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The Secret Code

DNA. The Secret Code. Genes. Genes are known to: Carry information from one generation to the next. Put that information to work by determining the heritable characteristics of organisms. Be easily copied, because all of a cell ’ s genetic information is replicated every time a cell divides.

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The Secret Code

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  1. DNA The Secret Code

  2. Genes • Genes are known to: • Carry information from one generation to the next. • Put that information to work by determining the heritable characteristics of organisms. • Be easily copied, because all of a cell’s genetic information is replicated every time a cell divides.

  3. Scientists call this the: DNA DNA Central Dogma of Molecular Biology! RNA RNA Protein Protein

  4. How do we know that all of our genetic information comes from DNA? Thanks to many scientists and many experiments over the last ≈ 80 years. Let’s take a look…….

  5. Griffith’s Experiment with Pneumonia in 1928 and the accidental discovery of Transformation • Frederick Griffiths was a bacteriologist studying pneumonia • He discovered two types of bacteria: • Smooth colonies • Rough colonies CONCLUSION: The smooth colonies must carry the disease!

  6. When heat was applied to the deadly smooth type… And injected into a mouse… The mouse lived! Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation

  7. Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation • Griffith injected the heat-killed type and the non-deadly rough type of bacteria. • The bacteria “transformed” itself from the heated non-deadly type to the deadly type.

  8. In 1944: Avery, McCarty, and MacLeodRepeated Griffith’s Experiment Oswald Avery Maclyn McCarty Colin MacLeod

  9. Avery, McCarty, and MacLeodAdded the non-deadly Rough Type of Bacteria to the Heat-Killed Smooth Type To the mixture, they added enzymes that destroyed… Carbohydrates Lipids Proteins RNA DNA

  10. S-Type Carbohydrates Destroyed S-Type Lipids Destroyed S-Type Proteins Destroyed S-Type RNA Destroyed S-Type DNA Destroyed Conclusion: DNA was the transforming factor!

  11. The Hershey-Chase Experiment: 1952 Protein coat Alfred Hershey & Martha Chase worked with a bacteriophage: A virus that invades bacteria. It consists of a DNA core and a protein coat DNA

  12. Protein coats of bacteriophages labeled with Sulfur-35 Phage • Hershey and Chase mixed the radioactively-labeled viruses with the bacteria Bacterium Phage The viruses infect the bacterial cells. Bacterium DNA of bacteriophages labeled with Phosphorus-32

  13. Protein coats of bacteriophages labeled with Sulfur-35 • Separated the viruses from the bacteria by agitating the virus-bacteria mixture in a blender DNA of bacteriophages labeled with Phosphorus-32

  14. Protein coats of bacteriophages labeled with Sulfur-35 • Centrifuged the mixture so that the bacteria would form a pellet at the bottom of the test tube • Measured the radioactivity in the pellet and in the liquid DNA of bacteriophages labeled with Phosphorus-32

  15. The Hershey-Chase results reinforced the Avery, McCarty, and MacLeod conclusion: DNA carries the genetic code! However, there were still important details to uncover…

  16. How did DNA:1. Store information?2. Duplicate itself easily? These questions would be answered by discovering DNA’s structure

  17. The Race to Discover DNA’s Structure Why do you think the bases match up this way? 1950 Chargaff’s Rule: Equal amounts of Adenine and Thymine, and equal amounts of Guanine and Cytosine Purine + Purine = Too wide Pyrimidine + Pyrimidine = Too Narrow Erwin Chargaff Purine + Pyrimidine = Perfect Fit from X-ray data

  18. The Race to Discover DNA’s Structure X-Ray diffraction image of DNA taken by Franklin in 1951 Maurice Wilkins Rosalind Franklin

  19. The Race to Discover DNA’s Structure 1953 Compiled data from previous scientists to build a double-helical model of DNA James Watson Francis Crick

  20. DNA • Deoxyribonucleic Acid (polymer) • A long molecule made up of units (monomers) called nucleotides. • Three components: • 5-carbon sugar: Deoxyribose • A phosphate group • A nitrogenous base Nucleotide

  21. DNA Structure • Called a double helix • Backbone (sides) : 5-carbon sugar and phosphate groups • Rungs: nitrogenous bases held together with hydrogen bonds

  22. Nitrogenous bases • Our alphabet has 26 letters • Can create many different words  many different sentences  billions of different books of information. • DNA alphabet has 4 letters • A, T, C, and G • Create 3 letter words  Amino acids  proteins  billions of different organisms

  23. Nitrogenous bases • Purines: • Adenine • Guanine • Pyrimidines: • Thymine • Cytosine

  24. DNA Replication • The double helix did explain how DNA copies itself • We will study this process, DNA replication, in more detail

  25. How does DNA replicate? Hypotheses: Conservative Semi-Conservative Dispersive

  26. Meselson-Stahl Experiment • Bacteria cultured in medium containing a heavy isotope of Nitrogen (15N)

  27. Meselson-Stahl Experiment • Bacteria transferred to a medium containing elemental Nitrogen (14N)

  28. Meselson-Stahl Experiment • DNA sample centrifuged after First replication

  29. Meselson-Stahl Experiment • DNA sample centrifuged after Second replication

  30. DNA Replication The “parent” molecule has two complementary strands of DNA. Each is base paired by hydrogen bonding with its specific partner (Chargaff’s rule): A with T G with C

  31. DNA Unzips The first step in replication is the separation of the two strands. An enzyme called DNA helicase unzips DNA

  32. New nucleotides added Each parental strand now serves as a template that determines the order of the bases along a new complementary strand. an enzyme called DNA polymerase adds the bases

  33. Gaps are closed The nucleotides are connected to form the sugar-phosphate backbones of the new strands. DNA ligase Each “daughter” DNA molecule consists of one parental strand and one new strand….semi-conservative

  34. Reviewing DNA Replication • And One more….. • And another…

  35. Here are some interesting Facts! • A single strand of DNA (one chromosome) is about 2 inches long when uncoiled. • Each human cell contains 46 chromosomes (6 to 9 feet of DNA) • Your body contains 75-100 trillion of cells. • All of your DNA (when uncoiled and tied together) could make about 6000 trips from the Earth to the Moon.

  36. A few more cool things about DNA • It takes about 8 hours for one of your cells to copy all of its DNA. • Our entire DNA sequence is called a Genome…and there is an estimated 3,000,000,000 DNA bases • This would take up about 3GB of storage • If you could type 60 wpm, 8 hours/day…it would take you 50 years to type this. • 99.9% of our DNA is the same…it is the 0.01% that makes you who you are!

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