Chapter 12: DNA and RNA

• Genes are made of DNA, a large, complex molecule. DNA is composed of individual units called nucleotides. Three of these units form a code. The order, or sequence, of a code and the type of code determine the meaning of the message.

1. On a sheet of paper, write the word cats. List the letters or units that make up the word cats.

2. Try rearranging the units to form other words. Remember that each new word can have only three units. Write each word on your paper, and then add a definition for each word.

3. Did any of the codes you formed have the same meaning?4. How do you think changing the order of the nucleotides in

the DNA codon changes the codon’s message?

12-1 DNA

Once it was known that traits were inherited the

next question was:HOW?

What molecule holds the genetic information?

Discovering the function and structure of DNA

Frederick Griffith, 1928

Oswald Avery, 1944

Alfred Hershey and Martha Chase, 1952

Erwin Chargaff

Rosalind Franklin, 1950’s

James Watson and Francis Crick, 1953

Griffith’s Experiment

• Rough Pneumococcus are harmless. They lack a gel capsule that would protect them from a host organism's immune system attack.

• Smooth Pneumococcus are pathogenic (they cause disease), and when injected, give a mouse fatal pneumonia.

• Griffiths injected several combinations of rough and smooth into hapless mice, and found...

Results of Experiment

– live rough --> mice okay – live smooth --> mice pushing up daisies – killed (boiled) rough --> mice okay – killed (boiled) smooth --> mice okay – live smooth + killed rough --> mice kick

the bucket – live rough + killed smooth --> MICE

CROAK! This was a surprise!

The Puzzle• When Griffiths autopsied the mice, he

found LIVE SMOOTH PNEUMOCOCCUS!!

• The heat killed bacteria had passed their disease-causing ability to the harmless rough bacteria.

• He called this transformation:

harmless, live rough bacteria TRANSFORMED into deadly smooth bacteria. But how?!

Conclusions

• The factor passed from the heat-killed smooth bacteria to the live rough bacteria must contain information to transform harmless bacteria into deadly bacteria.

• Perhaps this transforming factor was a gene!

Avery’s Experiment

• Oswald Avery (1944) –Repeated Griffith’s experiment in an

attempt to find molecule responsible for transformation

–Was it a Protein? Lipid? Carbohydrate? Nucleic acid?

Treated with enzymes toDestroy DNA

Treated with enzymes to destroy carbs, lipids,

protein and RNA

Heat-killed smooth bacteria mixture

Mouse Dies Mouse lives!

CONCLUSION: DNA STORES AND TRANSMITS GENETIC INFO.

The Hershey and Chase Experiment

• Used more advanced science to see if Avery was right about DNA

• Worked with bacteriophages (Viruses that attack bacteria)

• Side by side experiments were performed to see if it was the protein or the DNA of the virus that infected the host

• Conclusion: AVERY WAS RIGHT!

The DNA was passed into the host, not the protein coat.

The components and structure of DNA

Once Scientists agreed that it was definitely DNA that contained and passed on the genetic information the next question

was...What is DNA?

What does it look like?What is it made of? How does it work?

• DNA is composed of small units called nucleotides

• There are four different kinds: Adenine, Guanine, Cytosine and Thymine

• Chargaff realized that in DNA the # of Adenine was always similar to the number of Thymine. The # of Cytosine was always similar to the number of Guanine...

Chargaff’s Rule

• A always bonds to T

• C always bonds to G

Rosalind Franklin

• Franklin and Maurice Wilkins at King's College, (London, England) performed X-ray diffraction studies

• Franklin noticed a diffraction pattern in DNA that showed that DNA was 2-stranded and a helix-shape

Watson and Crick

• Used Franklin’s X-ray image to build a model of the DNA structure

• The Double helix shape of DNA explained how it could carry information and how that information could be copied