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Chapter 12
DNA
Section 12.1
Identifying the
Subsrance of Gene
Summarize the process of
bacterial transformation.
Describe the role of bacteriophages in identifying genetic
material.
Identify the role of DNA in
heredity.
Biological characteristics
are inherited through
genes that are passed
from parents to offspring.
the molecule make genes
Griffith’s Experiments
Streptococcus pneumoniae
S Strain (smooth) deadly
R Strain (rough) hramless
Griffith’s Experiments
Ran for tests on mice:
1)S-type injected into mice
-Mice die
2)R-type injected into mice
-Mice live
3)Heat-killed S-type injected
into mice
-Mice live
4)Heat-killed S-type and R-type
injected into mice
-Mice die
Transformation
•Conclusion
Some material from dead S-type
changed R-type into S-type
•Transformation
-one type of bacteria had been
changed permanently into another
-the transforming factor had to be a
gene
The Molecular Cause of
Transformation
•Which molecule was most important
for transformation in the heat-killed
bacteria?
•Avery
•Used enzymes to destroy
either the proteins, DNA, or
RNA of the cells and then
tried to transform the
bacteria.
R-type + S-type with
destroyed proteins
transformation
R-type + S-type with
destroyed RNA
transformation
R-type + S-type with
destroyed DNA
no transformation
Conclusion:
DNA must be needed to transform
bacteria cells, so it must be the key to
heredity
Bacterial Viruses
•viruses—tiny particles that can infect
living cells
•The kind of virus that infects bacteria
is known as a bacteriophage.
The Hershey-Chase
Experiment
bacteriophage made of just DNA
and a protein coat
Labeled DNA and protein coat of
virus with radioactive P and S
isotopes
The Hershey-Chase
Experiment
The Hershey-Chase
Experiment
•E.coli+Virus with labeled protein
no radioactivity in offspring
•E.coli + Virus with labeled DNA
radioactivity in offspring
•Conclusion:
DNA, not proteins, are passed on to
offspring
The Role of DNA
What is the role of DNA in heredity?
The DNA that makes up genes must
be capable of storing, copying, and
transmitting the genetic
information in a cell.
•Storing Information
Genes must contain the instructions that
cause a single cell to develop into a mature
individual.
•Copying Information
Before a cell divides, it must make a
complete copy of every one of its genes.
•Transmitting Information
When a cell divides, each daughter cell
must receive a complete copy of the
genetic information.
Section 12.2
the Structure of DNA
Identify the chemical
components of DNA
Discuss the experiments leading
to the identification of DNA as the
molecule that carries the genetic
code
Describe the steps leading to
the development fo the doublehelix model of DNA
The Components of DNA
•What are the chemical components
of DNA?
-Deoxynucleic Acid / polymer
-deoxynucleotide / monomer
-covalent bonds form between sugar
and phosphate groups.
Nitrogenous Bases and
Covalent Bonds
•four kinds of nitrogenous bases:
adenine (A)
guanine (G)
purines
cytosine (C)
thymine (T)
pyrimidines
•Chargaff’s Rules
[A] = [T] and [G] = [C]
Franklin’s X-Rays
• There are two strands in
the structure.
•The strands in DNA are
twisted around each other
like a helix.
•The nitrogenous bases are
near the center of the DNA
molecule.
The Work of
Watson and Crick
•They built threedimensional models of
the molecule.
•the double helix model
The Double-Helix Model
•Antiparallel Strands
-The two strands of
DNA run in opposite
directions.
-The nitrogenous
bases on both
strands meet at the
center of the
molecule
Hydrogen Bond and
Base Pairing
-between certain
nitrogenous bases
-Base Pairing
•Adenine with Thymine
•Guanine with Cytosine
-weak chemical bond
-allow two strands to
separate easily
Section 12.3
DNA Replication
Summarize the events of
DNA replication
Compare DNA replication in
prokaryotes with that of
eukaryotes
Replication of DNA (复制)
•The DNA in the chromosomes is
copied in a process called DNA
replication.
Replication of DNA (复制)
•separation of strands
replication forks
•base pairing
A-T; C-G
template
•bonding of bases
Replication of DNA (复制)
The Role of Enzymes
•break the hydrogen bonds
•unwind the two strands
DNA polymerase
•principle enzyme
•join individual nucleotides
•check new DNA strand
Telomeres
•the tips of
chromosomes
Telomerase
•add short, repeated
DNA sequences to
telomeres
Replication in Living Cells
•Prokaryotic cells
single circular DNA
• Plasmids
additional DNA rings
based between
different bacteria;
help protect against
their environment
•Eukaryotic cells
long DNA in the
nucleus
bigger
chromosomes
Prokaryotic DNA Replication
•Single starting point, ori
•Two directions; one large replication
bubble
•DNA attaches to the cell membrane to
steady itself
Eukaryotic DNA Replication
•Dozens or hundreds starting points;
many replication bubbles
•Chromatin condenses
That’s All
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