Chapter 5

CHAPTER 5: THE SOURCE OF HEREDITARY -

Genetically modified strain of mice “Doogie” – greater memory: modification + insertion of NR2B gene  improves functioning of nerve receptors  memory and learning.

5.1 Early Developments in Genetics -

History: early physicians and scientists investigations, artists sketching guide to anatomy, structure of  organs: principle: structure  function. Study of Genes + technology: Light microscope, electron microscope, X-ray diffraction, gel electrophoresis  better picture of mechanisms of gene action. Cytology and Genetics 2000 years ago: Greek philosopher Aristotle: hereditary: “power” of male’s semen Other scientists: female determined characteristics, male gamete set events in motion Other theory: heredita ry traits in blood: e.g. “bloodline” 1831: discovery of nucleus  towards understanding structure + function of cells + genes. 1865: Mendel published papers. Knowledge: egg + sperm = zygote. Accepted: factors blended = characteristics of offspring. Mendel didn’t know meiosis, structure or location of hereditary material, how genetic code worked  many theories about inheritance  – explained how traits were passed. Interpreted as experiment w/ garden peas. Same time. New techniques  – lens grinding = better microscopes  new branch: Cytology: study of cell formation, structure, function. Aided  1882: Walter Fleming: separation of threads in nucleus = mitosis Same ear – ear – Edouard van Benden: sperm and egg cells 2 chromosomes = fertilized egg 4 1887- August Weisman: special division theory: reduction division (meiosis)  framework for Mendel’s work 1900: rediscovered Mendel’s work.

5.2 Development of the Chromosom Chromosomal al Theory -

1902: American Walter S. Sutton, German Theodor Boveri  – independently – independently  – chromosome pairs segregate during meiosis, forming new pairs after. homologous chromosomes  supported Mendel’s explanation of  inheritance + paired factors. Today: alleles of a gene, each from each sex cell. Cellular evidence  – explained + supported union of 2 alleles in offspring, formation of new combos in offspring. Chromosomal behaviour  – gamete formation explained law of segregation + independent assortment Sutton and Boveri deduced  – factors (alleles)  chromosomes. (46 chromosomes = thousands of traits)  Sutton hypothesis: each chromosome  genes (on same chromosome: linked genes) Chromosomal Theory Development/refinement of microscope  cell biology + genetics. Later: biochemistry + nuclear physics Chromosomal theory of inheritance: 1) chromosomes  genes, units of hereditary, alleles w/ specific locus/position 2) paired chromosomes segregate (meiosis). Sex cells ½ #, unlike somatic cells. Each gamete: 1 of 2 Assort independently: chromosome pair  – no influence on movement of others. E.g.  AaBb   AB, aB, Ab, ab – equal frequency

5.3 Morgan’s Experiments and Sex Linkage

-

-

-

American geneticist Thomas Hunt Morgan (1866-1945)  – gender and inheritance Drosophila Melanogaster  – 1) reproduces rapidly, mating after leaving egg, 100 eggs each time. Study many in short period, larger number ideal for probability. 2) small  – many in single culture tube  small, solid nutrient. 3) Genders distinguishable. Males  – smaller, rounded abdomen, dark coloured posterior segment. Females – Females  – larger, pointed abdomen, pattern of dark bands. Mutations (heritable chance – chance  – molecular structure of DNA) linked to other traits (supported theory: genes  chromosomes). Examining eye color. White eyed male among red-eyed offspring  decided mutation  traced inheritance of allele of white eyes, White-eyed male x Red-eyed female  F1: all red eyes  Hybrid x Hybrid  F2 ¾ Red ¼ White – White  – all females red, half males white half males red  Cytology th 8 chromosomes: females 4 pairs, males 3 pairs: 4 pair, sex, partially homologous: X chromosome + Y chromosome. Conclusion: different genes. Explained: Y didn’t  eye colour (differences: on part of X not matching Y). = Traits on sex chromosomes = sex-linked traits

