4.4 Introduction to genetics


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Meiosis, karyotyping and mutations

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4.4 Introduction to genetics

  1. 1. 4. GENETICS
  2. 2. 4.1 Chromosomes,genes, alleles, and mutations
  3. 3. Inventory of your traitsTraits are observable characteristics, find out which traitsyour share with the rest of the class.First, fill out the inventory for yourselfSecond, tally up the results of your group and then addthem to the entire class.Third, draw a bar graph with the resultsHomework: make a graph using excel just like we did inclass
  4. 4. MeiosisMeiosis: A type of cell division that produces four cells, each withhalf the number of chromosomes as the parent cell.Homologous chromosomes: As a consequence of fertilization,humans have pairs of chromosomes with one chromosome in apair from each parent. The two chromosomes of each matchingpair are called homologous.Diploid: A nucleus with two chromosomes of each type. (2n)Haploid: A nucleus with only one chromosome of each type. (n)
  5. 5. The process of meiosisMeiosis I: Four stages. Previously, duringinterphase, the chromosomes replicate, sothat each chromosome has an identicalchromatid (2n). 1) Prophase I: Homologous chromosomes pair up and stick together along their length and exchange genetic material in the process called crossing over.
  6. 6. 2) Metaphase I:Homologous pairs(tetrads) move to theequator of the cell.Orientation of maternaland paternal on eitherside is random andindependent.3) Anaphase I:Homologous pairs areseparated and migrate toopposite poles of thespindle.
  7. 7. The process of meiosis4) Telophase I andCytokinesis: Each polenow has a haploiddaughter nucleus(eventhough eachchromosome consists oftwo chromatids). Noreplication occurs.Cytokinesis splits the cell.Nuclear membraneappears and chromosomesuncoil.
  8. 8. Meiosis II: Four stages as well.1) Prophase II: Chromosomes condense, nuclear envelopedisintegrates, and a spindle forms.2) Metaphase II: The chromosomes line up in the middle of thecell.
  9. 9. The process of meiosis3) Anaphase II: Centromeres separate and sisterchromatids are moved to opposite poles.4) Telophase II and cytokinesis: The chromatids reach thepoles, nuclear envelope forms, cytokinesis splits the cellagain, producing four haploid daughter cells as a finalresult.www.biology.com: Online activity 10. 4
  10. 10. Non-disjunctionIt occurs when homologous chromosomes fail to separate at anaphase.The result will be a gamete (sex cell) that either has an extrachromosome or is deficient in a chromosome.An abnormal number of chromosomes will often lead to a personpossessing a syndrome, a collection of physical signs or symptomsAn example is Down syndrome, or trisomy 21, which is due to a non-disjunction that leaves the individual with 3 chromatids of thechromosome 21, instead of 2Other examples of non-disjunction are Turner’s syndrome, Klinefelter’ssyndrome and Patau syndrome.
  11. 11. Karyotyping Karyotype: It is the number and type of chromosomes that the nucleus of a cell contains. To build a karyotype, the chromosomes are stained, a micrograph (microscopic photograph) is taken and then the chromosomes are arranged according to their size and structure, starting with the largest and ending with the smallest. As most cells are diploid, thechromosomes in a karyotype usually occur in pairs.
  12. 12. KaryotypingTwo procedures forobtaining the fetalchromosomes to producethe karyotype.1)Amniocentesis: involvespassing a needle throughthe mother’s abdominalwall, to withdraw a sampleof amniotic fluid from theamniotic sac of adeveloping fetus.2)Chorionic villussampling: This proceduresamples cells from theplacenta, specifically fromthe villi of thechorion, rather than theamniotic fluid.
  13. 13. Complete Human Karyotype
  14. 14. Class activityMake a Karyotype:Part I: Cut and paste the chromosomes in order. Rememberto look for similar sizes, band patterns, and location ofcentromeres.Part II: Cut and paste the chromosomes in order. Analyzethe karyotype and write down whether the patient is male,female and if they have a genetic disorder.
  15. 15. This graphic should help you locate bands andcentromeres.
  16. 16. Mutations2 kinds of mutations:a) Gene mutations: Produce changes in a single genea1) Point mutations: Involve changes in one or a fewnucleotides, they occur at a single point in the DNAsequence = substitutions.a2) Frameshift mutations: They shift the reading frame(every codon) of the genetic message, that changes everyamino acid that follows the point of mutation = insertionor deletion.
  17. 17. Mutationsb) Chromosomal mutations: Involve changes in thenumber or structure of chromosomes. They may changethe locations of genes on chromosomes and even thenumber of copies of some genes =deletions, duplications, inversions and translocations.
  18. 18. A. Deletion B. InversionC. Translocation D. Duplication