Download Chapter 9 DNA: The Genetic Material

Chapter 9
DNA: The Genetic Material
Mrs. Cook
Biology
Identifying the Genetic Material
• Griffith’s Experiments
- 1928 Frederick Griffith
- Worked with Streptococcus pneumoniae
- was trying to prepare a vaccine
 A substance that is prepared from
killed or weakened disease-causing
agents, and is introduced into the
body to protect the body against
future infections by the diseasecausing agent.
Identifying the Genetic Material
-Griffith worked with two forms of the S.pneumoniae
1st strain:
•has a capsule, which protects the cell
from the body’s defenses
•it virulent or able to cause disease.
• because of the capsule it grows with
smooth (S) edged colonies.
2nd Strain:
•lacks a capsule
•does not cause disease
• forms rough (R) colonies
Identifying the Genetic Material
- Griffith knew:
• Mice infected with the S bacteria grew sick
and died.
• Mice infected with the R bacteria were not
harmed
- Griffith Experiment:
• Injected mice with dead S bacteria and
mice remained healthy
• Prepared a vaccine by “heat killing”raised temperature of bacteria to the point
where they were alive but could no longer
reproduce.
Identifying the Genetic Material
• Then injected mice with “heat killed” S
bacteria, the mice lived.
• Then he mixed the harmless live R bacteria
with the harmless “heat killed” S bacteria
- The mice died
- The live R bacteria had acquired capsules
- The harmless R bacteria had become
virulent.
- We now call what happened
TRANSFORMATION- a change in genes
when cells take in a foreign genetic material
Identifying the Genetic Material
• Avery’s Experiments
-1944- Avery and his co-worker’s at the
Rockefeller Institute in NYC, demonstrated that
DNA is the material responsible for
transformation.
- Not all scientists accepted this.
- Most thought proteins were the genetic
material
- little was known about DNA at this time
Identifying the Genetic Material
• Alfred Hershey and Martha Chase Experiment
- 1952- worked at Cold Spring Harbor
Laboratory in NY.
- worked with viruses, which are much
simpler than cells, are composed of DNA or
RNA surround by a protective protein coat.
-A bacteriophage, which is a virus that,
infects bacteria and produce more viruses
when the bacterial cell ruptures.
Identifying the Genetic Material
- The experiment:
• The 1st bacteriophage’s outer protein coat
was labeled with radioactive Sulfur.
• The 2nd bacteriophage’s DNA was labeled
with radioactive Phosphorous.
• When they each infected a E.coli bacteria,
Hershey and Chase discovered that
- the protein coat of the bacteriophage
did not affect the E.coli bacteria
- the DNA of the 2nd bacteriophage had
been injected into the E.coli.
Identifying the Genetic Material
Identifying the Genetic Material
• Chargaff’s
Observations
- 1949, a
biochemist at
Columbia
University, NY city.
- observed that
adenine paired with
thymine, cytosine
pairs with guanine
in the chemical
make up of DNA.
Identifying the Genetic Material
• Rosalind Franklin’s Photographs
- developed X-ray diffraction
- photographed DNA
- 1st to theorize that DNA was a helix
Identifying the Genetic Material
• Watson and Crick
- 1953- used
finding’s from Chargarff
and Franklin
- used their
knowledge of chemical
bonding
- discovered that
DNA is a double helix, a
“spiral staircase” of two
strands of nucleotides
twisting around a central
axis
The Structure of DNA
• DNA is a Double Helix
• It is a Nucleic Acid made of two
long chains (also called strands) of
repeating subunits called
nucleotides
Phosphate
5 Carbon
Sugar
Nitrogen
Base
• Each Nucleotide is made of three
parts:
1. a Five Carbon Sugar
2. a Phosphate Group
3. a Nitrogenous Base
The Structure of DNA
• In DNA, the sugar is Deoxyribose
• The phosphate group consists of a
Phosphorous atom bonded to four oxygen
atoms
• The Nitrogenous Base contains nitrogen
atoms, and carbon atoms and is a BASEaccepts hydrogen ions.
• The alternating sugar and phosphate groups
are held together by strong covalent bonds and
make up the sides of the staircase/ladder.
Phosphate
Covalent
Bonds
5 Carbon
Sugar
Nitrogen
Base
The Structure of DNA
• Each full turn of Helix has 10 Nucleotide pairs
• The Nitrogenous Bases face toward the center
of the DNA Molecule and Bonds to the Bases
on the other side with weak hydrogen bonds.
• 4 different Nitrogenous Bases
- Thymine (T)
- Cytosine (C)
- Adenine (A)
- Guanine (G)
The Structure of DNA
• A and G have a double ring
of carbon and nitrogen
atoms and called Purines.
• C and T have a single ring
of carbon and nitrogen
atoms and are called
Pyrimidines
Chargaff’s Rule: A Purine must always pair with a
Pyrimidine!
The Structure of DNA
• Complementary Bases:
- Base Pairing Rules state that
C always bonds with G
A always bonds with T
- These pairs of bases are called
Complementary Base Pairs
- The order of the nitrogenous bases on a
chain of DNA is called Base Sequence.
The DNA Code
• The sequence of nucleotide pairs
controls how an organism will produce
its proteins, therefore DNA controls the
structure & function of the organism…it
is not random!
• All organisms share the same chemical
DNA. The difference is in the number,
kind and order of the nucleotides!
DNA Replication
• DNA Replication is the process by which DNA is
copied in a cell before a cell divides by Mitosis or
Meiosis. Also called Duplication.
• Three Major Steps:
1. Enzyme called Helicase separates the DNA
strands by breaking the hydrogen bonds
between the base pairs.
- The Y-shaped region that is formed is called
the Replication Fork.
DNA Replication
2. Enzymes called DNA Polymerases then add
complementary nucleotides (which are found
floating freely in the nucleus) to the original
strands
- at a rate of about 50 nucleotides per second!
3. Bases are added in opposite directions on the
strands.
- Synthesis moving away from the replication
fork leaves gaps which can later be filled in by the
enzyme DNA Ligase.
DNA
Replication
Replication
ForkHydrogen
bonds
broken by
Helicase.
DNA Replication
• While the new bases are being added, new
covalent bonds are formed on the outside
between the new phosphate and sugar
molecules
• New hydrogen bonds are formed between the
new complementary base pairs on the original
and new strands.
• DNA Polymerase falls off, resulting in two
separate and identical DNA molecules that are
ready to move to new cells in cell division.
DNA Replication
• After DNA Replication is complete, the end
result is the original DNA molecule and an
identical new DNA strand.
• This type of replication is called
Semi-Conservative Replication, because each
of the new DNA molecules has kept one of the
two original DNA strands.
DNA Replication
DNA Replication
DNA Replication
DNA Replication
• In Prokaryotes, remember that there is one
circular DNA molecule.
-DNA Replication begins in one spot
- Two replication forks are formed and proceed
in different directions until they meet.
DNA Replication
• In Eukaryotic, DNA is very long and straight.
- Replication beings at many points, or
origins, along the DNA
- at each origin, two replication forks move in
opposite directions
DNA Replication
• Any Error in DNA replication can lead to a
mutation.