-

R

R

r

(pure-breeding, red eyed female. Allele  – red eye  – dominant, located X chromosome) x X Y (White R r R eyed male: Y: no allele). Offsprings: Red eyes: females X X , males X Y F1 male x female => F2. Male offspring’s sex-linked sex -linked trait is from mother. Father supplies Y. R r R R R r F2 male X Y and X Y. Females: X X or X X . White-eyed females: female with 1 allele for white eyes x white-eyed male. Females 3 genotypes, males 2: can’t be homozygous for X gene – only 1 X In humans: recessive allele on X for red-green colourblindness. More males (only 1 X = only 1 recessive allele  expressed). E.g. human recessive lethal (trait: both alleles  death or severe malformation of  offspring) X-linked disorders  – more frequently in males. I.e. females live longer. Autosomal Dominant Genetic Disorders Controlled by dominant alleles on autosomal chromosomes. E.g. Huntington’s disease: lethal genetic disorder –  disorder – rare rare dominant allele: breakdown of certain areas of brain. No effective treatment. Males + females: equal frequency Autosomal Recessive disorders Recessive alleles on autosomes: needs 1 recessive allele from each parent. E.g. Cystic Fibrosis: most  common lethal genetic disease among Caucasians   thick mucus – affects – affects lungs and pancreas. Pedigree: symbols identifying males, females, individuals affected by trait, family relationships for genetic inheritance. X X

5.4 Looking Inside the Chromosome -

Deoxyribonucleic acid, DNA  – only molecule that can replicate  cellular reproduction. Contains instructions, transmitting hereditary info. Uniqueness  new combos of genes and mutations. E.g. instructions: new cells develop into specialized cells, mature cells  – replacement and repair of worn cell parts. Info  chemical messages, nucleus to cytoplasm. How genes affect expression of traits  how DNA regulates production  – cell protein (major structural/functional components). Searching for the Chemical of Hereditary Early 1940s  – biologists  – hypothesis: hereditary material  Chromosomes. Chromosomes: equal amounts of proteins  amino acids and nucleic acids. Altering sequence = n ew proteins. Nucleic acids  – similarities. Phosphate group + five carbon sugar molecule + ¼ nitrogenous bases = nucleotide. Polymer/long chain = DNA. At first: scientists: protein component of chromosome  genetic material w/ hereditary message, master molecule, directing arrangement of amino acids  – cytoplasm. Nucleic acids  monotonous repetitions of  1 sugar molecule, identical phosphates, 4 nitrogenous bases  – in all living things. Incorrect .

5.5. Discovering the Structure of DNA

-

James Watson  – child prodigy  – University of Chicago (15). Studied ornithology (study of birds), and genetics/molecular biology. 1951  – England’s Cambridge University, met Francis Crick, physicist.  interpret and synthesize experimental data.

-

Source of data: Cambridge laboratory of Maurice Wilkins: Rosalind Franklin: X-ray diffraction  structure of DNA molecule. Technique: photographed DNA molecule  clear images w/ helical structure + where phosphate sugars were located. Another clue: Scientists knew: DNA  sugars (deoxyribose), phosphate, 4 different nitrogenous bases. They didn’t know arrangement. New research – in any species: # of adenine molecules = thymine molecules, # guanine molecules = cytosine. I.e. arranged in pairs Watson’s background (emerging chemical data) + Crick’s background (significance – X ray diffraction results)  3d model of DNA molecule. 1953  scientific community. 1969  visually confirmed. Current version: extra info Models: useful tools. E.g. DNA: shows how different atoms interact. X-ray diffraction  picture  – how different chemical bonds interact. Visual devices  relationship/interactions of diff parts of molecule. Politics and Science Franklin  – X ray diffraction technique to view image. Watson + Crick, in England  model. American scientist Linus Pauling wanted to study, denied visa - identified communist sympathizer, support of  antinuclear movement.

