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Biology 1A03- Chapter 12 notes.docx

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Lovaye Kajiura

Chapter 12: Meiosis  Meiosis involves the production of 4 unique daughter cells Gamete- a haploid (n) reproductive cell that has one set of chromosomes Fertilization- occurs during the fusion of two gametes (one from each parent; sperm and egg) Colchicine- a chemical used to stop the cell cycle to take a digital picture of the chromosomes that they can be analyzed and matched up (staining them makes the matching up easy; matching allows scientists to understand what chromosomes cause what diseases). Sexual Reproduction in Humans  Brings 2 gametes (one from the male and one from the female) together  2 parents produce offspring (progeny)  Parents have 2n (diploid) nuclei  Gametes are n (haploid) and differ one from another gene in composition  Gametes have half the DNA content of each parent nucleus (n is 0.5 of 2n)  Each parent passes on half of its genes to its offspring  Offspring inherit a unique combination (mixture) of genes from both parents= Genetic Variation. Chromosomes  Composed of DNA and proteins  Carry genes (hereditary information)  Chromosomes of the same type, size and with the same genes at identical locations are called homologs or homologous chromosomes. Table 12.1 The Number of Chromosomes Found in Some Familiar Organisms The number of chromosomes varies for different species. Organism # of Different Types of Diploid Chromosome Chromosomes (haploid #) Number (2n) Human 23 46 Domestic Dog 36 72 Fruit fly 4 8 Chimpanzee 24 48 Bulldog ant 1 2 Garden pea 7 14 Corn (maize) 10 20 Genes  Units of heredity  Made of DNA (nucleic acid) located on chromosomes  Have specific sequences of nucleotides (monomers)  Most genes program cells to synthesize proteins  The actions of these proteins produce the organism’s inherited traits Alleles- are the different versions of a specific gene Sex chromosomes- are chromosomes that determine an individual’s sex Autosomes- are non-sex chromosomes Karyotype- a display of an individual’s chromosomes that is organized in terms of chromosome number, size and type. Box 12.1 Karyotyping Techniques To determine the karyotype the biologist would first obtain a sample of cells from the patient and grow them within a culture. When the culture cells begin to divide rapidly, they are treated with a compound called colchicine. This compound stops mitosis at metaphase by disrupting the formation of the mitotic spindle. By this point, the chromosomes are relatively easy to study, because they are condensed and consist of sister chromatids. These chromosomes are then stained and examined under a light microscope. Chromosomes can be distinguished by size, position of the centromere, the stripping or banding patterns that appear in response to some stains. Fig. 12.6 Human Chromosomes Unlike the orderly arrangement of chromosomes seen in a), condensed chromosomes that are undergoing mitosis are arranged randomly when first observed with the microscope. To determine a karyotype, a computer is used to separate the image of each condensed chromosome, place homologous pairs side by side, and arrange the homologs by number. b) shows the swapping of parts in the chromosome 9 and chromosome 22; this defect is called a translocation. This translocation causes a genetic change that leads to uncontrolled cell growth. c) shows the sex chromosomes seen in someone who has Klinefelter’s syndrome (two X chromosomes and one Y chromosome). Real World Examples Klinefelter Syndrome- XXY, XXXY, XXXXY, XXXXXY  Supplementary diagram CML (Chronic Myelogenous Leukemia)  Form of cancer  Portion of chromosome 22 (autosomal) switches with a small piece from chromosome 9  Treatment is available Why are Karyotypes Useful?  For genetic screening to identify chromosome defects in their number, size and type. Banding Patterns Locus- position of a gene along a chromosome (or along a DNA double helix) Summary Table 12.2 Vocabulary Used to Indicate the Chromosomal Makeup of a Cell 1. Prior to Meiosis  During Interphase, each chromosome in the 2n parent cell replicates Fig. 12.2 The Major Events of Meiosis Meiosis reduces chromosome number by half. In diploid organisms, the products of meiosis are haploid. Meiosis  Occurs in sexually reproducing individuals  Consists of two consecutive cell divisions, Meiosis 1 and Meiosis 2 Meiosis 1  Homologs from each pair of homologous chromosomes, separate and move to 2 different daughter cells. Meiosis 2  Sister chromatids of each chromosome separate and move to each of the daughter cells  At the end of meiosis 2 and cytokinesis, there are 4 haploid gametes each with one copy of each chromosome produced. Fig. 12.4 The Phases of Meiosis The phases of Meiosis are: Prophase 1, Metaphase 1, Anaphase 1, Telophase1 and Cytokinesis. 2. Early Prophase 1  Nuclear envelope breaks down and chromosomes condense  Replicated homologous chromosomes pair to form tetrads, this process is called synapsis Synapsis  Is precise  Homologous chromosomes align gene by gene  The exact mechanism which causes synapsis is still unknown, but it may involve a protein complex called the synaptonemal complex.  Synaptonemal complex- proteins draw the chromosomes together to cross over segments of the chromosomes; denature protein to determine what it effects. 3. Late Prophase 1  Crossing Over (recombination) occurs between two non-sister chromatids  Chiasma (singular)- one crossing over  Chiasmata (plural)- several crossing overs Fig. 12.7 A Closer Look at Chromosome Replication, Synapsis & Crossing Over Strain 1  Long chromosome 9, with knob  Kernels are coloured and waxy Strain 2  Short chromo
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