CH 10: Molecular
Biology of the Gene
DNA  RNA  Protein
Sections Covered
with my titles for each section
10.1: DNA as the genetic material
10.2/3: Structure of DNA and RNA
10.4/5: DNA replication
10.6-10.14: Transcription and translation
10.15: review
10.16: Mutations
History of DNA
 DNA

as the genetic material
Griffith (1928)
• Found that the genetic component of pathogenic
bacterial cells was not destroyed when the cells
were heated
• He did not follow-up on what that component was
and how/why it survived.
• Griffith Experiment
DNA as the Genetic Material
 Avery



(1944)
Most believed protein to be genetic material at
this time.
Avery found that pathogenic bacterial cells
treated with protein digesting enzymes could
still transform harmless bacterial cells.
Cells treated with a DNA digesting enzyme
could not.
DNA as the Genetic Material
 Avery


(1944)
Avery concluded that DNA and not protein
must be the genetic material.
Many refused to accept this conclusion.
• Thought his findings only applied to bacteria and
not eukaryotic cells.
DNA as the Genetic Material
 Hershey-Chase


Experiment (~1950)
Their work confirmed to the scientific
community that DNA was the genetic material.
Considered an “elegant” experiment.
• Very simple and demonstrates a great deal.

See page 183
Hershey-Chase Experiment
 They
took advantage in a chemical
difference between DNA and protein


DNA contains the elements:
C, H, O, N, P
Protein contains the elements: C, H, O, N, S
Hershey-Chase Experiment
Experiment utilized bacteriophages

Bacteriophages are viruses that infect
bacteria.
Knew that a virus’ genetic material enters
the host cell


•
as a result the bacterial cell makes more virus as
directed by the virus’ genetic material
Hershey-Chase Experiment
More on viruses…..


Viruses have two components:
•
An outer protein coat with nucleic acid inside
Hershey-Chase Experiment
The Experiment
Allowed one sample of viruses to infect
bacteria grown on a radioactive (RA)
sulfur-35 medium
1.
•
Viruses made had RA Sulfur-35 in their
protein coats.
Hershey-Chase Experiment
The Experiment
Allowed another sample of viruses to
infect bacteria grown on a radioactive
(RA) phosphorus-32 medium
2.
•
Viruses made had RA phosphorus-32 in
their DNA.
Hershey-Chase Experiment
The two RA viral cultures were isolated
and each was allowed to infect a new
(non RA) bacterial culture.
3.

4.
Exp’t was done in a liquid medium called the
supernatant.
Cultures were gently shaken in a blender
to shake the virus off of the outside of the
bacteria.
Virus infecting bacterial cell
Hershey-Chase Experiment
Each culture was centrifuged to separate
the liquid medium (supernatant) from the
infected bacteria.
The bacteria and the supernatant were
checked for radioactivity.
5.
6.

Whatever entered the bacteria is the genetic
material.
Hershey-Chase Experiment
What they found:

Bacteria infected with the virus with a RA S35 (protein) coat
•
•

The infected bacteria were NOT RA
The supernatant was RA
This is evidence that the protein did not
enter the bacteria and thus, could not be the
genetic material.
Hershey-Chase Experiment

For the bacteria infected by virus with RA P32 in their DNA
• The infected bacteria were RA
• The supernatant was not RA
• This is evidence that the DNA entered the bacteria
and thus, MUST be the genetic material.
• http://www.accessexcellence.org/RC/VL/GG/hersh
ey.php
Structure of DNA
What was known about DNA

Chemical components are:
• Deoxyribose – 5 carbon sugar
• Phosphate groups
• Nitrogenous bases
 Adenine
 Guanine
 Cytosine
 Thymine
Structure of DNA
 Nitrogenous

bases were of 2 types:
Purines: have a double-ring structure
• Adenine (A)
• Guanine (G)

Pyrimidines: have a single-ring structure
• Cytosine (C)
• Thymine (T)
• Page 185
Structure of DNA
 Chargaff’s


findings (1949)
Studied DNA from many organisms
Found that the amount of guanine is always
equal to the amount cytosine and the amount
of adenine is equal to the amount of thymine.
• G=C
• A=T
Structure of DNA
 X-Ray

