BIOL 1543 Custom Edition Lecture 11 Outline of Last Lecture I Like begets like more or less II Cells arise only from preexisting cells III Binary Fission IV THE EUKARYOTIC CELL CYCLE AND MITOSIS V The cell cycle consists of two major phases VI In the mitotic phase VII Interphase VIII Prophase IX Prometaphase X Metaphase Anaphase Telophase Cytokinesis XI Cytokinesis XII Review of the functions of mitosis XIII Anchorage cell density and chemical growth factors XIV Apoptosis XV Cancer XVI THE GENETIC BASIS OF CANCER XVII Carcinogen Outline of Current Lecture I A review of mitosis II Chromosomes occur in homologous pairs These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute III Gametes only have one set of chromosomes IV Meiosis V Meiosis I VI Meiosis II VII The steps of meiosis VIII A comparison of mitosis and meiosis IX Crossing over X How crossing over increases genetic variation XI ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE XII Down syndrome XIII Abnormal numbers of sex chromosomes XIV Human sex chromosome abnormalities Current Lecture I A review of mitosis II Chromosomes occur in homologous pairs a The somatic body cells anything other than egg or sperm of each species contain a specific number of chromosomes b Human cells 46 23 pairs of homologous chromosomes i 22 pairs are autosomes and one pair is the sex chromosomes ii The chromosomes of a homologous pair carry genes for the same characteristics at the same place or locus iii homologous chromosomes a pair of chromosome one from each parent that have corresponding gene sequences and pair during meiosis III Gametes only have one set of chromosomes a Cells with two sets of chromosomes are said to be diploid all somatic b Gametes eggs and sperm are haploid with a single set of chromosomes c Sexual life cycles involve the alternation of haploid and diploid stages i the zygote is the first diploid cell IV Meiosis a Meiosis reduces the chromosome number from diploid to haploid i Meiosis like mitosis is preceded by chromosome duplication AKA replication ii In meiosis though the cell divides twice to form four daughter cells b The purpose of meiosis is to produce gametes V Meiosis I a The first division meiosis I i Starts with synapsis the pairing of homologous chromosomes in tetrads four b In crossing over i Homologous chromosomes exchange corresponding segments c Meiosis I separates each homologous pair i And produces two daughter cells each with one complete set of chromosomes VI Meiosis II a Meiosis II is essentially the same as mitosis i There is no chromosome duplication ii The sister chromatids of each chromosome separate iii The result is a total of four daughter cells iv Each of which is haploid VII The steps of meiosis VIII A comparison of mitosis and meiosis a Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring i Each chromosome of a homologous pair differs at many points from the other member of the pair ii Random arrangements of chromosome pairs at metaphase I of meiosis lead to many different combinations of chromosomes in eggs and sperm iii Random fertilization of eggs by sperm greatly increases this variation Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 b Homologous chromosomes carry different versions of genes Combination 4 i The differences between homologous chromosomes are based on the fact that they can bear different versions of a gene at corresponding loci ii Bi convention the scientific community decided that Capital letters refer to dominant traits and lower case refer to recessive traits Coat color genes Eye color genes Brown Black C E C E C E c e c e Meiosis c White e Pink Tetrad in parent cell homologous pair of duplicated chromosomes Chromosomes of the four gametes IX Crossing over a Crossing over further increases genetic variability i Genetic recombination results from crossing over during prophase I of meiosis This increases variation further ii Crossing over An exchange of corresponding segments between homologous chromosomes iii crosses over at the chiasma X How crossing over increases genetic variation a new combinations are recombinant chromatids XI ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE a A karyotype is a photographic inventory of an individual s chromosomes i A karyotype is an ordered arrangement of a cell s chromosomes 1 presence of the Y chromosome indicates male b Accidents during meiosis can alter chromosome number i A person may have an abnormal number of chromosomes which causes problems ii Abnormal chromosome count is a result of nondisjunction 1 The failure of homologous pairs to separate during meiosis I leads to 0 normal gametes 2 The failure of sister chromatids to separate during meiosis II leads to only 2 normal gametes c Fertilization after nondisjunction in the mother i aneuploidy the condition of having more or less than the normal diploid number of chromosomes 2n 1 or 2n 1 only from the zygote forward XII Down syndrome a Down syndrome is caused by trisomy 21 an extra copy of chromosome 21 b The chance of having a Down syndrome child goes up with maternal age starting at 35yrs c trisomy 13 Patau syndrome XIII Abnormal numbers of sex chromosomes a Abnormal numbers of sex chromosomes do not usually affect survival i Nondisjunction can produce gametes with extra or missing sex chromosomes leading to varying degrees of malfunction in humans but not usually affecting survival 1 Kleinfelter syndrome for men XXY 2 Turner syndrome females X0 only one X XIV Human sex chromosome abnormalities