CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES Heredity transmission of traits from one generation to the next Variation differences in genes o Farmers use heredity variation to breed plants and animals for desired traits Genetics scientific study of heredity and variations INHERITANCE Genes hereditary units o Program specific traits that emerge as we develop from eggs to adults o Written in DNA Polymer of 4 diff nucleotides o Genes program cells to synthesize specific enzymes other proteins organisms inherited traits o Transmission of hereditary traits has molecular basis in the precise replication of DNA copies of genes that are passed from parent to offspring Gametes reproductive cells that are the vehicles that transmit genes from one generation to the next o Fertilization male female gametes unite Passes genes on of both parents to their offspring DNA is packaged into chromosomes within the nucleus Locus a gene s specific location along the length or a chromosome ASEXUAL REPRODUCTION o Mitosis A single individual is the sole parent passes copies of all its genes to its offspring Dna is copied allocated equally to 2 daughter cells The genomes of the offspring are exact copies of the parents genome Clone groups of genetically identical individuals Genetic differences in asexual reproduction results from DNA mutations SEXUAL REPRODUCTION the two parents MEIOSIS Two parents give rise to offspring that have unique combos of genes inherited from o Offspring vary genetically from their siblings and parents o Genetic variations are important consequences of sexual reproduction Life cycle generation to generation sequence of stages in the reproductive history of an organism from conception to production of its own offspring Somatic cell any cell other than those involved in gamete formation Karyotype resulting order of arranging chromosomes into pairs starting with the Homologous chromosomes 2 chromosomes in the pair have same length centrosome longest chromosome position staining pattern o Both chromosomes carry genes controlling the same inherited characters o Chromosomes X Y sex chromosomes Females XX Males XY Genes carried on X do not have counterparts on Y Y chromosomes has genes lacking on X o Autosomes other chromosomes o Occurrence of homolog pairs of chromosomes in each somatic cell is a consequence of our sexual origins o Inherit 1 chromosome from each pair from each parent 46 chromosomes 23 from each parent o n single set of chromosomes Diploid cell any cell w 2 chromosome sets 2n chromosomes For humans 2n 46 n 23 Haploid cell contain a single chromosome set o 22 autosomes 1 sex chromosome Unfertilized egg X chromosome Sperm X or Y chromosome fertilization union of gametes Fertilized egg zygote o Diploid o Contains 2 haploid sets of chromosomes Mitosis of zygote and its descendants generates all the somatic cells of the body Meiosis gametes sex cells o If gametes were made by mitosis they would be diploid like somatic cells o When two gametes fused the number of chromosomes would double o Meiosis reduces the of chromosomes in the gametes o Fertilization restores the diploid condition by combing 2 haploid sets VARIETY OF SEXUAL LIFE CYCLES Alternation of generation both diploid and haploid stages that are multicellular o Multicellular diploid stage sporophyte Spores sporophyte haploid cells Doesn t fuse with another cell Divides mitotically o Gametophyte multicellular haploid stage Give rise to gamete mitosis Fusion of 2 haploid gametes at fertilization diploid zygote o Then back to sporophyte generation MEIOSIS STAGES MEIOSIS 1 separates homologous chromosomes o Prophase 1 Chromosomes condense Homologs loosely pair along their lengths Aligned gene by gene Exchange of corresponding segments of DNA molecules by non sister Homologs are in SYNAPSIS held tightly together by proteins along chromatids is completed their lengths Synapsis ends in mid prophase Chromosomes pair and move apart slightly Sister chromatid cohesion Centrosome movement spindle formation nuclear envelope breakdown Microtubules attach to kinetochores homologs move toward metaphase plate o Metaphase 1 Pairs of homologs are arranged on the metaphase plate Both chromatids of one homolog are attached to kinetochore microtubules 1 chromosome in each pair facing each pole The other homolog are attached to microtubules from the opposite pole from one pole o Anaphase 1 Homologs move toward opposite poles guided by the spindle apparatus Sister chromatid cohesion persists at the centromere causing chromatids to move as a unit toward the same pole o Telophase 1 cytokinesis Each half of the cell has a complete haploid set of chromosomes Each chromosome is composed of 2 sister chromatids Include regions of non sister DNA Division of cytoplasm cytokinesis A cleavage furrow forms MEIOSIS 2 separates sister chromatids o Prophase 2 o Metaphase 2 Spindle apparatus forms Chromosomes move toward metaphase 2 plate Chromosomes are positioned on the metaphase plate Two sister chromatids of each chromosomes are NOT identical Kinetochores of sister chromatids are attached to microtubules extending from opposite poles o Anaphase 2 Breakdown of proteins holding sister chromatids together at the centromere allows the chromatids to separate Chromatids move toward opposite poles as individual chromosomes o Telephase 2 cytokinesis Nuclei form chromosomes condense cytokinesis forms Produces 4 daughter cells each with a haploid set of chromosomes Each of the 4 daughter cells is genetically distinct from the other daughter cells and from the parent cell CONDENSED SUMMARY OF MEIOSIS 1 Prophase replicated homologous chromosomes pair and exchange segments Metaphase chromosomes line up by homologous pairs Anaphase each pair of homologous chromosomes separates sister chromatids MEIOSIS 2 Telophase cytokinesis 2 haploid cells form each chromosome still consists of 2 Chromosomes sister chromatids finally separate Sister chromatids separate in anaphase 2 4 haploid daughter cells result containing unreplicated chromosomes PROPERTY DNA replication of divisions MITOSIS Occurs during interphase before mitosis begins One MEIOSIS Occurs during interphase before meiosis 1 begins Two Synapsis of homologous chromosomes Does not occur of daughter cells genetic composition Role in animal body 2 Each diploid 2n Genetically identical to parent cell Enables multicellular adult to arise from zygote Produces cells for growth repair asexual reproduction Meiosis 1
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