8.1 - The Discovery of DNA’s Function

Objectives: Identify the 4 properties required of every genetic material, Summarize the classic experiments determining that DNA is genetic material

Classic Experiments Determining that DNA Is Genetic Material

• DNA was first described by Mischer, who determined the following:

  • DNA is not a protein

  • DNA is rich in nitrogen and phosphorus

  • Never learned DNA function

• Griffin got a hint of DNA’s function (hereditary information)

  • Studied 2 bacteria, one harmless and one harmful, and found that heat destroyed the harmful bacteria’s ability to cause disease but not its hereditary material

Oswald, Avery, MacLeod, and McCarty identified that the principle must be a nucleic acid. It used enzymes that destroyed DNA and RNA to decide which one it was, and came to the conclusion it was DNA.

• Prior to this, scientists believed that proteins controlled heredity, not nucleic acids

  • DNA was assumed to be too simple

• They published their findings and concluded that DNA must be genetic material

Four Properties Required Of Genetic Material

Their discovery prompted new scientists to do further research:

• Confirmed DNA’s ability to carry genetic information

  • Realized that any molecule had to have a certain set of properties to store hereditary information. These are the following:

1. Hereditary information must be passed on from one generation to the next

2. Cells of a certain species should contain the same amount of it

3. Not susceptible to major change

4. Must be able to encode a huge amount of information to build life

How does DNA fulfill these requirements?

• Transmits full hereditary information

• Body cells of certain species had all the same amounts of DNA

• DNA is not involved in metabolism, therefore, its content does not change over time

• DNA’s structure allows it to encode large amounts of genetic information

8.2 - Discovery of DNA’s Structure

Objectives: Summarize the events that led to the discovery of DNA’s structure, identify the subunits of DNA and how they combine into a DNA molecule, explain how DNA holds information, describe base pairing

Subunits Of DNA and How They Combine into a DNA Molecule

DNA is a polymer of 4 different nucleotides:

1. Adenine

2. Guanine

3. Thymine

4. Cytosine

Each has 3 phosphate groups, deoxyribose sugar, and a nitrogenous base.

Events that led to the Discovery of DNA’s Structure

Chargaff made 2 important discoveries about DNA’s structure:

• Amounts of thymine and adenine are identical in any DNA molecule, and Amounts of cytosine and guanine are identical in any DNA molecule

  • Chargaff’s First RUle

• DNA of different species differs in proportion of adenine and guanine

Watson and Crickhat been theorizing about DNA’s structure.

• Used an X-ray to decipher shape, size, spacing between elements

• Since DNA was large, it was hard to crystallize w/ techniques of the time

  • Identified a repeating pattern and length

Base Pairing

• Thus, DNA is characterized by the following:

  • 2 chains/strands of nucleotides in opposite directions, in a double helix

  • Covalent bonds link deoxyribose to phosphate groups, leading to sugar-phosphate backbone

  • Hydrogen bonds hold the pair of strands together

    • Only Adenine to Thymine and Cytosine to Guanine

How DNA Holds Information

DNA sequence is the order of nucleotides bases, greatly differ from organism to organism. DNA molecules can hold hundreds of millions of nucleotides, thus, a sequence of nucleotides can hold a lot of information.