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Chapter 12

Chapter 12. DNA and RNA. 12-1: DNA. How was DNA discovered? Fredrick Griffith Oswald Avery Hershey & Chase Watson & Crick. Frederick Griffith. Griffith (cont.) Experimented with bacteria and mice Cultured both harmless and pneumonia causing bacteria.

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Chapter 12

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  1. Chapter 12 DNA and RNA

  2. 12-1: DNA • How was DNA discovered? • Fredrick Griffith • Oswald Avery • Hershey & Chase • Watson & Crick

  3. Frederick Griffith

  4. Griffith (cont.) • Experimented with bacteria and mice • Cultured both harmless and pneumonia causing bacteria. • Exposed mice to a mixture of both harmless and heat killed pneumonia causing bacteria • Mice came down with disease

  5. Conclusion: • Griffith called it transformation • Cells can be transformed when coming into contact with other types of cells • Harmless bacteria transformed into disease causing strains. • What was this disease causing agent and how did it transform the other cell?

  6. Oswald Avery • Set out to answer the previous question. • Repeated Griffith's work, took extract from heat killed bacteria. • Treated extract with enzymes that break down: • Proteins • CHO’s • Lipids • Even RNA • Same results • Then repeated with enzymes that break down DNA • Result: • No transformation

  7. Hershey and Chase (cont.) • Used radioactive markers to determine if proteins or DNA was injected by viruses • The tracers could be followed from the virus to the bacteria. • Injected with: • Phosphorus-32 (in DNA, not in proteins) • Sulfur-35 (in proteins, not in DNA) • Results: • Only Phosphorus-32 was transferred

  8. At this point, scientists knew DNA was the “culprit” for where genes are contained. • Still needed to know: • How did DNA carry genes generation to generation? • How did DNA code for traits? • How was copied?

  9. The Components and Structure of DNA • Nucleotides • 5-carbon sugar • Deoxyribose • Ribose • Phosphate • Nitrogen base

  10. Chargaff’s rule • Determined complementary nature of DNA.

  11. X-rays were used to “see” the general structure of DNA • Appeared like this:

  12. Watson & Crick • Determined the shape had to be a double helix • Two strands with complementary base pairs in between that are bonded together.

  13. 12-2: Chromosomes and DNA Replication • Chromosome structure • Histones • Proteins that chromatin is wrapped around • Chromatin • Condensed DNA • Nucleosome = histones + chromatin

  14. Duplicating DNA • “Replication” • Each new cell after mitosis gets exact copy

  15. Steps of replication • DNA is “unzipped” by protein called DNA helicase (breaks hydrogen bonds between compl. bases)

  16. DNA polymerase reads the sequence and adds base pairs that complement.

  17. DNA polymerase also proofreads along the existing strand.

  18. DNA ligases“put together” chunks of copied base segements.

  19. 12-3 RNA and Protein Synthesis • RNA • Structure • Single-stranded • Ribose-phosphate backbone • Contains nitrogen base uracil instead of thymine • It bonds with adenine on DNA

  20. Types of RNA • mRNA – messenger RNA • transcription • rRNA – ribosomal RNA • Make up components of the ribosomes • tRNA – transfer RNA • translation

  21. Transcription • DNA gives code to RNA for making proteins • Similar to replication except now code is copied to RNA (has uracil) • RNA polymerase unzips the DNA strand and begins to add bases that complement one of the strands.

  22. YouTube - Transcription

  23. How does it know where to start? • Promoters • Specific sequences of base pairs that RNA polymerase can only bind to in order to initiate transcription.

  24. RNA editing • Introns and Exons • Introns • Sequences that code for nothing • Exons • Sequences that directly code for sections of proteins • Sequences that are “expressed” as protein • Eventually enzymes go back and cut the introns out and splice together the exons to have a fully functioning mRNA.

  25. The Genetic Code • 20 different amino acids • Each is coded for by a segment of 3 base pairs • Codon • Most amino acids have multiple codons • Also codons for starting and stopping transcription

  26. Translation • Copying mRNA into a sequence of amino acids. • mRNA attaches to ribosome • Ribosome “reads” looks for a “start codon” • Two tRNA with “anticodon” that complement the strand are attached to mRNA by the ribosome.

  27. Temporary hydrogen bonds allow the tRNA to be bound to mRNA long enough to form a “peptide bond” between the two amino acids. YouTube - Translation

  28. Genes and Proteins • These proteins that are produced ultimately go to make: • Strucutral proteins (eye color, physical features) • Enzymes (control all cellular activities, ex: digesting lactose) • Hormones (producing testosterone/estrogen) • Combined all of these factors ultimately make us what we are.

  29. 12-4 Mutations • Mutations • Changes in the genetic sequence • Types of mutations • Point mutations • Occur in one (or few) base(s) of the DNA sequence • Include: • Substitutions • Sometimes little to no effect on amino acid sequence, however sometimes can be cataclysmic • Sickle-cell anemia

  30. Frameshift mutations • Caused by insertions or deletions of bases, shifting the way the mRNA is read. • Shifts the “reading frame” • Usually has dramatic effects on the formation of the protein – often rendering it useless

  31. Chromosomal mutations • Deletions • Duplications • Inversions • Translocations

  32. Significance of mutations • Most often mutations ultimately show little no effect on the protein that is supposed to be made. • However, when it does have an effect the new protein formed can be:

  33. Detrimental • Increases organisms chance of dying an not passing mutated gene on. • Beneficial • Increases the organisms chance of survival and reproductibility – therefore passing it down • CREATES GENETIC VARIABILITY! • This is often how asexually reproducing organisms evolve • Slow process

  34. 12-5: Gene Regulation • How does an organism “know” when to turn the gene on or off? • Example: How does your body turn on the lactase gene?

  35. Regulation of the lacoperon in E.Coli • Operon • Groups of genes that code for specific protein • lacoperon in E.Coli codes for protein that breaks down lactose

  36. Again, transcription begins at the sequence called the promoter • Just “below” the promoter are “operator” (O) sites • These sites are areas where “repressors” can bind • In most cases repressors are bound to O site, preventing transcription • Turning off the gene • Just like a room, when you are not in it – TURN OFF THE LIGHT!

  37. When lactose is present, it binds with the repressor changing its shape, forcing it off the O site • Allows RNA polymerase to begin transcription • YouTube - Lac Operon

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