Transcript Lesson Overview

12.1 Identifying the
Substance of Genes
Lesson Overview
Identifying the Substance of Genes
THINK ABOUT IT
How do genes work?
To answer that question, the first thing you
need to know is what genes are made of.
How would you go about figuring out what
molecule or molecules go into making a
gene?
Lesson Overview
Identifying the Substance of Genes
Griffith’s Experiments
Griffith isolated two different strains of the
same bacterial species. One was harmless,
one caused disease.
Lesson Overview
Identifying the Substance of Genes
Griffith’s Experiments
Lesson Overview
Identifying the Substance of Genes
Griffith’s Experiments
1.) Griffith took a culture of the S strain,
2.) Heated the cells to kill them
3.) Then injected the heat-killed bacteria
into laboratory mice.
4.) The mice survived.
Lesson Overview
Identifying the Substance of Genes
Griffith’s Experiments
In Griffith’s next experiment, he mixed
the heat-killed, S-strain bacteria with live,
harmless bacteria from the R strain and
injected the mixture into laboratory mice.
The injected mice developed pneumonia,
and many died.
Lesson Overview
Identifying the Substance of Genes
Griffith’s Experiments
What happened???
Lesson Overview
Identifying the Substance of Genes
Transformation
DNA transformed the good
bacteria into disease-causing
bacteria.
Lesson Overview
Identifying the Substance of Genes
Transformation
He called this process
transformation, because one type
of bacteria had been changed
permanently into another.
Lesson Overview
Identifying the Substance of Genes
Oswald Avery
• Used enzymes to kill off proteins, lipids, carbs
and RNA.
• Transformation still occurred.
• DNA was the transforming factor!
Lesson Overview
Identifying the Substance of Genes
Bacterial Viruses
Alfred Hershey and Martha Chase
Hershey and Chase studied viruses—
nonliving particles that can infect living
cells.
Lesson Overview
Identifying the Substance of Genes
Bacteriophages
The kind of virus
that infects bacteria
is known as a
bacteriophage,
which means
“bacteria eater.”
Lesson Overview
Identifying the Substance of Genes
The Hershey-Chase Experiment
Bacteriophage was composed of a DNA
core and a protein coat.
They wanted to determine which part
of the virus—the protein coat or the
DNA core—entered the bacterial cell.
Lesson Overview
Identifying the Substance of Genes
The Hershey-Chase Experiment
Hershey and Chase used
radioactive markers to see the DNA
and protein.
Lesson Overview
Identifying the Substance of Genes
Lesson Overview
Identifying the Substance of Genes
12.2 The Structure of DNA
Lesson Overview
Identifying the Substance of Genes
Nucleic Acids and Nucleotides
DNA’s nucleotides are made up of three
basic components:
a 5-carbon sugar called deoxyribose
a phosphate group
and a nitrogenous base.
Lesson Overview
Identifying the Substance of Genes
Nitrogenous Bases and Covalent Bonds
DNA has four kinds of nitrogenous
bases: adenine (A), guanine (G),
cytosine (C), and thymine (T).
The nitrogenous bases stick out
sideways from the nucleotide chain.
Lesson Overview
Identifying the Substance of Genes
Nitrogenous Bases and Covalent Bonds
The nucleotides can be joined together
in any order, meaning that any
sequence of bases is possible.
Lesson Overview
Identifying the Substance of Genes
Chargaff’s Rules
Erwin Chargaff 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].
The observation that [A] = [T] and [G] = [C]
became known as one of “Chargaff’s rules.”
Lesson Overview
Identifying the Substance of Genes
Franklin’s X-Rays
In the 1950s, British scientist Rosalind
Franklin used X-ray diffraction.
X-ray diffraction revealed an X-shaped pattern
showing that the strands in DNA are twisted
around each other like the coils of a spring.
Lesson Overview
Identifying the Substance of Genes
The Work of Watson and Crick
At the same time, James Watson, an
American biologist, and Francis Crick, a
British physicist, were also trying to
understand the structure of DNA.
They built three-dimensional models of the
molecule.
Lesson Overview
Identifying the Substance of Genes
The Work of Watson and Crick
Early in 1953, Watson was shown a copy of
Franklin’s X-ray pattern.
The clues in Franklin’s X-ray pattern enabled
Watson and Crick to build a model that
explained the specific structure and properties
of DNA.
Lesson Overview
Identifying the Substance of Genes
The Double-Helix Model
Common names for the structure of DNA:
- Double helix
- Twisted ladder
- Spiral staircase
The two strands of DNA are “antiparallel”
— they run in opposite directions.
Lesson Overview
Identifying the Substance of Genes
Hydrogen Bonding
Hydrogen bonds form between
the nitrogenous bases
G – C has 3 hydrogen bonds.
A – T has 2 hydrogen bonds.
Hydrogen bonds are weak.
Why is this important to DNA’s
function?
DNA’s strands have to separate
often!
Lesson Overview
Identifying the Substance of Genes
12.3 DNA Replication
Lesson Overview
Identifying the Substance of Genes
The Eukaryotic Cell Cycle
Lesson Overview
Identifying the Substance of Genes
THINK ABOUT IT
Before a cell divides, its DNA must first be
copied.
How might the double-helix structure of DNA
make that possible?
Lesson Overview
Identifying the Substance of Genes
The Replication Process
Before a cell divides, it duplicates its
DNA in a copying process called
replication.
Lesson Overview
Identifying the Substance of Genes
DNA Replication
The structure of DNA explains how it replicates
DNA “unzips” down the middle
Free floating nucleotides pair up with both sides of the
DNA molecule
A
T
C
G
A
T
A
G
C
T
Original
AT
TA
CG
GC
AT
A
T
C
G
A
T
A
G
C
T
AT
TA
CG
GC
AT
Exactly Copied DNA Strands
Lesson Overview
Identifying the Substance of Genes
DNA Replication
This process
produces two exact
DNA molecules
(Chromosomes) that
are the same
DNA never leaves
the nucleus
Lesson Overview
Identifying the Substance of Genes
The Role of Enzymes
How does this happen??
DNA replication is carried out by a series
of enzymes.
They first “unzip” a molecule of DNA by
breaking the hydrogen bonds between
base pairs.
Lesson Overview
Identifying the Substance of Genes
The Role of Enzymes
- DNA Polymerase
- DNA polymerase is an enzyme that joins
individual nucleotides to produce a new
strand of DNA.
- DNA polymerase also “proofreads”
Lesson Overview
Identifying the Substance of Genes
Replication in Living Cells
How does DNA replication differ in
prokaryotic cells and eukaryotic cells?
Replication in most prokaryotic cells starts
from a single point and proceeds in two
directions until the entire chromosome is
copied.
Lesson Overview
Identifying the Substance of Genes
Replication in Living Cells
How does DNA replication differ in
prokaryotic cells and eukaryotic cells?
In 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.
Lesson Overview
Identifying the Substance of Genes
Lesson Overview
Identifying the Substance of Genes