1. Chapter 12
DNA
Information and Heredity, The Cellular Basis of Life

2. Section 12-1: Identifying the Substance of Genes
Frederick Griffith’s Experiments (1920’s)
Bacterial Transformation
He was studying bacteria that cause pneumonia.
His experiment led to a discovery of a process a called transformation:
- Process in which one strain of bacteria changes into another strain.

3. Bacterial Viruses (Bacteriophages) The molecular cause of transformation What role did bacterial viruses play in identifying the genetic material ?
Oswald Avery (1940s)
His goal was to determine what molecule was used during transformation.
By a process of elimination using biological enzymes they discovered that D N A was the transforming factor.

4. Section 12-1: The Hershey - Chase Experiment (1950s)
They did an experiment involving viruses to prove the work of both Griffith & Avery.
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!.

5. Transformation

6. 12-1 The Role of DNA What is the role of DNA in heredity
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.
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.

7. Section 12-2: The Structure of DNA
The Components of DNA: The Nucleotide:
A nucleotide is the monomer of a nucleic acid such as DNA or RNA.
It has three components:
- A phosphate group
- A 5-carbon sugar called deoxyribose
- A nitrogenous base

8. Nucleotide
Phosphate / sugar / N base

9. 12-2: The Structure of DNA Components of DNA: The Nucleotide:
There are four different bases in DNA:
- Adenine (A) - Thymine (T)
- Guanine (G) - Cytosine (C)
These bases are placed in two groups:
- Purines (double ring)  A & G
- Pyrimidines (single ring)  C & T
Chargoff’s Rules of Base Pairing:
- A pairs with T - C pairs with G
- a purine will pair with a pyrimidine

10. 12-2: The Structure of DNA The Double Helix:
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.

11. 12-2 : The Structure of DNA The Double Helix:
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.

12. There are four levels of folding to produce the chromosome structure.
DNA
There are four levels of folding to produce the chromosome structure.
beads-on-a-string
30 nm fiber
looped domain
mitotic chromosome

13. DNA is wrapped around histones forming nucleosomes linked in a row.
beads-on-a-string
DNA is wrapped around histones forming nucleosomes linked in a row.

14. DNA
30 nm fiber
The beads-on-a-string level is coiled around itself in a solenoid spiral.

15. The solenoid spiral loops up and down to compress itself further.
DNA
looped domain
The solenoid spiral loops up and down to compress itself further.

16. DNA mitotic chromosome
The beads-on-a-string forming the solenoid spiral forming the looped domain give rise to the mitotic chromosome.

17. DNA

18. 12-3: DNA Replication
Coping the Code
Eukaryotic Chromosome Structure:

19. 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.

20. Section 12-3: Replication
The steps:
Within the replication bubble, two DNA polymerase work in opposite directions & make a complementary strand of DNA.
At the end of replication, there are two exact copies of DNA.

21. 12-3 Replication in Living Cells
How does DNA replication differ in
prokaryotic cells and eukaryotic cells?
Prokaryotes or bacteria 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.