Discovery and Structure DNA Discovery and Structure

Deoxyribonucleic acid 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.

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 and that proteins were molecules of heredity. ---They were wrong!

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.

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. 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. DNA structure was discovered by James Watson and Francis Crick in 1953. “photo 51”

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: Note: A & T are not labeled correctly in the animation Summary of DNA’s Structure: Alternating sugars & phosphates on the sides of the ladder. (Covalently-bonded) 2) Nitrogen bases in the rungs A-T , G-C (base-paring) 3) Weak bonds between bases (Hydrogen bonds)