Chapter 9 DN: he Molecule of Heredity What is DN? nswer: Molecule that carries the blueprint of life General Features: DN is packages in chromosomes (DN + Proteins) Gene = Functional segment of DN located at a particular place on a chromosome Gene Chromosome Discovery of nucleic acid 1869: Miescher isolated DN from pus! What is the genetic material? Protein vs DN Debated until the 1950 s. DN is made up of: Deoxyribose : the sugar Phosphate nitrogen rich base Protein complex 20 different amino acids Forms thousands of different molecules. DN simple Only 4 nucleotides Lactase Genetic material = DN wo major experiments 1920 s -1950 s Griffith s mice experiment 1. Griffith s dead mice 2. Radioactively labelled bacteriophages Bacteriophages are viruses that destroy bacteria. ypical bacteriophage infecting a bacteria cell 1
(Figure 9.1) Second experiment: radioactive viruses Viruses have protein coats, which enclose DN or RN. Only DN viruses was used in this experiment. he proteins were labelled with radioactive sulfur, the DN was labelled with radioactive phosphorus. Electron microscope photos of bacteriophages. virus particle labeled with 35 S 35 S remains outside cells Structure of DN: Nucleic acid composed of individual nucleotides DN (blue) being injected into bacterium virus particle labeled with 32 P DN (blue) being injected into bacterium 35 P remains inside cells Deoxyribose Know this structure, you will see it on the exam! Structure of DN: Nucleic acid composed of individual nucleotides 4 types of nucleotides (nitrogen base differences): Purines (double-ring bases) Pyrimidines (single-ring bases) How are the Nucleic cids arranged to form DN? 1) Chargaff (1949): DN contains equal amounts of adenine and thymine DN contains equal amounts of cytosine and guanine 2
How are the Nucleic cids arranged to form DN? Rosalind Franklin and Maurice Wilkens (early 1950s): Used X-ray diffraction DN is a helical molecule (corkscrew) DN has a uniform diameter (2 nm) DN consists of repeating sub-units How are the Nucleic cids arranged to form DN? Watson and Crick (1953) DN is two separate strands of linked nucleotides Phosphate groups bonded to sugar groups (backbone) ntiparallel (strands opposite one another) he order of nucleotides in DN can encode vast amounts of information: 10 nucleotides = > 1,000,000 possible sequences Organisms have millions (bacteria) to billions (plants / animals) of nucleotides Humans have 3 BILLION nucleotides. (similar to Figure 9.5) DN Replication Semiconservative Replication: he copying of DN is a critical event in a cell s life Cell reproduce by mitosis to produce two daughter cells from a single parental cell Each daughter cell must receive an exact copy of DN DN replication produces two DN double helices through process termed semiconservative replication Parental strands unwind and separate Daughter strands are formed using parental strands as templates Parental strand and new daughter strand wrap together to form new double helix 3
Semi-conservative replication DN Replication - Closer Look... Step 1: DN Strand Separation DN Helicase (enzyme) breaks hydrogen bonds holding nitrogen bases together (unzips) Requires energy (P) Multiple DN helicases work on single chromosome form replication bubbles; time-saving mechanism DN Helicase DN Replication - Closer Look... Step 2: DN Synthesis DN Polymerase (enzyme) joins nucleotide sub-units to form new strands of DN G C C G Forms complementary strands to parent strands (Recognizes nucleotide and adds appropriate base) C G G G G Polymerase C C C DN Replication - Closer Look... Step 2: DN Synthesis DN Polymerase (enzyme) joins nucleotide sub-units to form new strands of DN Forms complementary strands to parent strands (Recognizes nucleotide and adds appropriate base) DN Polymerase can only move in one direction One strand synthesized as continuous molecule Second strand synthesized in small pieces DN Polymerase DN Ligase: ttaches pieces together DN replication movie DN Replication - Mistakes Happen... DN replication is a rapid process 700 nucleotides / second DN movie DN polymerase mismatches 1 bp / 10,000 bps Proof-reading enzymes correct mistakes Overall mistake rate = 1 bp / 1,000,000,000 bps Mutations Other hazards damage DN Body temperature = 98.6 F = 10,000 bp lost / day Ultraviolet light 4
Sun Burns: 1st Degree 2nd Degree UV light from sun damages DN If damage is great enough, cell will commit cellular suicide BU If the damage is in region of DN controlling replication, cancer may result symmetry Border (irregular) Color (irregular) Diameter (large) 3rd Degree What happens if we don t have the DN repair enzymes? Xeroderma pigmentosum: a skin cancer predisposition disease. Extreme sensitivity to sunlight >> skin cancer Cause: genetic defects in the enzyme that repairs sunlight damage. DN damage due to sunlight Probability of cancer Xeroderma pigmentosum Possible inspiration for vampire myths. Patients with XP develop lesions when exposed to sunlight. utosomal recessive Both parents must be carriers. Only treatment is avoidance of UV light sources. 5