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  1. - Chapter 12 DNA

  2. 12-1 Identifying the Substance of Genes • To truly understand genetics, we must understand the chemical nature of genes.

  3. 1928 - Frederick Griffith • Disease causing bacteria, caused the mice to develop pneumonia and die. • The harmless strain of bacteria didn’t make the mice sick at all. • When the harmful bacteria were killed with heat and injected into mice, the mice didn’t get sick at all. • iText page 288

  4. Transformation • Griffith mixed the heat-killed, disease-causing bacteria with the harmless bacteria and injected the mixture into the mice. • Surprisingly, many of the mice got sick and died. • Somehow the heat-killed bacteria had passed their disease-causing ability to the harmless bacteria. • Griffith called this “transformation” because one strain of bacteria had been changed into another. • Some “factor” must have transferred from the heat-killed strain to the live cells.

  5. Griffith’s Experiment

  6. The Molecular Cause of Transformation Oswald Avery (1940’s) • Conducted experiments that used enzymes to digest specific molecules • Also conducted transformation experiments • Transformation only occurred in those experiments where DNA was left intact • Discovered that DNA is the nucleic acid that stores and transmits genetic information from one generation to the next.

  7. Bacterial Viruses It took several experiments to convince scientists about the chemical nature of a gene Two scientists, Hershey and Chase, worked with bacteria and viruses to identify genetic material. • A Bacteriophage is a kind of virus that infects bacteria. • It attaches to the surface of the bacteria and injects its genetic information in to it. • The viral genes produce many new bacteriophages which gradually destroy the bacteria . • The bacteria then bursts open, releases hundreds of new viruses.

  8. The Hershey-Chase Experiment • Used bacteria to discover whether genes were made of protein or DNA. iText Page 290

  9. The Role of DNA • Storing Information • The main job of DNA • Genes carry the information needed to produce traits and control the patterns of development in all organisms

  10. Storing Information

  11. The Role of DNA • Copying Information • Before cell division, a complete copy of every gene must be made • Transmitting Information • Genes are transmitted from one generation to the next • DNA molecules must be carefully sorted and passed along during cell division

  12. Copying & Transmitting Information

  13. 12-2 The Structure of DNA • Nucleotides are the building blocks (monomers) of Nucleic Acids • Each Nucleotide has 3 basic parts: • a 5-Carbon Sugar (deoxyribose) • a Phosphate Group • a Nitrogenous Base

  14. 4 Types of Nitrogenous Bases: • Adenine • Guanine • Cytosine • Thymine • The nucleotides in a strand of DNA are joined together by covalent bonds Nucleotides

  15. 1952 - Edwin Chargaff • Determined the rules that govern the proportion of DNA bases: A = T C = G

  16. 1952 – Franklin’s X-Rays • X-ray diffraction picture that led to discovery of the shape of the molecule – the Double Helix (Helix - strands twisted around each other like the coils of a spring)

  17. 1953 - James Watson and Francis Crick • The clues in Franklin’s X-Ray pattern helped Watson and Crick explain the scientific structure and properties of DNA • Won Nobel Prize for the discovery of the structure of DNA

  18. The Double-Helix Model • Antiparallel Strands • The 2 strands of DNA run in opposite directions (chemistry = antiparallel) • Enables • the nitrogen bases on both strands to come into contact at the center of the molecule • Allows each strand to carry a sequence of nucleotides arranged like letters

  19. The Double-Helix Model • Hydrogen Bonding • Hydrogen bonds are relatively weak chemical forces • Provide just enough force to hold the 2 strands of DNA together

  20. The Double-Helix Model • Base-Pairing • Bonds will only form between certain base pairs: A – T C – G • This explains Chargaff’s Rule and gives a reason why A = T and C = G.

