Topic 4: Genetics


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Topic 4: Genetics

  1. 1. Topic 4Genetics
  2. 2. Genes• Gene: a heritable factor that controls aspecific characteristic• Basic unit of inheritance• Genome: the whole of the geneticinformation of an organism
  3. 3. Chromosomes• Chromosomes: large DNA moleculesmade up of genes• Gene locus: the position of a gene on achromosome– In any particular type of chromosome thesame genes are arranged in the samesequence
  4. 4. Alleles• Allele: a form of a gene, differing fromother alleles of that gene by a few bases;occupying the same gene locus as theother alleles of that gene• Alleles can be recessive or dominant
  5. 5. Structure of Chromosomes• Chromatids: the two parts of achromosome• Centromere: connects the chromatids
  6. 6. Gene Mutation• Gene mutation: change to the basesequence of a gene• Base substitution: one base in a gene isreplaced by another base• Many gene mutations cause diseases– Example: sickle cell anemia
  7. 7. Haploid and Diploid• Diploid: two full sets of chromosomes; cellhas two of each type of chromosome• Haploid: only one full set of chromosomes;cell has one of each type of chromosome• Homologous: chromosomes with the samegenes in the same sequence, but notnecessarily the same alleles of the genes• Reduction Division: the number ofchromosomes is halved; change from diploidto haploid
  8. 8. Stages of Meoisis1. Prophase I2. Metaphase I3. Anaphase I4. Prophase II5. Anaphase II6. Telophase II
  9. 9. Prophase I• Homologous chromosomes pair up• Spindle microtubules grow from each poleto the equator• The nuclear membrane begins breakingdown
  10. 10. Metaphase I• The pairs of chromosomes line up on theequator• Spindle microtubules attach to differentchromosomes from each pair
  11. 11. Anaphase I• The homologous chromosomes are pulledfrom their pairings to opposite poles– This halves their chromosome number– Each chromosome still has two chromatidsheld with a centromere• Cell membrane will pull in to divide the cellin two between the poles
  12. 12. Prophase II• The cell has divided to form two haploidcells• New spindle microtubules grow from thepoles to the equator in both cells
  13. 13. Anaphase II• Plasma membrane around the chromosomesis gone• Spindle microtubules have attached to thechromosomes• Centromeres have divided• Chromatids pulled to opposite poles• Cell membrane pulls inward to divide cells
  14. 14. Telophase II• There are now four haploid cells from onediploid cell• Nuclear membranes have reformed• Each nucleus now has half as manychromosomes as the parent cell
  15. 15. Karyotypes• Karyotype: the number and appearance ofchromosomes in an organism• Living organisms that are members of thesame species usually have the samekaryotype
  16. 16. Deduce Gender• Amniocentesis:– Amniotic fluid is removed from amniotic sac aroundthe fetus– Needle draws out amniotic fluid that contains cell fromthe fetus• Chorionic villus sampling– Cells are removed from fetal tissues in the placenta– They are incubated with chemicals that stimulatethem to divide by mitosis– A fluid is used to spread out the chromosomes– A microscopic image is taken of the chromosomesand then arranged into pairs according to their sizeand structure (this process is karyotyping)
  17. 17. Analysis of Karyotypes• Can identify gender or abnormalities• Non-disjunction: non-separation ofchromosomes– Gametes are produced with either onechromosome too many or too few• Games with one chromosome too fewusually die; one chromosome too manysometimes survive
  18. 18. Mendel’s Monohybrid Crosses• Mendel crossed varieties of pea plants– Round and wrinkled peas• F1 Generation: first set of offspring• F2 Generation: offspring of the offspring• Inheritance is based on factors that can bepassed on from generation to generation
  19. 19. Genetic Terms• Homozygous: two identical alleles of agene• Heterozygous: two different alleles of agene• Dominant Allele: an allele that has thesame effect on the phenotype whether it isheterozygous or homozygous• Recessive Allele: an allele that effects thephenotype only in the homozygous state
  20. 20. Pedigree Charts• Shows the membersof a family and howthey are related toeach other• Circles = femalesSquares = males
