ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Fourteen Outline INTRODUCTION Patterns of Heredity Explained by Chromosomes and Meiosis Enhanced the Study of Humans as Biological Organisms WHERE DO CELLS STORE HEREDITARY INFORMATION? Hammerling's Experiments with Acetabularia fig 14.1 Initial experiment used a single genus as model organism Large green alga cell with distinct foot, stalk and cap Cap lacking nucleus amputated: cap regenerates Foot with nucleus amputated: no foot regenerated Concluded hereditary information in foot Second experiment used species that looked different fig 14.2 A. crenulata: disk-shaped cap, A. mediterranea: flower-shaped cap A. crenulata stalk onto A. mediterranea foot Regenerated cap looked similar to A. crenulata Amputated regenerated cap, next cap looked like A. mediterranea Further supported that hereditary information in foot Frog Nucleus Transplant Experiments Removed nucleus from frog egg: no development Added nucleus from another egg: development occurred Concluded nucleus directed development Carrot Experiments Mature carrot tissue fragmented Individual cells developed roots, became adult plants Concluded each cell has full set of genetic material, can generate entire adult WHICH COMPONENT OF THE NUCLEUS CONTAINS THE HEREDITARY INFORMATION? Genes Hold Hereditary Information The Griffith-Avery Experiments: Transforming Principle Is DNA Mice injected with various strains of bacteria fig 14.3 Virulent, coated bacteria lethal to mice Nonvirulent, coatless strain not lethal Dead coated bacteria not lethal to mice Dead coated and live coatless bacteria mixed and injected Mice died Transforming factor passed from one strain to other Transforming principle isolated, resembled DNA Activity unaffected by protein-digesting enzymes Activity lost in presence of DNase The Hershey-Chase Experiment: Some Viruses Direct Their Heredity with DNA Bacteriophage viruses attack bacteria, possess either DNA or RNA Lytic virus injects viral genetic material into bacteria Causes production and release of more viruses Genetic material DNA or protein fig 14.4 Labeled T2 bacteriophage DNA with 32P and protein coat with 35S Viruses infect bacteria, attached viruses shaken off Agitation removed 35S from bacterial preparation Found 32P injected into bacterial cells Concluded genetic material in bacteriophages was DNA The Fraenkel-Conrat Experiment: Other Viruses Direct Their Heredity with RNA Some viruses possess RNA, not DNA Tobacco mosaic virus (TMV) Holmes ribgrass virus (HRV) Genetic material RNA or protein Tobacco infected with hybrid: TMV protein coat and HRV RNA fig 14.5 Observed lesions characteristic of HRV Concluded hereditary material was RNA Other viruses also contain RNA, not DNA Most copy own DNA and insert into cell's DNA Retroviruses make intermediate double-stranded DNA THE CHEMICAL NATURE OF NUCLEIC ACIDS Nucleic Acid First Isolated from Cell Nuclei Composed of Nucleotides (P.A. Levine) General structure fig 14.6 Phosphate group PO4 Five carbon sugar Nitrogen containing base: purine or pyrimidine Purines = adenine, guanine Pyrimidines = thymine, cytosine Numbering scheme for sugar structure fig 14.7 A prime (.) indicates that the carbon is located on the sugar molecule Phosphate attaches to 5' carbon Base attaches to 1' carbon -OH attaches to 3' carbon Nucleotides Strung Together in Chains Phosphate at 5. C, hydroxyl at 3. C allow chains to form Sugars linked by phosphodiester bond fig 14.8 Nucleotide chain possesses definite direction One end of chain with free 5. phosphate group Other end of chain with free 3. hydroxyl group Sequences conventionally written in 5. to 3. direction Base Composition in Nucleotide Chains Initially thought all four bases were in equal amounts Assumed DNA a polymer of four repeating units DNA had structural role and protein had hereditary role Later found base amounts differed, depended on source tbl 14.1 DNA not a simple repeating polymer Chargaff's rules Proportion of adenine (A) equal to thymine (T) Proportion of guanine (G) equal to cytosine (C) Proportion of purine (A + G) equal to pyrimidine (C + T) THE THREE-DIMENSIONAL STRUCTURE OF DNA Franklin's X-Ray Crystallography fig 14.9 Pattern of diffractions caused by DNA fibers Not precise since DNA sample was in fibers not true crystals Initial analysis of DNA fig 14.10 Spring-like spiral with helical diameter of 2 nanometers Complete turn made every 3.4 nanometers Watson-Crick Analysis fig 14.11 Constructed models to determine shape Double helix fit all known data fig 14.12 Bases pointed inward toward one another Large purine always paired with small pyrimidine Hydrogen bonds between bases stabilize antiparallel strands fig 14.13 Model explained Chargaff's results fig 14.14 Adenine, thymine form two bonds Guanine, cytosine form three bonds HOW DNA REPLICATES Model Dependent on Complementarity of Strands Sequence of one chain determines sequence of its partner Each chain is complementary