The Discovery of DNA

What is DNA?

• DNA is a double stranded polymer of nucleotides (each consisting of a deoxyribose sugar, a phosphate, and four nitrogenous bases)
• Contains the genetic information of an organism
• Currently, we are discovering information about DNA and the role it can play in our every day lives
• But, this would not be possible without the historical discoveries made by scientists
• They discovered the role DNA plays in living things

In the Beginning.....

• By the start of the 1900s, the connection of chromosomes and the inheritance of specific traits had already been established
• Therefore the hereditary material was found in chromosomes
• It needed to meet the following criteria
• Contain material that controls the production of enzymes and other proteins
• Be able to replicate itself with great accuracy to maintain continuity
• Allow for occasional mutations as a means of introducing variation in species
• Chromosomes contained both proteins and nucleic acids
• Proteins were composed of 20 monomers that could be combined in endless ways to produce thousands of different proteins
• Nucleic acids were composed of only four bases and had very limited potential for combinations

The Discovery of DNA

• Friedrich Miescher (1869)
• Swiss biochemist that investigated the chemical composition of DNA using pus cells
• He discovered that the nuclei contained a large quantity of a substance that did not behave like protein
• At this time in history it was believed that protein was the hereditary material
• He called this substance “nuclein” as it was predominantly found in the nucleus
• Upon the discovery of the chemical composition in the 1920s it is now known as deoxyribonucleic acid (DNA)

Location of Hereditary Information

• Joachim Hammerling (1930s) was a Danish biologist that conducted an experiment with green algae, a single celled organisms, to determine the hereditary information
• Acetabularia was ideal because it contained three distinct regions
• A foot that contained the nucleus
• A stalk
• A cap
• He removed caps from some plants and foots from others and observed the regeneration

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• Found that those cells whose caps were removed were able to regenerate new caps while those whose foot was removed were unable to regenerate

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• He hypothesized that the hereditary information was found in the foot and possibly within the nucleus

Further Research

• To further his research he conducted another experiment with two species of Acetabularia which had different caps
• A. Mediterranea had a disk-shaped cap and A.crenulata had a flowerlike cap
• He grafted a stalk of A. crenulata with a foot of A. mediterranea
• The first cap that grew had a combination of traits from both plants
• He removed this cap and the one that formed was an A. mediterranea cap
• He concluded that the instructions for specifying the type of cap and for directing cellular substances to build a new cap are contained in the nucleus

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Identifying DNA as the Material of Hereditary

• They knew that the nucleus contained the hereditary material and that chromosomes were responsible for inheritance but there were unsure of the material
• Both protein and DNA are found in chromosomes
• Frederick Griffith’s work began in the 1920s in his attempt to find a vaccine against pneumonia caused by Streptococcus pneumoniae
• However his work led scientists to suspect that protein is not the hereditary material
• He discovered the process of transformation

Evidence of Hereditary Material Activity�Turn to Page 226 �Answer Observation and Analysis Questions

The Discovery of Transformation

DNA was the Transformation Material

• In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty published a study that supported the hypothesis that DNA was the hereditary material
• In their experiment they
• Prepared identical extracts of heat-killed S-strain bacteria
• They did this by growing cells, isolating the bacteria, disrupting the cell membranes and collecting the contents
• They then added 1 of 3 enzymes to the extract
• 1) Enzyme Destroyed Proteins
• 2) Enzyme Destroyed RNA
• 3) Enzyme Destroyed DNA

• They then mixed each extract with live R-strain bacteria
• They found that they only extract that did not cause transformation was the one treated with the DNA-destroying enzyme
• Therefore, they concluded that DNA was the transforming principle found in Griffiths experiment

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• However, scientists were still not convinced DNA was the hereditary material

DNA is the Hereditary Material

• In 1952, American microbiologist team of Alfred Hershey and Martha Chase designed an experiment to determine if DNA was the hereditary material
• Used bacteriophages that have an inner core of nucleic acid and an outer protein coat known as a capsid
• They used T2 bacteriophage strain of virus that contain a protein coat surrounding a piece of DNA
• When viruses infect a bacterial cell they attach to the outside and inject their genetic material
• Infected cell then manufactures new virus particles using the viruses genetic information
• Cell bursts and releases other viruses to infect other bacterial cells

Hershey and Chase Experiment

• They wanted to determine if it was the DNA core or the capsid that enters and produces new viruses
• Used radioactive isotopes to trace each type of molecule
• Protein contains sulfur and DNA does not so they used a radioactive sources of sulfur to label the protein capsid
• DNA contains phosphorus and T2 protein does not so they used a radioactive source of phosphorus to label the DNA

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• Since the viral DNA was transferred to the bacterial cells and that viral DNA held the genetic information needed for the viruses to reproduce it was concluded that DNA is the hereditary material