Crystallography Data on DNA
Maurice Wilkins and Rosalind Franklin
• Franklin’s data suggested that DNA was a long
thin molecule of 2 nm diameter
• Data also indicated a repeating pattern consistent
with a helix.
• Wilkins shared Franklin’s data and lab notes with
Watson and Crick without her permission.
Rosalind Franklin

As a scientist Miss
Franklin was
distinguished by extreme
clarity and perfection in
everything she undertook.
Her photographs are
among the most beautiful
X-ray photographs of any
substance ever taken.
Their excellence was the
fruit of extreme care in
preparation and mounting
of the specimens as well
as in the taking of the
photographs. -- J. D. Bernal
[1958 N]
Franklin’s X-Ray Data
Structure of DNA
"The instant I saw the picture my mouth
fell open and my pulse began to race....
the black cross of reflections which
dominated the picture could arise only
from a helical structure... mere
inspection of the X-ray picture gave
several of the vital helical parameters."
Watson
Structure of DNA
 In
1953 Watson, Crick, and Wilkins put the
pieces together and proposed their
famous double helix structure for DNA.

Watson, Crick, and Wilkins were awarded a
Nobel Prize for deciphering the structure of
DNA
Watson and Crick
Structure of DNA
 DNA

is a double-stranded helix
Each strand is a long chain of covalently
bonded nucleotides
• Phosphates can bond to carbon 5 or carbon 3 of
deoxyribose

Phoshpates link the sugars to form the backbone of the
chain
• Bases bond to carbon 1 of deoxyribose
• Page 187
Structure of DNA
 Each

strand has a 5’ and a 3’ end
Two DNA strands run in opposite directions
• One runs 5’  3’ and the other 3’ 5’
Structure of DNA
 The
two strands are joined by hydrogen
bonds between the bases
• Two H bonds form between A and T.
• Three H bonds form between G and C.
C
A
G
T
Structure DNA
DNA Replication
 DNA


replication – DNA synthesis
Occurs in the nucleus during ___ of the cell
cycle
Goal is to make an exact copy of the cell’s
DNA
• Put another way -- goal is to duplicate the
chromosomes.
Replication
Semi-Conservative Model
 Each
newly made piece of DNA is ½ old
DNA and ½ new DNA (page 188)
Simple animation of replication
DNA Replication-enzymes needed

Helicases
• Open the H bonds between the strands

Stabilizing proteins
• Hold the two strands apart
DNA Replication: enzymes needed

DNA polymerase III
• Adds nucleotides to the 3’ end of DNA
• Say…synthesizes DNA in the 5’  3’ direction
• It cannot initiate (start) a new DNA strand

DNA polymerase I
• Removes primer sequences and fills in the
gaps with DNA

Other DNA polymerases
• Proofread the DNA and correct mutations
DNA Replication-enzymes needed

“Primer” enzyme – not shown in text
• Starts synthesis in the 5’  3’ direction
• Makes a primer sequence to which DNA
polymerase III can add DNA

DNA ligase
• Joins newly made DNA segments after the primer
sequences have been removed and replaced by
DNA polymerase I.
DNA Replication
Helicases and stabilizing proteins open
and unwind small sections of DNA and
hold the strands apart.
1.
•
2.
Occurs at specific locations on the DNA –
called origins of replication
Primer enzymes synthesize primer
strands in the 5’  3’ direction on each
DNA strand.
DNA Replication
3.
DNA polymerase III adds DNA to each
primer sequence in the 5’  3’ direction.
DNA Replication
Proteins open more of the DNA
(replication fork opens more).
DNA synthesis continues in the 5’ 3’
direction on one strand. (leading strand)
4.
5.
•
Another primer is laid down on the other
strand and then DNA synthesis continues.
(lagging strand)
Primer sequences
DNA Replication
6.
7.
8.
Process continues until all of the DNA
has been replicated.
Primer sequences are cut out, the gaps
filled in with DNA
DNA ligase joins the new DNA
sequences.
http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swfAnimation