DNA

Griffith and Transformation

• In 1928 British scientist Frederick Griffith was trying to figure out how bacteria made people sick.
• His main concentration was Streptococcus pneumoniae, a bacteria that caused pneumonia.

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Fig. 16-2

Living S cells (control)

Living R cells (control)

Heat-killed S cells (control)

Mixture of heat-killed S cells and living R cells

Mouse dies

Mouse dies

Mouse healthy

Mouse healthy

Living S cells

RESULTS

EXPERIMENT

Avery’s Experiment

• In 1944, a group of scientist led by Canadian Biologist Oswald Avery decided to repeat Griffith’s experiment.
• Avery took some of the heated pathogenic and added some enzymes that destroyed proteins, lipids, and carbohydrates. He then stuck in with the nonpathogenic strain and injected some mice with it. They still all died.

• Avery then did the same thing but this time added enzymes that would destroy DNA. When he did that transformation did not occur and the mice survived.

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Hershey and Chase Experiment

• In 1952 American scientist Alfred Hershey and Martha Chase, performed some experiments with viruses and bacteria.
• They were using a certain type of virus that infects bacteria (bacteriophage).

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Viruses

• A virus is made up of a protein coat and a DNA core.
• A bacteriophage will attach to a bacteria and inject its genetic material into it and the bacteria will then incorporate the viruses genetic material and make more viruses unit it dies.

Hershey & Chase

Protein coat labeled

with ³⁵S

DNA labeled with ³²P

³⁵S radioactivity

found in the medium

³²P radioactivity found�in the bacterial cells

Which radioactive marker is found inside the cell?

Which molecule carries viral genetic info?

• Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.

DNA makeup

• DNA is a long molecule make up of units called nucleotides.
• Each nucleotides are made up of three things:

1) 5 carbon sugar called deoxyribose

2) A phosphate group

3) A nitrogenous base.

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Nitrogen bases

• There are four kinds of nitrogenous bases in DNA.
• Adenine and Guanine, belong to the purine group.
• Cytosine and Thymine, belong to the pyrimidines group.
• Purines have two rings, while pyrimidines have one ring.

• The backbone of a DNA chain is formed with sugar and phosphate groups.
• The nitrogenous base sticks out in any order it wants to.

Chargaff’s Rule

• Erwin Chargaff discovered that the percentages of guanine and cytosine bases are almost equal in any sample of DNA.
• The same was true for adenine and thymine.
• A=T
• G=C

The double helix

• Francis Crick and James Watson were the first to discover that DNA is a double helix.
• DNA is a double helix, in which two strands were wound around each other.

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Base pairing

• Hydrogen bonds form between certain nitrogenous bases and provide just enough force to hold the two strands together.
• Between G and C is a triple hydrogen bond and between A and T is a double hydrogen bond.

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