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Chapter 12

Textbook and Class Notes Collaborated - Week 5+6 - Unit 3 - Chapter 12 Bio 1A03

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Xudong Zhu

Unit Three: Gene Structure and Expression Chapter 12: Meiosis  Ultimate explanation – explains why something happens; evolutionary nature  Proximate explanation – explains how something happens; mechanic in nature  Sexual reproduction allows for genetic variation  A sperm (male reproductive cell) and an egg (female reproductive cell) unite in a process called fertilization  Meiosis – nuclear division that leads to a halving of chromosome number o Formation of gametes (egg and sperm cells) o Occurs in sexually reproducing individuals 12.1 How Does Meiosis Occur?  Gamete – a haploid (n) reproductive cell that has one set of chromosomes  Fertilization – occurs during the fusion of two gametes (one from each parent)  Sexual Reproduction in Humans o Brings 2 gametes (one from the male and one from the female) together o 2 parents produce offspring (progeny) o parents have 2n (diploid) nuclei o gametes are n (haploid) and differ one from another in gene composition o gametes have half the DNA content of each parent nucleus (n is ½ of 2n) o each parent passes on have of its genes to its offspring o offspring inherit a unique combination (mixture of genes from both parents  genetic variation  Chromosomes o Composed of DNA and proteins o Carry genes (hereditary information) o Homologs/Homologous Chromosomes – chromosomes of the same type size and with the same genes at identical locations  Carry the same genes but may carry different alleles  Genes o Units of heredity o Made of DNA (nucleic acid) located on chromosomes o Have specific sequences of nucleotides (monomers) o Most genes program cells to synthesize proteins o The actions of these proteins produce the organisms inherited traits  Alleles – are the different versions of a specific gene  Sex Chromosomes – are chromosomes that determine an individuals sex o Female (human) have XX o Male (human) have XY  Autosomes – are non-sex chromosomes  Haploid Number and Ploidy o Haploid number “n” – the number of distinct types of chromosomes in a given cell o Ploidy – the combination of the number and n o Diploid (2n) – when an organisms cells contain two versions of each chromosome o Haploid (n) – when an organisms cells contain one version of each chromosome  Do not contain homologous chromosomes; have one allele of each gene  Karyotype – the number and type of chromosomes present; a display of an individuals chromosomes that is organized in terms of chromosomes and type o Used to organize chromosomes by length o Currently organize chromosomes by number o Chromosomes are only readily visible when they are condensed during mitosis and meiosis  Obtain cells during cell division o Collect cells and grow the cells in culture o Treat cells with colchicine, which disrupts the formation of the mitotic spindle, stopping mitosis at metaphase  Chromosomes are easily studied because they are condensed and consist of sister chromatids o Chromosomes are then stained and examined with a light microscope o Karyotypes are useful for genetic screening to identify specific chromosomal defects in their number, size and type o Researchers can distinguish condensed chromosomes by size, position of the centromere and by striping or banding patterns that appear in response to some stains  Banding Patterns – Locus – positions of a gene alone a chromosome (or along a DNA double helix) o Spectral Karyotyping (SKY) or Chromosome Painting  Done with fluorescent dyes that are attached to short DNA molecules  The dyed pieces of DNA bind to particular regions of particular chromosomes  By using a combination of dyes, technicians can give each pair of homologous chromosomes a distinctive suite of colors  Allows physicians to diagnose an array of chromosomal abnormalities  Figure 12.6a, page 266; Karyotype of Human Chromosome o Real World Examples  CML (Chronic Myelogenous Leukemia)  Translocation- swapping of chromosome segments  A large portion of Chromosome 22 switches with a small piece from Chromosome 9, easily recognizable shortened chromosome 22 (called the Philadelphia chromosome)  Is a cancer, which affects the cells that give rise to white blood cells  Figure 12.6b, page 266  Klinefelter syndrome  Males have an abnormal of sex chromosomes, XXY  Males have very small testes and are sterile  Frontal baldness absent  Fewer chest hairs  Female-type pubic hair pattern  Poor beard growth  Long legs  Males display feminine body contours (enlargement of breast tissue, narrow shoulders, wide hips)  Have normal intelligence  Occur in about 1 in 1000 male births  Figure 12.6c, page 266  Cri Du Chat Syndrome  Deletion on chromosome 5  Age range > 10 years old  1 in 50000  Abnormal glottis and larynx  Size of deletion on chromosome 5 influences physical and mental skills  Anatomical malformations, G1 