DNA

The Hereditary Material

The Discovery of DNA

• 1869 – Friedrich Miescher investigated chemical composition of DNA using pus cells.

• At the time, proteins were thought to be hereditary material.

• Miescher discovered the nuclei of cells contained a substance that does not act like protein. He called this substance nuclein.

• This caused debate – Is protein or nuclein the hereditary material?

Location of Hereditary Information

• 1930s Hammerling Experiment

• Experiment using Acetabularia, one-celled green alga

• Hypothesized that hereditary material was found in the foot and possibly the nucleus of the Acetabularia.

Hammerling’s Experiment

• Acetabularia Experiment• Observations:

– Caps that were removed regrew

– Feet that were removed, did not regrow

– Caps that grew matched the species of the foot not the stalk.

• Conclusions:– Hereditary information was found in the nucleus in the

foot of the cell.

The Transforming Principle

• The nucleus contains the genetic information but scientists were still uncertain whether the chromosomes consisted of both proteins and DNA.

• 1920s - Frederick Griffith was trying to develop a vaccine against pneumonia. He obtained unexpected experimental observations that led him to believe that protein is not the hereditary material.

• Griffith Experiment

•Some substance (the Transforming Principle) must have been transferred from the dead bacteria to the living non-virulent bacteria making it virulent

•1944 – Avery, McCarty, MacLeod later showed that the Transforming Principle was DNA

Hershey and Chase Experiment

• 1952 – Hershey and Chase conducted experiments using bacteriophage T2 that infects a bacterial host.

• View experiment

Conclusion:

DNA, not protein is the hereditary material.

DNA Structure

• 1860s - Miesher discovered that nuclein is slightly acidic and composed of large amounts of P and N.

• 1920s – Determined that DNA comprises a deoxyribose sugar, a phosphate, and a nitrogen base

DNA Structure Animation

DNA Structure Contributors

• Chargaff – adenine = thymine, cytosine = guanine

• Franklin – X-ray diffraction pattern suggesting a double helix shape

• Watson and Crick – Deduced structure of DNA using work of Chargaff, Franklin and Wilkins

DNA Replication and Repair

DNA Replicates semi-conservatively

Meselson and Stahl

• 1958 - Meselson and Stahl devised an experiment to suggest that DNA replication is semi-conservative.

• Meselson and Stahl Experiment

• Experiment 2

DNA Replication and Repair

DNA Replication is a process comprising of:1. Separating DNA strands

2. Building Complementary Strands

3. Quality Control

DNA Replication Animation