Transcript Chapter 9

Chapter 9
Sections 9-1 and 9-2
Section 9-1
Identifying the Genetic Material
Mendel’s experiments and results answered
why you resemble your parents – because
you have copies of their chromosomes, which
contain their genes
New Question – What are genes made of?
Griffith’s Experiments
Griffith, a bacteriologist, was trying to
prepare a vaccine against the pneumoniacausing bacterium, Streptococcus
pneumoniae
VACCINE – substance that is prepared
from killed or weakened microorganisms
and is introduced into the body against
future infections by the microorganisms
There are two types/strains of S.
pneumoniae
– First Strain – enclosed in a capsule made of
polysaccharides. Makes bacterium virulent
(disease causing). Smooth in appearance.
Known as the S strain.
– Second Strain – no polysaccharide capsule.
Does not cause disease. Rough in
appearance. Known as the R strain.
Mice infected with the S strain die, and the
mice infected with the R strain do not.
To determine if the capsule was what was
causing the disease, Griffith designed an
experiment with both strains of the
bacterium.
Griffith first injected mice with dead S
bacteria. The mice remained healthy.
He then injected mice with “heat-killed” S
bacteria (capsule still in tact). Mice
remained healthy.
This proved that the capsule did not cause
the disease, but what did?
Griffith then mixed the R strain bacteria
with the heat-killed S strain. What do you
think happened?
The mice died.
Griffith examined the blood of those mice
and found NO R strain bacteria. He only
found S strain.
The R strain had acquired the capsule
from the S strain, changed, and became
virulent.
Griffith’s experiment allowed him to
discover what is known as
TRANSFORMATION – a change in
phenotype caused when bacterial cells
take up foreign genetic material.
Now… What causes transformation?
Avery’s Experiment
There was a series of experiments that
demonstrated that protein-destroying
enzymes did not stop the process of
transformation, but DNA-destroying
enzymes did stop it.
Avery and his colleagues showed that
DNA is the material responsible for
transformation.
Hershey and Chase’s Experiment
It was then known that viruses were made
of DNA or RNA surrounded by a protein
coat for protection.
Bacteriophage or Phage – a virus that
infects only bacteria. When they infect
bacteria, they produce more virus which is
released when the bacterial cell ruptures.
Hershey and Chase used bacteriophage
T2.
They knew the only place sulfur was found
was in the protein coating of the phage,
and they also knew that phosphorus was
only found in the DNA.
They used these chemical differences to
carry out the experiment.
Step One
– They grew T2 with E. coli that contained
radioactive sulfur and another batch grown in
radioactive phosphorus.
– As the phage grew, it would incorporate each
of the radioactive compounds.
Step Two
– The radioactive labeled phage was used to
infect two separate batches of E. coli.
– The radioactive material could be followed
through the biological processes, and this
would show what was being transferred into
the bacteria.
Step Three
– After a few minutes, they tore the radioactive
labeled phage off the bacterial cells.
– They then used a centrifuge to separate the
bacteria and phages.
– Upon examining the upper layer (phage
material), they found most of the radioactive
sulfur was still part of the phage.
– The radioactive phosphorus was found in the
bacterial cells.
– This showed that the viral DNA had been
injected into the bacteria.
Step Four/Conclusions
Hershey and Chase concluded that the
DNA of viruses is injected into the bacterial
cells, and most of the proteins remain
outside the cell.
This showed that DNA was the hereditary
material.
9-1 Review
Griffith discovered transformation, which is
a change in phenotype caused when
bacterial cells take up foreign genetic
material.
Hershey and Chase determined that DNA
was the material that carries hereditary
information.
Section 9-2
The Structure of DNA
Watson and Crick determined the structure
of DNA.
DNA is a molecule that is a double helix –
two strands twisted around each other, like
a winding staircase.
Each strand is made of linked nucleotides
(a 5 carbon sugar, a nitrogen base, and a
phosphate group).
The 5 carbon sugar in DNA is
deoxyribose.
There are four different nitrogen bases:
Adenine, Cytosine, Thymine, and
Guanine.
Adenine – A
Guanine – G
Thymine – T
Cytosine - C
PURINES: Made of two rings – adenine
and guanine
PYRIMIDINES: Made of one ring –
thymine and cytosine
Chargaff’s Discovery – The amount of
adenine in a DNA molecule always equals
the amount of thymine in the same
molecule. This is the same for cytosine
and guanine. The amount of both pairs
found in each molecule of different
organisms will vary.
Pyrimidines
Purines
Wilkins and Franklin – the first to use XRay diffraction to photograph DNA fibers.
The pictures showed that DNA resembled
a tightly coiled helix and was composed of
two or three stands of linked nucleotides.
Watson and Crick – First to create the
three-dimensional model of the DNA
molecule.
DNA Base-Pairing Rules
A purine on one strand is always paired
with a pyrimidine on the opposite strand.
ADENINE ALWAYS PAIRS WITH
THYMINE!!! Forms two hydrogen bonds
with each other.
GUANINE ALWAYS PAIRS WITH
CYTOSINE!!! Forms three hydrogen
bonds with each other.
DNA Base-Pairing Rules
The strictness of base-pairing results in
two strands that are complementary to
each other; that is, the sequence of bases
on one strand determines the sequence
on the other strand.
TCGAACT
Original Strand
AGCTTGA
Complementary Strand
DNA Base-Pairing Rules
Problem 1
AGCCTGACGGATTCCGAT
TCGGACTGCCTAAGGCTA
Problem 2
GCAATTGACCATGGAATCG
CGTTAACTGGTACCTTAGC
9-2 Review
http://www.biologycorner.com/bio4/notes/D
NA-structure.php
DNA is a double helix made of two strands
of linked nucleotides.
Purines – A and G and Pyrimidines – C
and T
A always pairs with T AND c always pairs
with G
HOMEWORK
Section Review Questions p. 195 (1-5)
Active Reading Worksheets for 9-1 and
9-2