  21. 12-3 DNA Replication The Replication Process • During DNA replication, the DNA molecule separates into two strands. • Each strand of the double helix of DNA serves as a template, or model for the new (complementary) strand • Adenine always pairs with Thymine • Guanine always pairs with Cytosine iText Page 298

  22. DNA Replication

  23. Replicating DNA

  24. The Role of Enzymes DNA replication is carried out by a series of enzymes: • Enzymes “unzip” a molecule of DNA by breaking the Hydrogen bonds between base pairs and unwinding the two strands • Each strand serves as a template for the attachment of the complimentary bases • DNA Polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA • DNA Polymerase also “proof-reads” each new strand so that is a near-perfect copy of the original strand

  25. DNA Replication

  26. Telomeres • DNA at the tips of chromosomes • Difficult to replicate • A special enzyme, telomerase, solves this problem. • Helps prevent genes from being damaged or lost during replication • Common in rapidly dividing cells (stem cells, cancer cells) • Switched off in adult cells

  27. Replication in Living cells • Occurs during the S phase of the Cell Cycle • Carefully regulated • Location: • Prokaryotes • Single, circular DNA molecule in the cytoplasm • Eukaryotes • Have up to 1000 times more DNA • Nearly all DNA is found in the nucleus • Consists of chromosomes formed from chromatin containing DNA and histone molecules

  28. DNA is found in the nucleus organized into chromosomes

  29. Chromosomes contain both DNA and proteins • The proteins are called histones

  30. Prokaryotic DNA Replication • Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied

  31. Eukaryotic DNA Replication • In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.

  32. Ch. 13 - RNA & Protein Synthesis

  33. The Role of DNA • Watson and Crick’s discovery of DNA structure led to the understanding of how DNA could be copied. • How DNA actually works was not yet understood. • The discovery of another nucleic acid, RNA was then discovered. • It led to the understanding of how RNA put the genetic code into action.

  34. Genes contain coded DNA instructions that tells cells how to build proteins. • The first step is to copy the genetic instructions from DNA into RNA. • RNA then uses these instructions to direct the production of proteins, which help to determine an organism’s characteristics.

  35. Comparing RNA and DNA • There are 3 important differences between RNA and DNA • The sugar • The number of strands • One nitrogenous base • DNA is like the valuable master plan that builders use. It never goes to the job site where it might be damaged or lost. • Instead, inexpensive, disposableblueprints (like RNA) are used on the job.

  36. The DNA master plan is used to prepare the RNA blueprints • The DNA stays safely in the nucleus • The RNA goes to the protein-building sites in the cytoplasm ---- the ribosomes

  37. Master Plans and Blueprints

  38. DNA and RNA

  39. Functions of RNA • Messenger RNA (mRNA) – is made from the DNA template and it takes the message out of the nucleus to the ribosome • Transfer RNA (tRNA)- brings amino acids to the ribosome to be strung together to make a protein • Ribosomal RNA (rRNA) – makes up the ribosomes, where protein is made

  40. Types of RNA

  41. RNA Synthesis • Transcription • DNA segments are used as templates to produce complementary RNA molecules • Requires an enzyme (RNA Polymerase) • Promoters • Regions of DNA that have specific base sequences • Shows RNA Polymerase exactly where to begin making RNA

  42. Transcription

  43. Transcribing DNA into RNA

  44. mRNA

  45. RNA Synthesis • RNA Editing – RNA is edited before it is ready to be read • Introns – • Portions that are cut (edited) out and discarded • They are left “IN” in nucleus • Exons – • The remaining pieces that are spliced back together to form the final RNA • They get to “EXit” the nucleus

  46. Introns and Exons

  47. Ribosomes & Protein Synthesis • DNA and RNA work together in the process of protein synthesis. • Protein Synthesis is the making of new proteins from DNA blue prints by RNA molecules

  48. The Genetic Code • Proteins are made by joining amino acids into long chains of polypeptides • The 4 Bases (A, C, G and U) form a language which is called the genetic code • The Genetic Code • is read 3 “letters” at a time • Ex.: AUG-AAC-UCU • each “word” (codon) is 3 bases long and corresponds to a single amino acid

  49. How to Read Codons • A genetic code table is used to decode codons: • Start at the middle of the circle with the 1st letter of the codon, and move outward • Next, move out to the 2nd ring to find the 2nd letter of the codon • Find the 3rd and final letter among the smallest set of letters in the third ring Then, read the amino acid in that sector