and cardiac complications  Severly mentally challenged o Requires special care and schooling An Overview of Meiosis  Summary Table 12.2, page 260; Vocabulary  Before undergoing meiosis, cells replicate each of their chromosomes – at the start of the process, chromosomes are in the same state they are in prior to mitosis o Each chromosome contains two identical sister chromatids – contain the same genetic information and are physically joined at the centromere and their entire length o Unreplicated chromosomes consist of a single DNA molecule and its associated protein – single chromosome o Replicated chromosome consists of two sister chromatids –single chromosome  Meiosis consists of two cell divisions o Meiosis I – Diploid (2n) parent cell produces two haploid (n) daughter cells  Homologs from each pair of homologous chromosomes, separate and move to 2 different daughter cells  End result – daughter cells have one of each type of chromosomes instead of two – half as many chromosomes as parent cell o Meiosis II – equivalent to mitosis occurring in a haploid cell  Sister chromatids from each chromosome separate  Sister chromatids of each chromosome separate and move to each of the daughter cells  At end of meiosis II and cytokinesis, there are 4 haploid gametes each with one copy of each chromosome produces  End result – have one of each type of chromosomes, but not eh chromosomes are unreplicated o Figure 12.2 a and b, page 261; Meiosis  Two gametes fuse during fertilization forming a zygote, full complement of chromosomes is restored o Figure 12.2c, page 261; Meiosis  Gametogenesis – formation of egg or sperm cells  Maternal or paternal chromosomes – homologous chromosomes from mother or father (respectively)  Figure 12.3, page 262; Ploidy Changes during the Life Cycle of a Dog  Reduction Division o Reduces chromosome number from 2n (diploid) to n (haploid) o Daughter cells have half the number of parent chromosomes o Union of two haploid (n) gametes during fertilization restores the diploid (2n) number Phases of Meiosis I  The Phases of Meiosis I o Prophase I o Metaphase I o Anaphase I o Telophase I o Cytokinesis 1. Prior to Meiosis  During interphase, each chromosome in the 2n parent cell replicates 2. Early Prophase I o Nuclear envelope breaks down o Chromosomes condense o Spindle apparatus forms o Synapsis – replicated homologous chromosomes pair to form tetrads o Tetrad – consists of two homologous chromosomes, each consisting of two sister chromatids o Non-sister chromatids – chromatids from different homologs (one from mother, one from father) o Is precise o Homologous chromosomes align gene by gene o The exact mechanism which causes synapsis is still unknown, but it may involve a protein complex called the synaptonemal complex  When homologs synapse, two pairs of non-sister chromatids are brought close together and are held there by the synaptonemal complex (network of proteins)  Figure 12.7, page 267; Replication, Synapses and Crossing Over o Regions of homologous chromosomes that are similar at the molecular level attract one another via mechanisms that are being researched o Spindle fibers attach to the kinetochores at the centromeres of chromosomes 3. Late Prophase I o Non-sister chromatids begin to separate but remain joined at certain locations o Crossing Over (recombination) – maternal and paternal chromatids break and rejoin each chiasma, producing chromatids that have both paternal and maternal segments o Occurs between two non-sister chromatids o Chiasma (sing. abbreviated Xma) – one crossing over o Chiasmata (pl. abbreviated Xta) – several crossing over o Evidence for Crossing Over (Creighton and McClintocks Experiments)  Harriet Creighton and Barbara McClintock studied Zea mays (corn  Strain 1  Long chromosome 9, with knob  Kernels are colorless and waxy  Strain 2  Short chromosome 9, no knob  Kernels are colorless and starchy  Strain 1 x Strain 2 (strain 1 is crossed with strain 2)  Chiasmata form between non-sister chromatids  Some offspring have 1. Long chromosomes 9, no knob Kernels are colorless and waxy 2. Short chromosome 9, no knob Kernels are colored and starchy 3. Same as parental types  Conclusion o Physical exchange of DNA (knob) took place o They are displayed in phenotypes o Crossing over has occurred 4. Metaphase I o Tetrads line up along the metaphase plate by spindle fibres o Each tetrad moves to the metaphase plate independently of the other tetrads o The alignment of maternal and paternal homologs from each chromosome is random 5. Anaphase I o Homologs separate and start migrating to the opposite poles of the cell o Disjunctional segregation occurs during Anaphase I 6. Telophase I and Cytokinesis o Chromosomes finish their movement to the opposite sides of the cell o Cytokinesis divides the cytoplasm to form 2 haploid daughter cells  End Of Meiosis I o One chromosome of each homologous pair is distributed to a different dau
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