DNADNA

Discovery and StructureDiscovery and Structure

What is DNA?

• Deoxyribonucleic acid

• DNA is the molecule of genes.

• It is found in the chromosomes.

• It holds the instructions to making all the proteins in an organism.

• Proteins make up the structures of cells.

• Proteins, in the form of enzymes, also control all cell processes.

• DNA carries the instructions from mother to daughter cells, from parents to children. Therefore it is the molecule of heredity.

mitosis sexual reproduction

Who discovered DNA?• Deoxyribonucleic acid (DNA) was first isolated in

1869 by the Swiss scientist Friedrich Miescher. He called the white, slightly acidic chemical that he found in cells "nuclein." He identified DNA as part of a chromosome in 1871.

Did DNA or Protein makeup a molecule of heredity?

By the 1920s it was thought that genes were made of protein, the other main ingredient of the chromosome. Many scientists at the time thought DNA was too simple to carry genetic information. ---They were wrong! thought that proteins were molecules of heredity.

How did scientists discover that DNA was the hereditary molecule?

Scientists such as the Griffith’s bacteria experiment in 1928, and Avery’s bacteria experiment in 1944, pointed toward DNA.

The famous Hershey-Chase experiment finally ended the debate.

Oswald Avery

Hershey-Chase: The “Waring Blender” experiment – 1952.

Alfred Hershey and Martha Chase

• They experimented with bacteriophages, viruses that infect bacterial cells.

• These viruses are basically composed of protein surrounding DNA.

• Bacteriophage viruses force bacteria cells to make new virus particles.

• The experiment would see if the virus’ proteins or the virus’ DNA caused the bacteria to produce the new virus particles.

• Hershey and Chase grew the viruses in a mixture of radioactive isotopes of elements found mainly in either the DNA (phosphorus-32) or in the Protein (sulfur-35).

• The viruses were mixed with bacteria. After a few minutes, the viruses were separated from the bacteria.

• They used a blender to do this which is why it is sometimes referred to as the “Waring Blender Experiment.”

The bacteria were then tested for phosphorus-32 (32P) and sulfur-35 (35S).

Nearly all the radioactivity in the bacteria was phosphorus-32 (32P), the isotope that marked the DNA!

Conclusion: The genetic material of the bacterophage viruses was not the protein but DNA.

DNA is the molecule of heredity!

Rosalind Franklin and Maurice Wilkins examined DNA in the laboratory to determine its structure.

X-Ray Diffraction (X-Ray Crystallography) photograph of DNA, taken by Rosalind Franklin was very important to the scientists who eventually discovered the structure of DNA.

“photo 51”

B form of DNA from Calf Thymus Gland

DNA structure was discovered by James Watson and Francis

Crick in 1953.

ORIGINAL DNA MODEL BUILT BY WATSON & CRICK

Watson & Crick in their Cambridge office c. early1950s

Watson, Crick and Wilkins shared the Nobel Prize in Physiology or Medicine in1962.

Unfortunately, Franklin had died of breast cancer prior to the award.

Rosalind Franklin, seen here serving coffee in evaporating dishes at her Parisian laboratory in the late 1940s, went on to publish an impressive array of scientific papers in the decade she had remaining to her.

MMMMM! Evaporating Dish Coffee!

What is DNA’s Structure?What is DNA’s Structure?• DNA is a biological polymer made up of

subunits called nucleotides.

• DNA Nucleotides are made up of:

a) 5-carbon deoxyribose sugar

b) phosphate group

c) nitrogen-containing base

By the way ---the sugar and base alone make up a nucleoside

There are two classes of nitrogen bases:

• Purines (double ring structure)

adenine (A) guanine (G)

• Pyrimidines (single ring structure) cytosine (C) thymine (T)

Mnemonic Device

• All that is Good is Pure.

• C (see) The pyramids.

Adenine Guanine purines

Cytosine Thymine pyrimidines

Another discovery that helped Watson & Crick

Erwin Chargaff - demonstrated that guanine and cytosine are always in equal amounts (as well as thymine & adenine)

Rosalind Franklin’s X-Ray evidence showed, among many other things, that DNA has the shape of a double helix or “twisted ladder.”

Summary of DNA’s Structure:1) Alternating sugars & phosphates on the

sides of the ladder. (Covalently-bonded)

2) Nitrogen bases in the rungs

A-T , G-C (base-paring)

Note: A & T are not labeled correctly in the animation

Mnemonic Device

AT the Grand Canyon A to T G to C

3) Weak bonds between bases (Hydrogen bonds)