1. DNA
Biology 11

2. The genetic code
The genetic code is the way in which cells store the program that they pass from one generation to the next.
So what is the genetic code made of?
DNA!

3. The genetic code
As of 1928, scientists still did not know about DNA (deoxyribonucleic acid), but that was about to change:
A British scientist by the name
of Frederick Griffith was doing
an experiment with mice and
pneumonia causing bacteria.
He made an astonishing
discovery…

4. The Genetic Code

5. The genetic code
Griffith called the process that caused the harmless bacteria into the disease-causing bacteria transformation.
Griffith hypothesized that when the live bacteria and the heat-killed bacteria were mixed together, a factor was transferred from the heat-killed cells to the live cells.

6. The genetic code
In 1944, a group of scientists in New York, led by Oswald Avery, decided to see if they could discover what molecules made up Griffith’s transforming factor.
Avery and his colleagues made
an extract from the heat-killed
bacteria, then treated it with
enzymes that destroyed lipids,
proteins and carbohydrates;
transformation still occurred!

7. The genetic code
Next, they treated it with enzymes that destroyed RNA (ribonucleic acid) and transformation occurred.
But, when they treated the extract of the heat-killed bacteria with enzymes what would break down DNA, transformation did not occur.
DNA was the transforming factor!
DNA carries the genetic code.

8. The genetic code
Because scientists are generally skeptical, many others tried experiments to show that DNA was the genetic code.
One of the most important experiments was done by Alfred Hershey and Martha Chase in
They used bacteriophages (viruses that infect bacteria) which are composed of DNA and a protein coat.
This is what happened…

9. The genetic code

10. The structure of dna
DNA is a polymer made up of units called nucleotides.
Each nucleotide has three parts.
A 5-carbon sugar called deoxyribose.
A phosphate group.
A nitrogenous base.

11. The structure of dna DNA contains four nitrogenous bases: The purines:
Adenine (A)
Guanine (G)
The pyrimidines:
Cytosine (C)
Thymine (T)

12. The structure of dna The nucleotides are joined together in a
chain with the sugar
and phosphate
groups forming the
“backbone”.

13. The structure of dna
In the 1950’s Rosalind Franklin, also British, started working with DNA. Using X-rays, she was able to determine that the fibers that make up DNA are twisted.

14. The structure of dna
At the same time as Franklin (and her colleague Maurice Wilkins), two other scientists in Cambridge were working on determining the structure of DNA; they were James Watson and Francis Crick.

15. The structure of dna
Watson and Crick had been working on 3-D models of DNA, and after seeing Franklin’s X- ray image, they realized that they needed to twist their models and found that DNA forms a double-helix.

16. The structure of dna
Watson and Crick then figured out complimentary base pairing.
They knew that a guy named Chargaff had observed that in any sample of DNA, the number of thymine molecules was equal to the number of adenine molecules, and the same was true for cytosine and guanine.
So this was the final result:

17. The structure of dna