Bio Unit 8 - DNA

• DNA is the same in all organisms

DNA Discovery

• 1869 - Friedrich Miescher

  • studied Leukocytes - white blood cells

  • discovery named “Nuclein” —> Deoxyribonucleic Acid (DNA)

Frederick Griffith’s Experiment

• Streptococcus Pneumoniae bacteria

  • goal: create pneumonia vaccine

• Two strains of bacteria:

  • R Strain - Nonvirulent: rough appearance in dish

    • mouse lives

  • S Strain - Virulent: smooth appearance in dish

    • Smooth capsule [coating] protects bacteria from immune systems

    • mouse died

• 2 Trials:

  • Heat (killed smooth strain bacteria)

    • mice lives, no disease

  • Mixed Rough strain with heat-killed Smooth strain

    • killed mice

    • living smooth strain bacteria in mouse

• Conclusion: Rough strain bacteria took “Transforming Principle” from heat-killed Smooth bacteria = Virulents

• Breakdown:

  1. (deadly) Smooth strain bacteria killed by excess heat

  2. Cell membrane ruptures = DNA released

  3. (harmless) Rough strain makes up deadly DNA

  4. DNA incorporated into genome

     Genome: entire set of DNA instructions in cell

  5. Transforms into deadly strain

• debate over what was transforming the material

Oswald Avery’s Experiment

• 1944 - Avery repeats Griffith’s experiment with 2 added enzymes

  1. Added Enzymes that destroyed carbohydrates, lipids, and proteins to heat-killed Smooth strain

  • transformation still occured

    1. Added Enzymes that destroyed nucleic acids to heat-killed Smooth strain

  • transformation didn’t occur

• Conclusion: DNA = the genetic material

Hershey-Chase Experiment

• 1952 - Alfred Hershey and Martha Chase

  • used Bacteriophage to prove if DNA or proteins = the genetic materials

    • Bacteriophage: viruses that infect bacteria and use the host cell to replicate itself

    • Protein Coat + DNA

• Experiment:

  • protein coat - red radioactive sulfur

  • virus DNA - blue radioactive phosphorus

• Results:

  • infected cells only contained the blue radioactive phosphorus (DNA)

• Conclusion:

  • Virus injects DNA to replicate

  • Avery results confirmed

Nucleotide Structure

• 1929 - Phoebus Levene discovers Deoxyribose

  • first to describe nucleotide structure

  • Each nucleotide contains:

    • Phosphate Group:

    • Deoxyribose Sugar: 5 -carbon sugar

    • Nitrogenous Base: make up ladder rungs

      • Adenine (A)

      • Guanine (G)

      • Cytosine (C)

      • Thymine (T)

    • Purines - Double-ring nitrogenous bases

      • Adenine

      • Guanine

    • Pyrimidines - Single-ring nitrogenous bases

      • Cytosine

      • Thymine

      • Uracil (RNA only)

    • proposed Tetranucleotide Hypothesis (wrong)

      • DNA contains equal amounts of each nitrogenous base

      • 4 Nucleotides bonded together

• Base Pairing

  • 1949 - Erwin Chargaff analyzed percentages of each base in DNA

  • A = T

  • C = G

  • Hydrogen bonds hold bases together (weak)

    • 3 bonds between G & C

    • 2 bonds between A & T

Rosalind Franklin

• 1952 - 1st photographed 3D structure of DNA using x-ray diffraction

  • Photo 51

  • James Watson and Francis Crick

    • 1953 - stole Franklin’s photo to build the 1st correct 3D model of DNA

      • DNA = double stranded double helix

    • got credit for it

DNA Structure

• 2 Repeating Strands of Nucleotides

  • strands supported by a Repeating Phosphate-sugar backbone

  • bases paired in the middle (rungs)

• Antiparallel - strands run in opposite directions

  • 5’ to 3’ and 3’ to 5’

• # of carbon atoms in deoxyribose = strand running direction

  • count clockwise from carbon after central oxygen

    • oxygen in central ring

  • 5’ : free phosphate group (circle)

  • 3’ : Un-bonded sugar

Function of DNA

1. stores genetic information

2. copies itself (same DNA of same cells)

3. can express genetic information

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Nucleotide

DNA Replication

• process where the cell copies itself

  • each new cell has an identical copy

  • cells must divide because large cells don’t have enough DNA to give instructions to the cell

    • hereditary information is contained in Genes

  • Parent Strand —> 2 Replicated Strands

• Semiconservative Replication

  • 2 Replicated Strands: Each with 1 old strand and 1 new strand

  • old strand = template for new strand to be created

• Origin of Replication

  • Eukaryotic cells have multiple origins (speeds up)

  • Replication Bubble (formed here)

    • site of replication

      • Replication forks

        • at either ends of bubble

        • move in opposite directions —> bubble grows with forks

        • continues until DNA is copied

• Replication Enzymes

  • Topoisomerase: unwinds DNA double helix

    • stops DNA from snapping

  • Helicase: unzips DNA strands by breaking hydrogen bonds between nitrogenous bases

  • Single Strand Binding Proteins (SSBs): keep the 2 seperated strands apart

    • (DNA naturally wants to rejoin)

  • Parent Strand: original DNA strand/template for new strand

  • DNA Polymerase: adds the DNA Nucleotides to the new strand

    • adds complementary bases together

    • only adds nucleotides from 5’ to 3’ direction

      • creates leading and lagging strands

      • Leading Strand: made continuously towards replication fork (5’ to 3’)

      • Lagging Strand: made in fragments away from replication fork

        • Okazaki Fragments: small segments of DNA created through lagging strand

          • DNA Ligase: Bonds Okazaki Fragments together to make 1 continuous DNA strand

    • DNA Primase: adds a short strand of RNA

    • RNA Primer: shows DNA Polymerase where to start adding nucleotides

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