Bio 1107 Chapter 13 03 26 2013 Chapter 13 Meiosis and Cell Life Cycles Heredity the transformation of traits from one generation to the next Genetics scientific study of heredity and heredity variation 13 1 Genes hereditary units that are given from parents to offspring Genes translated into features traits Gametes reproductive cells Male sperm cells female eggs Majority of DNA in a Eukaryote is packaged into chromosomes in the nucleus Locus a genes specific location along the length of a chromosome Asexual Reproduction a single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes Offspring are clones Clone a group of genetically identical individuals Sexual reproduction two parents give rise to offspring that have unique combinations of genes inherited from the two parents 13 2 offspring Life cycle the generation to generation sequence of stages in the reproductive history of an organism from conception to production of its own Karotype a display of condensed chromosomes arranged in pairs Homologous chromosomes have the same length centromere position and staining pattern Sex chromosomes X and Y chromosomes not homologous Autosomes the other chromosomes Diploid cells Any cell with two chromosome sets abbreviated 2n For humans diploid number is 46 2n 46 Haploid cells single sets of chromosomes as found in gametes abbreviated n For humans haploid number is 23 n 23 The set of 23 consists of 22 autosomes plus a single sex chromosome Fertilization union of gametes culminating in fusion of their nuclei Zygote resulting fertilized egg Diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines Mitosis of the zygote and its descendent cells generates the somatic cells in the body Gametes are the cells not produced by mitosis Develop from specialized cells called germ cells in the gonads ovaries in females and testes in males Three types of life cycles alternation between meiosis and fertilization Humans Gametes are only haploid cells Meiosis occurs in germ cells during production of gametes Gametes undergo no further cell division prior to fertilization After fertilization diploid zygote divides by mitosis to produce a multicellular organism that is diploid Alternation of generations plants and some algae Includes both diplod and haploid stages that are multicellular Sporophyte multicellular diploid stage Spores haploid cells that are products of meiosis in the sporophyte The haploid spore does not fuse with another cell It instead divides mitotically to generate a multicellular haploid stage called the gameophyte Cells of gameophyte give rise to gametes by mitosis Fusion of two haploid gametes at fertilization results in a diploid zygote which develops into the next sporophyte generation Summary Sporophyte generation produces a gameophyte as its offspring and the gameophyte generation produces the next sporophyte generation Life cycle of Fungi and Protists multicellular offspring forming After gametes fuse and form a diploid zygote meiosis occurs without a Meiosis does not produce gametes but instead produces haploid cells These haploid cells divide by mitosis and give rise to either unicellular descendents or a haploid multicellular organism The haploid organism carries out further mitosis to produce the cells that develop into gametes Only diploid stage found in these species is the single celled zygote Either haploid or diploid cells can divide by mitosis depending on the cell Only diploid cells can undergo meiosis because haploid cells have a single set of chromosomes that cannot be further reduced 13 3 Meiosis reduced the number of chromosomes sets from diploid to Meiosis is preceded by duplication of chromosomes Single duplication followed by two consecutive cell divisions meiosis I and cycle haploid meiosis II The two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents Homologs may have different alleles versions of genes at corresponding loci Meiosis in an Animal Cell Mitosis v Meiosis All events unique to meiosis occur in meiosis I Synapsis and Crossing Over Duplicated chromosomes pair up during prophase I and the synaptonemal complex forms to hold them in synapsis Crossing over occurs at chiasmata Homologous pairs at the metaphase plate Chromosomes are positioned at the metaphase plate as homologs insteads of individual chromosomes like in mitosis Separation of homologs During anaphase I the duplicated chromosomes of each homologous pair move toward opposite poles but sister chromatids remain attached In anaphase of mitosis sister chromatids separate More info on Meiosis Sister chromatid cohesion in meiosis is released in two steps one at anaphase I and the second at anaphase II The combination of crossing over and sister chromatid cohesion result in the formation of chiasma Chiasma hold homologs together as the spindle forms for the first meiotic divison allows homologs to separate At onset of anaphase I the release of cohesion along sister chromatid arms At anaphase II the release of sister chromatid cohesion at the centromeres allows sister chromatids to separate Meiosis I is called reductional division because it halves the number of chromosome sets per cell from 2 to 1 Meiosis II is called the equational division because the sister chromatids separate to form haploid cells 13 4 Genetic Variation produced in sexual life cycles contributes to evolution Mutations are the original source of genetic diversity Result in the creation of different alleles Independent Assortment of Chromosomes occurring at metaphase I Crossing Over resulting in the production of recombinant chromosomes Recombinant chromosomes individual chromosomes that carry genes DNA derived from 2 different parents On average one to three crossovers occur per chromosome pair depending on size of chromosome and position of centromeres 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