mirror image of other Replication Is Semiconservative DNA replication model based on Meselson-Stahl experiments fig 14.15 Double strands unzip from one another Separated strand serves as template for new strand Each strand is copied to make two new double helices Labeled generations of bacteria with heavy nitrogen 15N Transferred onto media containing lighter nitrogen 14N Initial bacteria all heavy: two heavy strands Later ones intermediate: one heavy, one light strand Later grouped into intermediate and light classes Intermediate group had one strand of each Light group had two light strands Two Strands of DNA Are Replicated in Opposite Directions Replication begins at one or more origins of replication DNA duplex opened and untwisted by helicase enzyme Forms replication bubbles where DNA strands are separated fig 14.16 Actual replication occurs at Y shaped ends of replication fork fig 14.17 Catalyzed by DNA polymerase RNA primer constructs initial 10 sequence RNA complement DNA polymerase recognizes primer and adds to it RNA nucleotides replaced with DNA nucleotides Replication occurs only in 5. to 3. direction Strands are elongated by different mechanisms Replication of leading strand, 5' to 3' strand New strand grows from 3' end Elongates towards replication fork Lagging strand, 3. to 5. strand replication Elongates away from replication fork Synthesized discontinuously in small batches 5' 3' synthesis catalyzed by DNA polymerase Segments called Okazaki fragments DNA ligase attaches fragment to lagging strand Overall replication process is termed semidiscontinuous Comparing Prokaryotic and Eukaryotic DNA Replication Bacterial DNA double helix in form of single circle fig 14.18 Duplex nicked at single site Displace strand on one side form one replication fork Displace strand on two sides form two replication forks Forks proceed around circle creating a daughter DNA loop When complete, two circles of DNA are present Eukaryote DNA is not circular, but in chromosomes Each chromosome has many replication forks Each zone replicated as discrete replication unit fig 14.19 Zones average 100,000 base pairs in length Advantage of this method is speed Large amount of DNA requires sophisticated controls THE EUKARYOTIC CHROMOSOME Nucleus Contains a Large Amount of DNA fig 14.20 Too fragile to stay extended at all times Need efficient packaging to fit inside Histones Package DNA into Nucleosomes and Chromatin Single DNA molecule wrapped around cluster of eight histones Cluster binds to 146 nucleotide base pairs to form a nucleosome fig 14.21 Resembled beads (nucleosomes) on a string (linker DNA) H1 histone protein further condenses material into chromatin Euchromatin and Heterochromatin Both found in cell during interphase Heterochromatin is tightly packaged, can't be transcribed Less densely packaged euchromatin can be transcribed The Chromosome Further condensing occurs at beginning of mitosis Probably assisted by H1 histones Most transcriptionally inactive form of DNA Packaging ensures surviving mitotic process Chromosome has centromere and telomeres at ends of DNA Full complement of chromosomes seen in karyotype fig 14.22 Stained chromosomes show banded pattern Homologous bands identified in related species How Many Genes Are on a Chromosome? Example: Saccharomyces, brewer's yeast chromosome III Identified 182 genes, half with no known function Most genes are transcribed since 160 different mRNA's detected Requires far more to identify gene functions than to map chromosome GENES: THE UNITS OF HEREDITARY INFORMATION Garrod Investigated Alkaptonuria, a Genetic Disorder Abnormal urine turns black on exposure to air Contains homogensic acid that oxidizes and blackens Acid in normal urine broken down by enzymes Postulated that affected patients lack enzymes Concluded that information in DNA coded for enzymes The One Gene-One Enzyme Hypothesis Beadle and Tatum examined bread mold Set out to create mutations in chromosomes Creating genetic differences fig 14.23 Used x-rays to induce mutations in mold spores Allowed progeny to grow on complete medium Contains all possible nutrients Strains unable to produce own nutrients still grew Identifying mutant strains Grow progeny on minimal medium to test for deficiencies Cells unable to make metabolite would not grow Identified numerous growth-deficient mutants Pinpointing biochemical deficiencies Individually replace chemicals to determine deficiency Determine enzymes involved in deficiencies Arginine mutants clustered in three areas fig 14.24 Each site coded for different enzyme in pathway Postulated one gene-one enzyme (now polypeptide) hypothesis How DNA Encodes Proteins Sanger identified amino acid sequence of insulin First demonstration of protein structure Information for enzymes is ordered list of amino acids Ingram analyzed normal and sickle-cell hemoglobin fig 14.25 Single amino acid substitution between hemoglobins Alleles for genes altered in only one amino acid