BIO 100 Chapter 9 notes
BIO 100 Chapter 9 notes BIO 100
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This 4 page Class Notes was uploaded by Lauren Tebbe on Wednesday September 28, 2016. The Class Notes belongs to BIO 100 at Eastern Kentucky University taught by Kenneth Blank in Fall 2016. Since its upload, it has received 7 views.
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Date Created: 09/28/16
Chapter 9 BIO notes 1 The Basics of Meiosis o Meiosis is a type of nuclear division that is important in sexual reproduction. o Serves 2 main functions: reduce chromosome number, and shuffle chromosomes and genes to make genetically different gametes known as sperm (males) and eggs (females). o Random fertilization results in variation making each offspring unique 2 Homologous Chromosomes o A picture of a chromosome is called a karyotype o Chromosomes occur in pairs since we have one from each parent giving us 23 pairs. o 22 pairs are called autosomes and the remaining are called sex chromosomes since they contain gender determining genes. o In the sex chromosome, the larger part is X and the smaller is Y. Females have 2 X and males have a single X and Y. o The 23 pairs (46) chromosomes make the diploid (2n) number half this number is haploid (n) number which identifies the different chromosomes in a cell and diploid goes with the fact we have 23 pairs. o Members of a chromosome pair are called homologous chromosomes or homologues. This is described by having the same size, shape, and location of centromere. o Each homologue may have different versions of a gene called alleles which occur at the same location on each homologue. 3 The Human Life Cycle o Life cycle with sexually reproducing organisms refers to all the reproductive events that occur in each generation o Human life cycle involves 2 types of nuclear division: mitosis and meiosis. o Meiosis reduces the chromosome number from the diploid to the haploid so that the gamete has one chromosome from each homologous pair of chromosomes. o Male: meiosis is part of spermatogenesis in the testes producing sperm o Females: meiosis is part of oogenesis in the ovaries producing eggs o These create the zygote which has a diploid number of homologous chromosomes. o Zygote because a human through mitosis o Meiosis followed by fertilization keeps the chromosomes number constant. 4 Overview of Meiosis o Results in four daughter cells because it consists of 2 divisions called meiosis I and II o Meiosis I is where homologous chromosomes of each pair come together lined up side by side and this takes place because of an event called synapsis. o Synapsis make a tetrad which is an association of 4 chromatids and the chromosomes of a tetrad stay in close proximity until they separate. o There are no rules that restrict what chromosomes go with what daughter nucleus so there is variation. o One role of meiosis is to reduce the number of chromosomes. o Chromosomes that are half the number of chromosomes but still duplicated are called dyads because they are composed of 2 sister chromatids. o In Meiosis II the sister chromatids of each dyad separate and become daughter chromosomes and have the haploid number of chromosomes o Meiosis also shuffles the genetic information between homologous chromosomes. When a tetrad forms, the nonsister chromatids exchange genetic material which is called crossing-over. o Nonsister chromatids share genetic info, sister chromatids have different combos of alleles so gametes are genetically different o Crossing-over increase the diversity of gametes and offspring o Meiosis is important because they help keep chromosome number constant and it introduces genetic variation 5 The Phases of Meiosis I o Same stages of mitosis occur in meiosis I and II (prophase, metaphase, anaphase, and telophase 6 The First Division - Meiosis I o During prophase 1, nuclear envelope fragments and nucleolus disappear and the spindle appears. Homologues undergo synapsis and produce tetrads. Crossing-over between nonsister chromatids happen during synapsis and jumbles up the alleles. o During Metaphase I, tetrads attach to the spindle and line up at the spindle equator with homologous chromosomes facing different poles giving variations of gametes. Shuffles chromosomes. o Homologous chromosomes separate during anaphase I o Reformation of nuclear envelope during telophase and cytokinesis where the daughter nuclei are haploid. o No replication of DNA occurs between meiosis I and II during a period of interkinesis. 7 The Second Division - Meiosis II o Events are the same as those for mitosis except the cells are haploid. o Prophase II is where the spindle appears and the nuclear envelope fragments and the nucleolus disappear. Dyads are present and each attach to the spindle. o Metaphase II is where the dyads line up at spindle equator and sister chromatids face opposite poles. o Anaphase II is where sister chromatids of each dyad separate and move toward poles. o Telophase II is where the spindle disappears and nuclear envelope forms. o Cytokinesis is where plasma membrane pinches off into haploid chromosomal cells. 8 Meiosis Compared with Mitosis o Meiosis requires 2 consecutive nuclear divisions; mitosis only requires 1. o Meiosis produces 4 daughter nuclei and there are 4 daughter cells after cytokinesis and there are only 2 after mitosis. o In meiosis, 4 daughter cells are haploid with half the chromosome number as parent and in mitosis the daughter cells have the same number. o In meiosis the cells are different genetically and in mitosis they are identical to the parent cells. 9 Meiosis I Compared with Mitosis o Synapsis occurs in prophase I of meiosis and does not occur in mitosis. o Tetrads align in spindle equator in metaphase I in meiosis and in mitosis the tetrads align separately at spindle equator. o In anaphase I of meiosis homologous chromosomes of each tetrad separate and dyads move to opposite poles and sister chromatids do not separate but in anaphase of mitosis, sister chromatids separate and move to opposite poles. 10 Meiosis II compared to Mitosis o Only difference is that in meiosis II the cells have haploid number of chromosomes. 11 Mitosis and Meiosis Occur at Different Times o Meiosis only occurs at certain times in the life cycle and only in specialized tissues. o The function is to provide gamete variation and keep chromosome number constant. o Mitosis occurs in all tissues during embryonic development and during growth and repair and its purpose is to keep chromosome number constant and same genetic material 12 Changes in Chromosome Number o 2n = 46 which is the normal number of chromosomes o Nondisjunction is what happens when people are born with abnormal numbers of chromosomes because they failed to separate correctly o 24 chromosomes instead of 23 the result is a trisomy because one type of chromosome is present in 3 copies o 22 chromosomes instead of 23 then it is monosomy because one type of chromosome is present in a single copy. 13 Down Syndrome o This is called trisomy 21 because it is a condition where an individual has 3 copies of chromosome 21. o Easily recognized: short stature, eyelid fold, stubby finger, wide gap between first and second toes, large fissured tongue, round head, pal, crease, and mental disabilities. 14 Abnormal Sex Chromosome Number o An inactive X chromosome mass is called a Barr Body o On the SRY gene located on short arm of Y chromosome produces testis-determining factor and creates male genitals. Individuals lacking a functional SRY on their Y chromosome have Swyer syndrome. o Turner syndrome (45 XO) is a female that is short, broad chest, webbed neck, genitals are underdeveloped, do not go through puberty or menstruate and breasts do not develop o Klinefelter syndrome is a male where the genitals are underdeveloped, no facial hair, breasts develop, large hands, feet, arms, ad legs
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