1. Bell Ringer!
Does the DNA of all organisms have the same structure?

2. DNA: The Genetic Material
Structure of DNA
DNA: The Genetic Material

3. Objectives Describe the three components of a nucleotide
Develop a model of the structure of a DNA molecule
Evaluate the contributions of Chargaff, Franklin, and Wilkins in helping Watson and Crick determine the double-helical structure of DNA.
Relate the role of base-pairing rules to the structure of DNA

4. Franklin
Rosalind Franklin's most famous piece of evidence is this image: Photo 51. It depicts an X-ray diffraction of DNA. Taken in 1951, the image eventually led to the conclusion that DNA was composed of a double helix. Such a structure consists of two helices (corkscrew structures) running in parallel.

5. Watson and Crick
Using stick-and-ball models, Watson and Crick tested their ideas of how the DNA molecule might fit together. Their first attempt in 1951 failed, but they kept at it. They also used information from X-ray pictures to give them ideas for the structure. Rosalind Franklin and Maurice Williams were two scientists that were experts in taking these pictures. Crick and Watson were able to gain some valuable information by studying pictures taken by Franklin and Williams.  In 1953, Crick and Watson were able to put together an accurate model of the DNA structure. The model used a twisting "double helix" shape. This model would help scientists throughout the world in learning more about genetics.

6. DNA: The Genetic Material
The spiral structure in the picture is a large organic molecule. What type of organic molecule is it?

7. DNA: The Genetic material
Each cell has its own job.
There are over 200 cell types in the body - that is 200 different jobs!
But how does each cell know what job to do ?
Our cells are told what to do by a very special molecule called DNA
Your cells' instructions are molecules of DNA

8. DNA structure DNA = Deoxyribonucleic Acid
The deoxyribo part of the name refers to the name of the sugar that is contained in DNA, deoxyribose
The nucleic acids are a type of molecule that store information
DNA is made of a very long chain of nucleotides. In fact, in you, the smallest DNA molecule has well over 20 million nucleotides

9. DNA Structure
Watson and Crick determined that a DNA molecule is a Double Helix
Double Helix- Two strands twisted around each other, like a winding staircase
Nucleotides- the subunits that make up DNA. Each nucleotide is made of three parts: a phosphate group, a five-carbon sugar molecule, and a nitrogen containing base.
The five-carbon sugar in DNA nucleotides is called Deoxyribose.

10. DNA Double Helix

11. NUCLEOTIDES Nucleotides are composed of three main parts:
a phosphate group.
a 5-carbon sugar (deoxyribose in DNA).
a nitrogen-containing base.

12. A Winding Staircase, continued
The nitrogen base in a nucleotide can be either a bulky, double-ring purine, or a smaller, single-ring pyrimidine
Pyrimidines: Cytosine and Thymine
Purines: Guanine and Cytosine

14. Nucleotides
The only difference between each nucleotide is the identity of the base.
There are only four possible bases that make up each DNA nucleotide:
adenine (A)
guanine (G)
thymine (T)
cytosine (C)
The various sequences of the four nucleotide bases make up the genetic code of your cells.

15. The Genetic Code
It may seem strange that there are only four letters in the “alphabet” of DNA.
But since your chromosomes contain millions of nucleotides, there are many, many different combinations possible with those four letters.

16. Discovering DNA’s Structure
Chargaff’s Observations
In 1949, Erwin Chargaff observed that for each organism he studied, the amount of adenine always equaled the amount of thymine (A=T)
Likewise, the amount of guanine always equaled the amount of cytosine (G=C)
However, the amount of adenine and thymine and of guanine and cytosine varied between different organisms.

17. Discovering DNA’s Structure
Pairing Between Bases
An adenine on one stand always pairs by hydrogen bonds with a thymine on the opposite strand , and a guanine on the one strand always pairs by hydrogen bonds with cytosine on the opposite strand.
These base-pairing rules are supported by Chargaff’s observations.
The strictness of base-pairing results in two strands that contain complementary base pairs.

18. DNA Structure
Double Helix Visual

19. Question to think?
How is the process through which DNA was found to be the genetic material an example of the scientific method in action? Are there aspects of this discovery that deviate from the scientific method?