DNA: Deoxyribonucleic Acid

I. Molecule for HeredityA. Scientists needed a molecule that could carry 3

main functions:1. Carry genes from generation to generation2. Had to put to work information to determine heritable

characteristics3. Easily copied

II. DNA ExperimentsA. To understand genetics biologists had to discover

the chemical nature of the gene and how exactly this molecule works.

B. Several experiments (scientists listed)were carried out to better understand the molecule of heredity:1. Griffith2. Avery3. Hershey-Chase4. Franklin5. Watson & Crick

1. Griffith’s Experimentsa. British scientist in 1927

was trying to figure out how bacteria made people sick (i.e. pneumonia)

b. Griffith isolated 2 strains of bacteria (Diplococcus pneumoniae)1. smooth – disease causing &

2. rough – harmless bact.

c. Injecting mice with both strains he observed:1. smooth bacteria killed mice while rough harmless

bacteria did not do anything2. when heating the smooth bacteria and then injecting

mice the mice survived3. Mixing heat-killed smooth bacteria with live harmless

bacteria, all mice died

d. The harmless strain changed permanently to the disease causing strain due to transformation (a factor was transformed from one cell to another)1. that factor must be a gene

2. Avery & DNA : What’s responsible?

a. Replicated Griffith’s work in 1944 to try to determine which molecule in the heat-killed bacteria was the most important for transformation

b. They carefully treated the heat-killed bacteria with enzymes that destroyed proteins, lipids, carbohydrates, and RNA, the bacteria was still transformed

c. Only when DNA was destroyed did transformation not occur

d. Therefore they concluded DNA is the factor that is transformed from one generation to the next

3. Hershey-Chase : DNA or Protein??

a. Wanted to ensure that it is DNA & not protein that is the key genetic factor that is being passed down from generation to generation

b. Tagging bacteriaphages (viruses that infect bacteria) with radioactive tags would determine what the chemical nature of a gene truly is.1. DNA – contains no sulfur 2. Protein – contains no phosphorous

• c. Which radioactive tag was found to have entered the bacteria after the viruses infected them; tags for phosphorous were inside the bacteria meaning DNA is responsible for genetic material (1952)

4. Franklina. Used X-ray crystallography to identify the

structure of the DNA molecule.b. the X-pattern in the photograph showed

that the DNA molecule was twisted in a shape known as a helix

5. Watson & Crick1953

a. Using Franklin’s x-rays Watson & Crick were able to make a 3D model of the double helix model DNA1. Double helix looks like a twisted ladder

b. With further testing they were able to identify that hydrogen bonds were responsible for holding DNA together.

II. Components of DNA

A. DNA is a large molecule made up of units known as nucleotides.

B. Each Nucleotides is composed of 3 parts:1. 5 carbon sugar deoxyribose (bonds side of “ladder”)2. Phosphate group (bonds side of “ladder”)3. Nitrogenous base (“ladder rungs”)

C. There are 4 bases:1. Purines (2 rings): Adenine & Guanine2. Pyrimidines (1 ring): Cytosine & Thymine

III. Chargaff’s Rule

A. Erwin Chargaff had discovered that the percentages of Guanine (G) & Cytosine (C) were almost equal in any given sample of DNA; the same thins was true for Adenine (A) & Thymine (T)In other words G = C and A = T Guanine always bonds with cytosine and adenine always

bonds with thymine.