a
double
helix.
3´
T
T
A
G
C
C
T
G
A
A
T
A
A
C
G
G
C
A
T
T
A
C
G
C
T
A
T
G
C
T
A
G
C
T
A
3´Cartoon of 5´
base pairing
5´
3´
A
G C
CG
T
T
C
G
G
A T
A
T
G
A
A
T
G
C
A
T
5´
Cartoon of
double helix
3´
5´
3´
Space-filling model of
double helix
Figure 16.6 Base pairing in DNA
Unnumbered Figure (page 292) Purine and pyridimine
Distinguishing between Models of DNA Replication

Three different models of how DNA might replicate were
proposed based on DNA structure.
• Semi-conservative replication
• Conservative replication
• Dispersive replication
Figure 16.8 Three alternative models of DNA replication
Distinguishing Between Models of DNA Replication

The Meselsohn and Stahl experiment determines which
model is correct.
•
was fed to growing E. coli cells to mark DNA (“heavy” DNA),
then cells were switched to 14N.
15N
• Newly synthesized DNA will incorporate 14N (“light” DNA.
15N
15N
15N
14N
14N
15N
Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 1)
Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 2)
Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 3)
Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 4)