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Chapter 12
DNA
Information and Heredity, The Cellular Basis of Life
Section 12-1: Identifying the
Substance of Genes
Frederick Griffith’s Experiments (1920’s)
Bacterial Transformation
1. He was studying bacteria that cause
pneumonia.
2. His experiment led to a discovery of a
process a called transformation:
- Process in which one strain of
bacteria changes into another strain.
Bacterial Viruses (Bacteriophages)
The molecular cause of transformation
What role did bacterial viruses play in identifying the genetic material ?
Oswald Avery (1940s)
1. His goal was to determine what molecule
was used during transformation.
2. By a process of elimination using
biological enzymes they discovered that
D N A was the transforming factor.
Section 12-1:
The Hershey - Chase Experiment (1950s)
1. They did an experiment involving viruses to
prove the work of both Griffith & Avery.
2. Viruses are only made of two things:
- protein coat
- nucleic acid core
3 In step 1 of their experiment, they tagged the
DNA core with radioactive P-32 to see if that
was the transforming factor. It was the
transforming factor!!.
4. In step 2 of their experiment, they tagged the
protein coat with radioactive S-35 to see if that
was the transforming factor. It was not!.
Transformation
12-1 The Role of DNA
What is the role of DNA in heredity
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The DNA that makes up genes can:
1. Store information in cells.
2. Copy this information in cells.
3. Transmit this information when cells divide.
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Study the book – cell analogy on page 342 of your textbook.
A book can store information, you can copy information from this book and
the book after being copied can be transmitted or given to others.
Section 12-2: The Structure of DNA
The Components of DNA: The Nucleotide:
1. A
nucleotide
is the monomer
of a nucleic acid such as DNA or RNA.
2. It has three components:
- A phosphate group
- A 5-carbon sugar called deoxyribose
- A nitrogenous base
Nucleotide
• Phosphate / sugar / N base
12-2: The Structure of DNA
Components of DNA: The Nucleotide:
3. There are four different bases in DNA:
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
4. These bases are placed in two groups:
- Purines (double ring)  A & G
- Pyrimidines (single ring)  C & T
5. Chargoff’s Rules of Base Pairing:
- A pairs with T- C pairs with G
- a purine will pair with a pyrimidine
12-2: The Structure of DNA
The Double Helix:
1. The structure of DNA was discovered
through the work of three people:
A. Rosalind Franklin (1950s)
- she used a technique called
x-ray diffraction to show that
DNA has two strands that form a
helix.
12-2 : The Structure of DNA
The Double Helix:
1. The structure of DNA was discovered through
the work of three people:
B. James Watson & Francis Crick (1953)
- They expanded on Franklin’s work by
building a 3-D model of DNA
- Their model had a double helix
structure
- The helix had a sugar-phosphate
backbone
- The bases were in the middle and
held together by hydrogen bonds.
DNA
There are four levels of folding to produce the
chromosome structure.
1. beads-on-a-string
2. 30 nm fiber
3. looped domain
4. mitotic chromosome
DNA
beads-on-a-string
DNA is wrapped around histones forming
nucleosomes linked in a row.
DNA
30 nm fiber
The beads-on-a-string level is coiled around
itself in a solenoid spiral.
DNA
looped domain
The solenoid spiral loops up and down to
compress itself further.
DNA
mitotic chromosome
The beads-on-a-string forming the solenoid
spiral forming the looped domain give rise to
the mitotic chromosome.
DNA
12-3: DNA Replication
Coping the Code
Eukaryotic Chromosome Structure:
Section 12-3: DNA Replication
What is replication?
This is the process of making a copy of the
genome during the S Phase of the cell cycle.
The steps:
DNA helicase “unzips” the double helix by
breaking the H bonds between the bases.
- Two helicase enzymes work in opposite
directions & form a replication bubble.
- The site where the helicase is doing the
unzipping is called a replication fork.
Section 12-3: Replication
The steps:
2. Within the replication bubble, two DNA
polymerase work in opposite directions &
make a complementary strand of DNA.
3. At the end of replication, there are two exact
copies of DNA.
12-3 Replication in Living Cells
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How does
DNA
replication differ in
prokaryotic cells and eukaryotic cells?
bacteria
• Prokaryotes or
cells normally have a
single circular strand of DNA. Replication starts from a
single point and proceeds in 2 directions.
• In Eukaryotes or cells with a nucleus, replication begins
at dozens or even hundreds of places and proceeds in
both directions.