Chapter 7

Chapter 7

Nucleic Acids and Protein


Chapter 7

7-1 DNA

• The Genetic Code

▫ Various Scientists contributed to the idea that

DNA carries the genetic code.

▫ The genetic code is the way in which cells store the

information needed to pass onto the next


Science History

• 1928 – Frederick Griffiths - Transformation

▫ Bacterial causes for disease

Used mice to see affect

Smooth colonies causes diseases

Rough colonies don’t

▫ Used heat to kill the bacteria then injected them

No disease

Mixed live and heat-killed bacteria

1944 – Rockefeller Institute

• Avery, McCarty and Macleod

▫ Used enzymes to destroy lipids, proteins and


▫ Transformation still occurred

▫ Repeated the experiment with enzyme that

destroys RNA

▫ Transformation still occurred

▫ Repeated with enzyme that destroyed DNA

▫ No transformation

1952 – Hershey and Chase

• Used viruses called bacteriophages that infect


▫ Bacteriophages [bacteria eaters]

Structure – DNA core and a protein coat

Attach themselves to the surface of a bacterium then

inject a material into the cell

Material begins replicating itself until cell is full and


How a bacteriophage works

Radioactive isotopes

• Hershey and Chase used two different

radioactive isotopes to “label” each part

▫ Phosphorous – 32 on DNA

▫ Sulfur – 35 on protein coat

• Mixed labeled viruses with bacteria and

discovered: Phosphorous-32 inside the cell

Structure of DNA

• Polymer formed from nucleotides

▫ Nucleotides are made up of three parts

5-carbon sugar called deoxyribose

A phosphate group

Nitrogenous base

▫ Only four bases found in DNA

Adenine and Guanine form purines

Cytosine and Thymine form pyrimidines

1950’s Rosalind Franklin and Wilkins

• Shot X-rays at long strands of DNA

• Showed that DNA fibers are twisted

• Spaced out in long regular intervals

• Not enough to determine the structure but

important clues!

1950’s Watson and Crick

• Tried building 3-D models

• Visited Franklin and observed her X-ray


• Began twisting their models to try and form

similar pattern to Franklins photograph


• Observed that the number of adenine molecules

were equal to the thymine molecules

• Same was true of cytosine and guanine

• This attraction is known as “base pairing”


• Process of DNA copying itself before the cell can


• Only adenine attaches to thymine

• Only cytosine attaches to guanine

• DNA “unzips” so the exact same molecule is

produced when a new DNA strand is made

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