Chapter 12�DNA Structure and Function

DNA Scientist

• Johann Friedrich Miescher discovered nucleic acid in 1868
• Linus Pauling discovered the helical structure of proteins in 1951
• 1953 Watson and Crick discovered the structure of the master molecule of life– DNA

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Fred Griffith

• 1928 working with the pneumonia-causing bacterium (S strain - pathogenic and R strain – nonpathogenic)
• Found that nucleic acid was the causes the person to be sick
• Performed four experiments:
• Injected mice with R Cells – mice lived
• Injected mice with S Cells – mice died
• S Cells were heat-killed then injected then into mice; live S cells were found in the blood
• Griffith explanation – heated S strain did not have hereditary information it was destroyed
• Transformation: process of changing the genetic material from one source to another

Conclusion

• Living bacteria acquired genetic information from dead bacteria - particularly the instructions for making capsules, thus transforming the naked bacteria into incapsulated bacteria.
• The Transforming agent was discovered to be DNA. DNA was isolated and added to live naked bacteria, and they were transformed into the incapsulated kind.

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Oswald Avery

• Took Griffith experiment one step further and found that both proteins and nucleic acid was DNA

Hershey and Chase

• Used the bacteriophage (a virus) to show that viruses are composed of DNA or RNA
• 1. Hershey and Chase forced one population of phages to synthesize DNA using radioactive phosphorous.
• 2. The radioactive phosphorous "labeled" the DNA.
• 3. They forced another group of phages to synthesize protein using radioactive sulfur.
• 4. The radioactive sulfur "labeled" the protein.
• 5. Bacteria infected by phages containing radioactive protein did not show any radioactivity.
• 6. Bacteria infected by phages containing radioactive DNA became radioactive.
• 7. This showed that it was the DNA, not the protein, that was the molecule of heredity.

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Experiment A

Experiment B

DNA contains phosphorous but not sulfur.

Proteins contain sulfur but not phosphorus

DNA Structure

• DNA is composed of four kinds of nucleotides which consist of:
• Five carbon sugar– deoxyribose
• Phosphate group
• One of four bases: adenine (A), guanine (G), Thymine (T) and Cytosine (C)
• Nucleotides are similar, but thymine and cytosine are single-ring pyrimidines; A and G are double ring purines

Chargaff

• In 1949 found that the four kinds of nucleotide bases making up DNA molecule differ in relative amounts from species and species
• Adenine =Thymine

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• Cytosine=Guanine

Rosalind Franklin

• Used X-ray diffraction techniques to produce images of DNA molecules
• DNA exist as a long, thin, molecule of uniform diameter
• Structure is highly reptitive
• DNA is helical (twisted ladder)

Watson and Crick

• Used numerous sources of data to build models of DNA
• Following features were
• Single-ringed thymine was hydrogen bond with double ringed adenine and single-ringed cytosine with double ringed guanine, along the entire length of the molecule
• Backbone was made of chains of sugar-phosphate linkages
• The molecule was double stranded and looked like a ladder with a twist to form a double helix

DNA Structure

• The sugar and phosphates make up the "backbone" of the DNA molecule.
• The phosphate is attached to the 5' carbon (the 5 is a number given to sugar molecules). The DNA strand has a free phosphate on th 5' end, and a free sugar on the 3' end - these numbers will become important later.

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Continue…

• DNA is composed of subunits called nucleotides, strung together in a long chain -- Each nucleotide consists of: a phosphate, a sugar (deoxyribose), and a base
• The two sides of the helix are held together by Hydrogen bonds

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Hydrogen

Bond

Nitrogen Bases

DNA is composed of subunits called nucleotides, strung together in a long chain -- Each nucleotide consists of: a phosphate, a sugar (deoxyribose), and a base

Bases come in two types: pyrimidines (cytosine and thymine) and purines (guanine and adenine)

DNA

DNA Replication and Repair

• Steps of DNA replication
• 1. DNA helicase (enzyme) unwinds the DNA. The junction between the unwound part and the open part is called a replication fork.
• 2. DNA polymerase adds the complementary nucleotides and binds the sugars and phosphates. DNA polymerase travels from the 3' to the 5' end.
• 3. DNA polymerase adds complementary nucleotides on the other side of the ladder. Traveling in the opposite direction.

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• 4. One side is the leading strand - it follows the helicase as it unwinds.
• 5. The other side is the lagging strand - its moving away from the helicase
• Problem: it reaches the replication fork, but the helicase is moving in the opposite direction. It stops, and another polymerase binds farther down the chain.
• This process creates several fragments, called Okazaki Fragments, that are bound together by DNA ligase.

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• 6. During replication, there are many points along the DNA that are synthesized at the same time (multiple replication forks). It would take forever to go from one end to the other, it is more efficient to open up several points at one time.
• Hyperlink\DNAReplication.swf

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REPLICATION IS CALLED SEMI-CONSERVATIVE, BECAUSE ONE HALF OF THE ORIGINAL STRAND IS ALWAYS SAVED, OR "CONSERVED"

Monitoring and Fixing the DNA

• DNA polymerase, DNA ligases and other enzymes engage in DNA repair
• DNA polymerase “proofread” the new bases for mismatched pairs, which are replaced with correct bases