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DNA

DNA. The Molecular Basis of Inheritance http://player.discoveryeducation.com/index.cfm?guidAssetID=3692BEA5-C588-4A4E-AB35-0296ED8BE7DB. Identifying the Genetic Material. 1928 Fredrick Griffith (English Bacteriologist) Trying to find a vaccine for pneumonia

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DNA

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  1. DNA The Molecular Basis of Inheritance http://player.discoveryeducation.com/index.cfm?guidAssetID=3692BEA5-C588-4A4E-AB35-0296ED8BE7DB

  2. Identifying the Genetic Material • 1928 Fredrick Griffith (English Bacteriologist) • Trying to find a vaccine for pneumonia • Vaccine: prepared from killed/weakened microorganisms introduced into the body to produce immunity • Griffith worked with 2 strains of Streptococcus pneumoniae bacteria • S strain • Polysaccharide Capsule • “Smooth” edged colonies • Virulent – able to cause disease • R strain • No Capsule • “Rough” edged colonies • Nonvirulent - does not cause disease

  3. Griffith’s Experiment • Griffith’s Conclusion: Something had passed from heat killed bacteria to the nonvirulent R strain making them virulent… he called this the “transforming principal” • Griffith did not know what it was, but many scientists thought it was proteins

  4. Today we know… • Transformation – cells take up foreign genetic material, changing their own genes (used for genetic engineering) • Heat killed S bacteria – enzymes were denatured therefore the DNA could not be copied • Proteins are denatured at 600C and DNA is denatured at 900C • DNA of heat killed S bacteria survived and transformed DNA of R bacteria

  5. The Search for what caused the Transformation… • 1944 – Oswald Avery, MacLeod, & McCarty (American Bacteriologists) • Experiment: • Added protease to “R and heat-killed S” mixture Result  Mice died • Added DNAase to “R and heat-killed S” mixture Result  Mice Lived • Conclusion: • DNA, not protein, is the transforming factor in Griffith’s experiment

  6. More Evidence that DNA is the Genetic Material… • 1952 – Alfred Hershey & Martha Chase (NY) • Used T2 bacteriophages (phage) – virus that infects bacteria • Composed of nucleic acid surrounded by a protein coat • Viruses infect specific host • Viruses are not living • Not composed of cells • Cannot reproduce on their own • Do not grow and develop

  7. Background Info on Viruses

  8. Hershey & Chase Experiment • Experiment: • Grew T2 w/radioactive Sulfur 35S (protein coat takes in 35S) • Grew another group of T2 w/ radioactive Phosphorus 32P (DNA takes in the 32P) • 35S-labeled and 32P–labeled phages were used to infect E.Coli bacteria • Separated phages from bacteria using a blender and a centrifuge… the bacterial cells at bottom and viral parts at the top • Results: • 35S-labels still in viral parts • 32P-labels mostly in the bacterial cells, and new phages also contained 32P DNA • Conclusion: • Viral DNA (not protein) enters bacteria and carries instructions on how to make more phages • Without a doubt, DNA is the hereditary material!

  9. Hershey & Chase Experiment

  10. Structure of DNA • By 1950’s most scientists were convinced that • Chromosomes carry genetic material • Genes are on chromosomes • Genes are made of DNA • Basic Structure of DNA • Composed of nucleotides • Nucleotides made of 3 parts deoxyribose, phosphate, N base • 2 types nitrogen bases: • Purines – double ring of C and N • Adenine • Guanine • Pyrimidines – single ring of C and N • Cytosine • Thymine

  11. Discovering DNA’s Structure • Erwin Chargaff (NYC) • 1947 – DNA composition varies among different species • 1949 -Chargaff’s Rules- Discovered regularity of ratios: • # Adenines = # Thymines • (ie. Humans A =30%, T=30%) • # Guanines = # Cytosines • (ie. Humans G = 20%, C = 20%) • 1952 Rosalind Franklin & Maurice Wilkins (England) • Developed X-ray crystallography photographs of DNA • Suggested “helix” shape of 2-3 chains of nucleotides

  12. April 25th, 1953 • James Watson & Francis Crick (England) • Built the 1st accurate 3D (tin and wire) model of DNA • “Double Helix” – spiral staircase • Purine is always linked by h-bond to a pyrimidine • 2 strands of DNA are complimentary to each other • 2 strands are anti-parallel • 1962 Awarded the Nobel Prize

  13. More on DNA • Ex. If the sequence of bases on one strand is AATGCGCAT, than the complimentary strand will be: ________________ • Human DNA has 3 billion base pairs.. Less than 1% of our DNA makes us different from one another!

  14. Origin of Replication • Prokaryotic Cell – single origin of replication where proteins separate the 2 strands and create a replication bubble, replication proceeds in both directions from the replication fork • Eukaryotic Cells – hundreds or thousands of replication bubbles form to speed up the copying process, replication proceeds in both directions from the replication fork

  15. DNA Replication • Watson and Crick proposed that the complimentary strand of DNA serves as a template for which the other strand is built…experiments confirmed this 5 years later • DNA Replication: Process of Synthesizing new molecules of DNA • Helicase breaks H-bonds and opens up the double helix forming replication forks (point at which DNA separates) • At the replication fork, DNA Polymerases continuously adds complimentary nucleotides to exposed bases • Process continues until all DNA has been copied, end result is 2 new molecules of DNA each identical to the original and composed of one new and one old strand • http://www.youtube.com/watch?v=L9RjNNfgaEQ

  16. DNA Synthesis

  17. Proofreading • DNA polymerase only moves to the next nucleotide if the previous nucleotide was a correct match • If mismatched, DNA Polymerase backs up, removes the mismatched nucleotides and replaces it with the correct one(s). • Only 1 error per 1 billion nucleotides! • http://www.youtube.com/watch?v=oJZH4lV3h6I&mode=related&search=

  18. DNA Replication & Aging • Every time DNA is copied, DNA polymerase cannot complete replication on the ends • Eukaryotic DNA has a non-coding, repeating nucleotide sequence on the ends called telomeres that protects genes from being eroded over successive replications • It is believed that telomeres are directly related to the aging process

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