1. Chapter 12 Section 1 DNA: The Genetic Material
Ms. Diana and Ms. Su’ad

2. Discovery of the Genetic Material
Since Mendel’s work was rediscovered, scientists have tried to figure out which of the macromolecules- nucleic acids (DNA) or proteins was the actual source of genetic information.
Scientists knew that genetic information was found on chromosomes and the main components of chromosomes are DNA and protein.

3. Griffith’s Experiment
The first major experiment that led to the discovery of DNA was performed by Frederick Griffith in 1928.
He studied two strains of the bacteria Streptococcus pneumoniae, which causes pneumonia. He found that one strain could be transformed (changed) into the other form.
Smooth strain (S) – has a sugar coat and causes pneumonia.
Rough strain (R) – does not have a sugar coat and does not cause pneumonia.

4. Griffith’s Experiment
Step 1 – When the mouse was injected with the live S strain, the mouse died.
Step 2 – When the mouse was injected with the R strain, the mouse lived.
Step 3 – When the mouse was injected with the killed S strain (by using heat), the mouse lived.
Step 4 – When a mixture of live R cells and killed S cells were injected into the mouse, the mouse died.
It became obvious that there was a disease-causing factor that was passed from the killed S bacteria to the live R bacteria.
Griffith concluded then, that there had been a transformation from live R bacteria to live S bacteria.

5. Griffith’s Experiment

6. Avery Experiment
In 1944, Oswald Avery and his colleagues identified the molecule that transformed the R strain of bacteria into the S strain.
Avery separated different macromolecules, such as DNA, proteins and lipids, from killed S cells. Then he exposed live R cells to each of the macromolecules separately.
When the live R cells were exposed to the S strain DNA, they were transformed into S cells. Avery concluded that when the S cells in Griffith’s experiments were killed, DNA was released.
Some of the R bacteria kept this DNA in their cells, and this changed the bacteria into S cells.

7. A Closer Look

8. Hershey and Chase
In 1952, Alfred Hershey and Martha Chase published results of experiments that provided definite evidence that DNA was the actual transforming factor.
Their experiments involved a bacteriophage, a type of virus that attacks bacteria.
Two components made the experiment perfect for confirming that DNA is the genetic material.
1. The bacteriophage is used in the experiment was made of DNA and protein.
2. Viruses cannot replicate themselves. They must inject their genetic material into a living cell to reproduce.

9. A Closer Look

10. DNA Structure
Scientists were convinced that DNA was the genetic material. The questions that remained were how nucleotides came together to form DNA and how DNA could communicate information remained.
Nucleotides are the subunits of nucleic acids and consist of a five-carbon sugar, a phosphate group, and a nitrogenous base.

11. DNA Structure
The two nucleic acids found in living cells are DNA and RNA.
DNA nucleotides contain the sugar deoxyribose, a phosphate, and one of four nitrogen bases:
Adenine
Guanine
Cytosine
Thymine

12. RNA Structure
RNA nucleotides contain the sugar ribose, a phosphate, and one of four nitrogen bases:
Adenine
Guanine
Cytosine
Uracil

13. Chargaff
Erwin Chargaff analyzed the amount of adenine, guanine, thymine, and cytosine in the DNA of various species.
He found that the amount of guanine nearly equals the amount of cytosine. He also found that the amount of adenine nearly equals the amount of thymine within a species.
This finding is known as Chargaff’s rule: C=G and T=A.

14. The Structure Question
Four scientists joined together to search for the structure of DNA. Soon, Chargaff’s data became important and clear.
The four scientists are:
Rosalind Franklin, a British chemist
Maurice Wilkins, a British physicist
Francis Crick, a British physicist
James Watson, an American biologist

15. X-ray Diffraction
Wilkins was working at King’s College in London with a technique called X-ray diffraction (aiming X rays at a DNA molecule).
In 1951, Franklin joined the staff and used the same technique and the famous Photo 51.
It shows that DNA was a
double helix, or twisted ladder
shape, formed by two strands
of nucleotides twisted around
each other.

16. Watson and Crick
Watson and Crick were working at Cambridge University when they saw Franklin’s picture.
Using Chagraff’s data, they measured the width of the helix and the spaces between the bases.
Together, they built a model of the double helix that looked like this:

17. DNA Structure DNA is often compared to a twisted ladder.
The rails represent the alternating deoxyribose and phosphate.
The steps represent the pairs of bases (cytosine-guanine or thymine-adenine).

18. DNA ORIENTATION
Another unique feature of the DNA molecule is the direction (orientation) of the two strands.
On the top rail, the sugars run from 5’ to 3’ (“five prime to three prime”).
On the bottom rail, the sugars run from 3’ to 5’ (“three to five prime”).
Since they run in opposite directions, they are said to be antiparallel.

19. DNA Orientation

20. The Announcement
In 1953, Watson and Crick surprised the scientific community by publishing a one-page article in the journal Nature that showed their DNA structure.
In the same journal, Wilkins and Franklin presented evidence that supported Watson and Crick’s structure.

21. Chromosome Structure
In prokaryotes, the DNA molecule is contained in the cytoplasm and consists mainly of a ring of DNA and some proteins.
Eurkaryotic DNA is organized into individual chromosomes.
The length of a human chromosome ranges from 51 million to 245 million base pairs.
In order for the DNA to fit into the nucleus, the DNA tightly coils around a group of proteins that look like beads called histones.

22. A Closer Look

23. Questions