BioNotesChapter13MeiosisandSexualLifeCycles.pdf BIOL 1020 - 001
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This 4 page Class Notes was uploaded by Emma Karlson on Saturday October 24, 2015. The Class Notes belongs to BIOL 1020 - 001 at Auburn University taught by Dr. Zanzot in Summer 2015. Since its upload, it has received 21 views. For similar materials see Principles of Biology in Biological Sciences at Auburn University.
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Date Created: 10/24/15
Chapter 13 Meiosis and Sexual Life Cycles Overview Variations on a Theme Living organisms are distinguished by their ability to reproduce their own kind Genetics is the scientific study of heredity and variation Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Mother gives you something that your father did not the mitochondria in you Inheritance of Genes Genes are the units of heredity and are made up of segments of DNA Genes are passed to the next generation via reproductive cells called gametes sperm and eggs Each gene has a specific location called a locus on a certain chromosome Most DNA is packaged into chromosomes Comparison of Asexual and Sexual Reproduction In asexual reproduction a single individual passes genes to its offspring without the fusion of gametes A clone is a group of genetically identical individuals from the same parent In sexual reproduction two parents give rise to offspring that have unique combinations of genes inherited from the 2 parents Set of Chromosomes in Human Cells Human somatic cells any cell other than a gamete have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of chromosomes from a cell The 2 chromosomes in each pair are called homologous chromosomes or homologs Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters Sex chromosomes 0 Determine the sex of the individual 0 Called x and y In humans 0 Females have a homologous pair of X chromosomes XX 0 Males have one X and one Y chromosome The remaining 22 pairs of chromosomes are called autosomes A gamete sperm or egg contains a single set of chromosomes and is haploid n For humans the haploid number is 23 n23 Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg ovum the sex chromosome is X In a sperm cell the sex chromosome may be either X or Y Behavior of Chromosome Sets in the Human Life Cycle Fertilization is the union of gametes the sperm and the egg The fertilized egg is called a zygote and has one set of chromosomes from each parent The zygote produces somatic cells by mitosis and develops into an adult The Variety of Sexual Life Cycles The alternation of meiosis and fertilization is common to all organisms that reproduce sexually The tree main types of sexual life cycles differ in the timing of meiosis and fertilization Concept 133 Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis meiosis is preceded by the replication of chromosomes Meiosis takes place in 2 sets of cell divisions called meiosis I and meiosis 11 The 2 cell divisions result in 4 daughter cells rather than 2 daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell The Stages of Meiosis After chromosomes duplicate two divisions follow 0 Meiosis I reductional division homologs pair up and separate resulting in 2 haploid daughter cells with replicated chromosomes 0 Meiosis II equational division sister chromatids separate The result is four haploid daughter cells with unreplicated chromosomes A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from 2 diploid to one haploid producing cells that differ genetically from each other and from the parent cell Three events are unique to meiosis and all three occur in meiosis I o Synapsis and crossing over in prophase I Homologous chromosomes physically connect and exchange genetic information 0 At the metaphase plate there are paired homologous chromosomes tetrads instead of individual replicated chromosomes 0 At anaphase I it is homologous chromosomes instead of sister chromatids that separate Concept 134 Genetic variation produced in sexual life cycles contributes to evolution Mutations changes in an organism s DNA are the original source of genetic diversity Mutations create different versions of genes called alleles Reshuf ing of alleles during sexual reproduction produces genetic variation Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Three mechanisms contribute to genetic variation 0 Independent assortment of chromosomes 0 Crossing over 0 Random fertilization Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2 where n is the haploid number For humans n23 there are more than 8 million 223 possible combinations of chromosomes Crossing Over Crossing over produces recombinant chromosomes which combine DNA inherited from each parent Crossing over begins very early in prophase I as homologous chromosomes pair up gene by gene In crossing over homologous portions of two non sister chromatids trade places Crossing over contributes to genetic variation by combining DNA from 2 parents into a single chromosome Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum unfertilized egg The fusion of 2 gametes each with 84 million possible chromosome combinations from independent assortment produces a zygote with any of about 70 trillion diploid combinations The Evolutionary Significance of Genetic Variation Within Populations Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population which originates form mutations Definitions you should know Genetics gene chromosome locus Gamete sexual reproduction asexual reproduction clone Somatic cell karyotype homologous chromosomes Haploid vs diploid sex chromosome vs autosome Zygote fertilization life cycle What should you know about meiosis Purpose and products of meiosis Stages of meiosis o Prophase I metaphase I anaphase I telophase I interkinesis o Prophase II metaphase II anaphase II telophase 11 Differences between meiosis and mitosis Mechanisms of variation in meiosis Crossing over Independent assortment Fertilization Can you draw and label all the steps in meiosis
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