5.6 Structure of DNA

-

-

DNA molecule: nucleotides: deoxyribose sugars, phosphates, nitrogenous bases. Double helix/spiral ladder: sugar + phosphate molecules (backbone), nitrogenous bases (rungs) paired by hydrogen bond (weak  – forms between + of hydrogen atom and  – on electronegative nitrogen/oxygen atom at end of  another molecule). Pair = complementary base pair. Estimate: 3.5 bill base pairs of DNA, 30 000 genes on 46 chromosomes. Nitrogenous bases w/ double rings, purines (adenine and guanine) always combine w/ nitrogenous bases w/ single rings, pyrimidines (cytosine and thymine). Adenine molecule always + thymine (2 hydrogen bonds), guanine molecule always + cytosine (3 hydrogen bonds)

5.7 Genes that Change Position -

Barbara McClintock (American)  – theory: Jumping Gene Theory (genes can move positions). Old theory  – until 1980s – 1980s – believed genes + chromosomes = fixed. Experiments w/ Indian corn. Colour variation in kernels  hypothesized elements: transposons (specific segments of DNA  – move along chromosomes). Inserting some genes  new position of chromosome inactivates genes affection pigment production. 1940s – 1940s – not well accepted. 1983 – 1983  – Nobel Prize. Recombinant DNA Bacterium – Bacterium – inserts genes along circular chromosome. Transposon can move  – chromosome of bacterium to another. Inserted genes can integrate w/ plasmid (in bacterium)  – secondary structure, small ring of  genetic material (cons. Extra DNA) Some bacteria: conjugation: sexual reproduction  – genetic material exchanged; 2 cells fuse, plasmids passed. Possible to insert DNA from 1 into another  genetic engineering. 1 technique  recombinant DNA – DNA  – DNA fragments from 2+ organisms spliced together. DNA from 1  – cut at specific sites by restriction enzyme  DNA fragments w/ unpaired nitrogenous bases/ “sticky ends”. Same w/ plasmid DNA. 1 f ragment  another, bonds - complementary nitrogenous bases. Negative conjugation: disease-causing bacteria  evolved genes resistant to antibiotics. Normally: interfere w/ chemical reactions in harmful microbes. Now  – genes of resistance.

5.8 Genetic Research and Technologies Human Genome Project mid 1980s: plans  mapping entire genetic makeup of human being. Began U.S.A. Oct 1990  – James st Watson (1 of 1 directors) Human genome(complete set of instructions in DNA): 30 000 genes w/ 23 pairs of chromosomes w/ 3 bill pairs of nucleotides  DNA. Beginning: only 4500 genes + sequence of nucleotides that made up genes. Completed May 2000: many scientists, diff countries, improvements in sequencing techniques. British biochemist Frederick Sanger  – DNA sequencing technique for project: pieces of DNA  – replicated + changed so fragments (ending with ¼ nucleotides)  – detected by laser. Automated equipment  exact number of nucleotides in chain. Computer  – combines data + reconstructs original sequence Quantity of DNA – DNA  – initially: cloning human DNA in single celled organisms. Now: American biochemist Kary Mullins  millions of copies of 1 molecule of DNA, few hours Add to list of genes for her editary disorders (cystic fibrosis, muscular dystrophy, Huntington’s chorea  new drugs + genetic therapies Other hand: controversial ethical questions, legal dilemmas, societal problems imputable Huntington’s Chorea Incurable brain disorder  – strikes prime of life  debilitating mental breakdown, eventual death Trapped inside own bodies, unable to communicate 1993: Dr Michael Hayden – Hayden  – UBC  team isolated gene Now: simple DNA test  screen people w/ family history, 98% accuracy  potential sufferers or parents who might pass on Another technique: animal modeling: lab mice  – genetically manipulated  like people w/ Huntington’s. Studied  how and why disorder  brain cells to die prematurely. Goal: treatments  – alleviate effects Cystic Fibrosis Inherited disorder  – 1 gene  protein CFTR (cystic fibrosis transmembrane conductance regulator): must inherit 2 defective alleles (from each parent). 1/25 of European ancestry carries  – most common recessive genetic disorder Hospital for Sick Children  – samples from families w/ more than 1 child suffering  – Dr Lap-Chee Tsui + team identified gene 1989  – mapped 2 modifier genes in animals  – alter severity of cystic fibrosis. Still investigating impact on treatment. U of T: Dr Christine Bear  – used cystic fibrosis mouse model  possible  – correct defect  delivering normal CFTR protein. Working  method to provide protein therapy to correct defect. Field of genetics  daily medical practice. Dr. Judith Hall UofT: head of pediatrics: clinical geneticist + experience w/ families affected by c ystic fibrosis. Communicates genetic reasons to children + families. Muscular Dystrophy Group  – genetic disorders: weakening + deterioration of muscles. Some forms  because  – defects on autosomal chromosomes, occurs in males + females. Other forms: sex-linked, males. Emery-Dreifuss muscular dystrophy  males, recessive X linked gene. Even female carriers  – normal dominant allele + recessive defective allele: mild symptoms. Hospital for Sick Children: Dr. Ron Worton: located Duchenne muscular dystrophy gene: 1987. Sex linked form  – boys age 2-6. Progressive damage in muscles of pelvis, upper arms, legs. Calf muscles enlarge: enzyme creatinine kinase leaks  swelling. Age 12: wheelchair. Other forms: both males and females equally: defective genes on autosomes. Some forms: recessive and dominant.

