Biology 111: Concepts of Biology Ch. 9
Biology 111: Concepts of Biology Ch. 9 BIOL 111
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This 7 page Class Notes was uploaded by Megan Giesler on Sunday February 21, 2016. The Class Notes belongs to BIOL 111 at University of North Dakota taught by Christopher Felege in Spring 2016. Since its upload, it has received 36 views. For similar materials see Concepts of Biology in Biological Sciences at University of North Dakota.
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Date Created: 02/21/16
Biology 111: Concepts of Biology Chapter 9 9.1 The Basics of Meiosis • Meiosis serves 2 major functions • Reducing chromosome number • Shuffling chromosomes in the cell to produce genetically different gametes • Homologous chromosomes • Members of a pair of chromosomes • Also called homologues • Have the same size, shape, and construction (location of centromere) • Contain the same genes for the same traits • A child receives one member of each homologous pair from each parent • Homologous pairs may contain different versions of the same gene • Alleles • Alternate forms of a gene • Both males and females have 23 pairs of chromosomes • 23 pairs or 46 total chromosomes = diploid (2n) • Haploid number (n) in gametes • 23 total chromosomes • 22 pairs of autosomes • Chromosomes not involved in sex determination • 1 pair of sex chromosomes • XX female or XY male • Human life cycle • Life cycle • In sexually reproducing organisms refers to all the reproductive events that occur from one generation to the next • Involves both mitosis and meiosis • Mitosis involved in continued growth of a child and repair of tissues throughout life • As a result, somatic (body) cells are diploid • Meiosis reduces the chromosome number from diploid to haploid • Gametes (egg and sperm) have only 1 member of each homologous pair • Spermatogenesis produces sperm in the testes • Oogenesis produces eggs in the ovaries • Egg and sperm join to form diploid zygote • Overview of meiosis • Results in 4 daughter cells • Before meiosis I, each chromosome has duplicated • Two divisions • Meiosis I • Homologous pairs line up during synapsis resulting in tetrad • Homologous chromosomes of each pair then separate • Meiosis II • No duplication of chromosomes (no interphase) • Chromosomes are dyads—composed of two sister chromatids • Sister chromatids are separated • Two daughter nuclei separate • Crossing-over • During prophase I, homologous chromosomes pair up and form a tetrad, a process called synapsis. • Each tetrad consists of two chromosomes, with each chromosome containing two chromatids, for a total of four chromatids. • When a tetrad forms during synapsis, chromatids from homologous chromosomes (non-sister chromatids) may exchange genetic material. • Increases variability of the gametes and, therefore, the offspring • The importance of meiosis • Chromosome number stays constant in each new generation by producing haploid gametes • Generates genetic variations • Crossing-over • Every possible combination of chromosomes can occur in daughter cell • Fertilization produces new combinations • (2 ) or 70,368,744,000,000 chromosomally different zygotes are possible, even assuming no crossing-over 9.2 The Phases of Meiosis • Meiosis involves two divisions: meiosis I and meiosis II Each division is broken down into four phases: • Prophase (I and II) • Metaphase (I and II) • Anaphase (I and II) • Telophase (I and II) 9.3 Meiosis Compared with Mitosis • Meiosis • Meiosis occurs only at certain times of the life cycle of sexually reproducing organisms and only in specialized tissues. • Two consecutive nuclear divisions • Results in 4 daughter cells • The daughter cells are: • Haploid and have 23 chromosomes • Not genetically similar to each other or parent cell • Can possibly get the best or worst genes from parents • The other two are leave a copy of parents chromosomes • Mitosis • For growth and repair • Only one nuclear division • Results in 2 daughter cells • Daughter cells have 46 chromosomes • Are genetically identical to each other and parent cell • Happens more often • Occurs in all tissues during embryonic growth • Also occurs during growth and repair • Meiosis I compared to mitosis • During prophase I of meiosis, synapsis occurs. • During metaphase I of meiosis, tetrads align at the spindle equator, with homologous chromosomes facing opposite spindle poles and the paired chromosomes have a total of four chromatids each; during metaphase in mitosis, dyads align separately at the spindle equator. • Sister chromatids do not separate during anaphase I; during anaphase of mitosis, sister chromatids separate, becoming daughter chromosomes that move to opposite poles. • Meiosis II compared to mitosis • The events of meiosis II are just like those of mitosis except that in meiosis II, the cells have the haploid number of chromosomes. • Mitosis and Meiosis Occur at Different Times • Meiosis occurs only at certain times of the life cycle of sexually reproducing organisms and only in specialized tissues. 9.4 Changes in Chromosome Number • Nondisjunction Meiosis I • Both members of a pair go into the same daughter cell Meiosis II • Sister chromatids fail to separate • Trisomy 3 copies of a chromosome • Down syndrome (trisomy 21) • Monosomy Single copy of a chromosome • Down syndrome Trisomy 21 Recognizable characteristics • Short stature, eyelid fold, stubby fingers, mental disabilities Chance of a woman having a Down syndrome child increases rapidly with age, starting at about 40. • Abnormal sex chromosome number Too few or too many X or Y chromosomes Newborns with abnormal sex chromosome numbers are more likely to survive than those with abnormal autosome numbers. • Extra X chromosomes become Barr bodies • Inactivated Y determines maleness • SRY (sex-determining region Y) gene on Y chromosome Turner syndrome (45, XO) • Absence of second sex chromosome • Female Klinefelter syndrome (47, XXY) • Extra X inactivated as Barr body • Male
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