Identifying the Substance of Genes
Learning Objectives
 Summarize the process of bacterial transformation.
 Describe the role of bacteriophages in identifying genetic
material.
 Identify the role of DNA in heredity.
Griffith’s Experiments
R strain:
S strain:
causes disease harmless
Mouse dies
of pneumonia.
Mouse
lives.
S strain:
heat killed
Mouse
lives.
Mixture:
R, dead S
Mouse dies
of pneumonia.
Live S strain
Avery’s Experiments
• Scientists discovered that the nucleic acid DNA
stores and
transmits genetic information from one generation of
bacteria to the next.
• DNA was the transforming factor
• Avery's team repeated Griffith's experiments because they wanted to
determine which molecule in the heat-killed bacteria was the
transforming factor.
Bacterial Viruses
Bacteriophage: a kind of virus that infects bacteria
DNA
head
tail sheath
tail fiber
Avery’s
Experiment
worked because
bacteriophages
and bacteria
would share
DNA
Hershey-Chase Experiment
Bacteriophage with
radioactive tag in DNA
Phage infects
bacterium.
Radioactivity
inside bacterium
Bacteriophage with
radioactive tag in
protein coat
Phage infects
bacterium.
No radioactivity
inside bacterium
Hershey-Chase Experiment
• Hershey and Chase’s experiment with bacteriophages
confirmed Avery’s results, convincing many scientists that
DNA was the genetic material found in genes—not just in
viruses and bacteria, but in all living cells.
• Used radioactive isotopes Sulfur-35 and Phosphorus-32
• Phosphorus is abundant in DNA, while sulfur is abundant
in protein
• If they would have found both P-32 and S-35 in the
bacteria them they could have concluded that both the
virus’s protein coat and its DNA were injected into the
bacteria. However this was not the case.
The Role of DNA: Storing Information
DNA stores information needed by every living cell.
The Role of DNA: Copying and Transmitting
Copying
Information
Transmitting
Information
The Role of DNA: Summary
1. Store information
2. Copy information for daughter cells
3. Transmit information to daughter cells
The Structure of DNA
Learning Objectives
 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 of the
double-helix model of DNA.
Nucleotide Structure
• DNA is made up of nucleotides joined into long strands or
chains by covalent bonds.
•Nucleotides are joined together to form the DNA chain by
links between deoxyribose molecules and phosphate groups
• Nucleic acids are made up of building blocks called
nucleotides.
Phosphate
group
Base
Deoxyribose
sugar
Nitrogenous Bases
Adenine
Guanine
Cytosine
Thymine
Nucleic Acid Structure
One nucleotide
Covalent bond
between nucleotides
Chargaff’s Rule
[A] = [T] and [C] = [G]
Erwin Chargaff, in carrying out biochemical studies,
had discovered that the percentages of adenine [A]
and thymine [T] bases are almost equal in any
sample of DNA. The same thing is true for the other
two nucleotides, guanine [G] and cytosine [C].
Franklin’s X-rays
• DNA is a helix.
• Likely two strands to the molecule
• Nitrogenous bases near the center of the molecule
The Work of Watson and Crick
DNA is a double helix, in which two strands of nucleotide
sequences are wound around each other.
The Double Helix: Antiparallel Strands
The two strands in a DNA molecule run in opposite
directions.
The Double Helix: Hydrogen Bonding
Hydrogen bonds
The Double Helix: Base Pairing
• The two strands of DNA are held together by hydrogen
bonds between the nitrogenous bases adenine and thymine
and between guanine and cytosine
.
• Adenine and Guanine can’t pair together because they
both have long bases.
DNA Replication
Learning Objectives
 Summarize the events of DNA replication.
 Compare DNA replication in prokaryotes with that of
eukaryotes.
Review of DNA Structure
nitrogenous
bases
sugar-phosphate
backbone
double helix
DNA Replication and the Cell Cycle
S
DNA replication occurs during the
cell cycle.
phase of the
Copying DNA
replication fork
DNA polymerase
Direction
of replication
Direction
of replication
new nucleotides
being added
DNA polymerase joins
individual nucleotides by
adding base pairs to
produce a new strand of
DNA and proofreads the
new strand.
DNA Replication
The blue strand represents the
original
DNA strand.
The red strand represents the
new
DNA strand.
Therefore DNA replication results
in two DNA molecules, each with
one new strand and one original
strand.
Telomeres
• Telomeres: the tips of eukaryotic chromosomes
• Telomerase adds short, repeated DNA sequences to
telomeres as the chromosomes are replicated.
Telomeres
Chromosomes
Chromosomes include
-histones
-nucleosomes
-DNA
Eukaryotic DNA Replication
Unreplicated DNA
Replication
forks
Eukaryotic cells replication may
begin at dozens or even
hundreds of places on the DNA
molecule, proceeding in both
directions until each
chromosome is completely
copied.
New DNA
Prokaryotic DNA Replication
new DNA
replication fork
Replication in most
prokaryotic cells begins
at a single starting point
and proceeds in two
directions until the entire
chromosome is copied.
Regulatory Proteins
binds to the prokaryotic
chromosome to start
DNA replication
replication fork
unreplicated DNA