5.9 DNA Fingerprinting -

DNA fingerprinting test: Alec Jeffreys (geneticist from University of Leicester. Found: long stretches of DNA molecule are similar in e veryone: less than 1% unique.

-

Particular segments  – unique sequences of nitrogenous bases  – no function. Belief: nonsense codes, repeat as a chemical type of stuttering DNA fingerprinting test: DNA isolated from skin, hair, semen, blood vs. DNA from blood sample from suspect. DNA samples cut w/ restriction enzymes (possible to cut at specific points)  reproducible DNA fragments. Differences in molecular structure = location an d number of cuts that can be made are distinct  – profile of DNA is unique. Transferred  nylon sheet, radioactive marker identifies unique sequence of  DNA chain  placed onto X-ray film. Black bands appear (where markers attach  – segments used to establish identity)  Print from film, used to compare samples.

5.10 Gene Therapy -

-

-

3500+ genetic diseases liked to defective genes: e.g. cystic fibrosis, diabetes, hemophilia, Huntington’s chorea, sickle cell anemia Treatments: e.g. diabetics – insulin – insulin injections can control affliction. No true cures. Medical advances + modified diet + restricted behaviour + environmental adjustments  reasonably productive life. Potential of transforming the genetic gene Gene therapy: defective genes  – replaced with normal genes. 3 strategies: 1. Gene insertion: normal gene  position by a virus, or o ther agent, on chromosome of affected cell (not every cell uses gene) to function in the intended manner. 2. Gene modification: defective gene modified chemically  recode genetic message. More delicate + needs greater knowledge of chemical composition of normal + defective genes. 3. Gene surgery: defective gene removed + replaced w/ normal gene. First clinical use: 1990: 4 year old girl  – treatment  inherited disorder of immune system  enzyme deficiency  – insufficient amounts of enzyme ADA (adenosine deaminase). Genetically modified virus carried normal ADA gene  immune cells. = cells produced required enzyme. Viruses reproduce  DNA into normal body cells, reprogramming  production of new viruses. Can be modified  beneficial genetic material. Trick: inserting in appropriate place (no unexpected results) 1999: McGill Michel Tremblay, Dr. Brian Kennedy  found gene  controls product - enzyme tyrosine phosphatase, involved in regulating blood glucose levels. Significant role  diabetes mellitus (Type II diabetes) and obesity.  lead to gene therapy techniques to control problems. 2000: joint team: South Korea + Canada  treatment of juvenile (type I) diabetes in mice  gene therapy.