  21. 21. Sex Chromosomes and Gender• Two sex chromosomes determine thegender of a child• X chromosome is relatively large andcarries many genes• Y chromosome is much smaller andcarries only a few genes• XX = Female, XY = Male• Women pass on X; Men pass on X or Y
  22. 22. Sex Linkage• Sex Linkage: association of acharacteristic with gender, because thegene controlling the characteristic islocated on a sex chromosome
  23. 23. Choosing Symbols for Alleles• One dominant and one recessive allele:– A letter is chosen (Example: B)– Dominant allele is capitalized (B); recessive allele islower case (b)• Co-dominant alleles:– A letter is chosen (Example: C)– This letter and a superscript letter represent eachallele (Example: Cw and CJ)• Sex-linked dominant and recessive alleles– Letter X shows X chromosome; Y shows Ychromosome– Example: XH XG
  24. 24. Using Pedigree Charts• Can deduce whether a character iscaused by a dominant or recessive alleleand whether or not it is sex-linked• Can deduce the genotypes of individuals
  25. 25. Using Test Crosses• Test cross: an individual that might beheterozygous is crossed with an individualthat is a homozygous recessive• This allows you to deduce the genotype ofthe unknown organism being tested
  26. 26. Polymerase Chain Reaction• Polymerase Chain Reaction: DNA iscopied repeatedly to produce many copiesof the original molecules• DNA from very small samples can beamplified using a DNA polymeraseenzyme from Thermus aquaticus
  27. 27. Gel Electrophoresis• Gel Electrophoresis: method of separatingmixtures of proteins, DNA or other molecules thatare changed• Stages:1. The mixture is placed on a thin sheet of gel2. An electric field is applied to the gel by electrodes onboth ends3. Positively and negatively charged particles will movetowards their respective electrodes• The rate of particle movement depends on thesize and charge of the molecules– The smaller the moelcule, the faster
  28. 28. DNA Profiling• Satellite DNA: short sequences of basesthat are repeated many times• Using PCR and Gel Electrophoresis, theDNA can be distinguished• Can be used for forensic investigations(court cases) and paternity investigations(finding the father)
  29. 29. Genetic Modification• Genetically Modified Organisms: Organismsthat have had genes transferred to them• Two examples:– Transfer of a gene for blood clotting fromhumans to sheep, where it is produced in thesheep’s milk– Transfer of a gene for resistance to theherbicide glyphosate from a bacterium to acrop, allowing the crop to be sprayed with theherbicide
  30. 30. Clones and Cloning• Clone: A group of genetically identicalorganisms or genetically identical cellsderived from a single parent cell• Reproductive cloning: creating moreorganisms from a parent with a desirablecombination of characteristics• Therapeutic cloning: producing skin orother tissues needed to treat a patient
  31. 31. Plant and Animal Cloning• Plant cloning is simple: take a piece fromthe root, stem, or leaf• Animals are not as simple: it is hard topredict which embryos will develop intoanimals with desirable characteristics andshould therefore be cloned
  32. 32. Cloning Techniques• Stages1. Udder cells taken from a donor sheep2. Unfertilized egg cells taken from another sheep3. Nucleus is removed from each egg cell4. Egg cells are fused with donor cells usingelectricity5. Fused cells become an embryo6. Embryo implanted into a surrogate mother7. Lam successfully born genetically identical tosheep who donated udder cells
  33. 33. Therapeutic Cloning in HumansArguments for therapeutic cloning: Arguments against therapeutic cloning:Embryonic stem cells can be usedfor therapies that save lives andreduce sufferingEvery human embryo is a potentialhuman being, which should be given achance of developingCells can be removed from embryosthat have stopped developing, sowould have died anywayMore embryos may be produced thanneeded, so some may have to be killedCells are removed at a stage whenembryos have no nerve cells andcannot feel painThere is a danger of embryonic stemcells developing into tumor cells
  34. 34. Human Genome Project• Human Genome Project: aims to find thelocation of all of these genes on the humanchromosomes and the base sequence of allof the DNA that makes them up• International cooperative project• It will make it easier to study how genesinfluence human development, easieridentification and curing of genetic diseases,and new insights into the evolution ofhumankind