Variations on a Theme Inheritance hereditary transmission of traits from one generation to the next Can be variation Genetics scientific study of hereditary variation Meiosis and fertilization maintain a species chromosome count during the sexual life cycle Offspring Acquire Genes from Parents by Inheriting Chromosomes Inheritance of Genes Genes coded information in hereditary units Genetic link to parents Program specific traits that emerge as we develop Genetic program written in language of DNA Inherited information is passed on in form of each gene s specific sequence of DNA nucleotides Most genes program cells to synthesize specific enzymes and other proteins whose actions produce inherited traits Gametes vehicles that transmit genes from one generation to the next During fertilization male and female gametes sperm eggs unite to pass on genes of both parents to their offspring DNA of a eukaryotic cell is packaged into chromosomes within nucleus Lotus a gene s specific location along the length of a chromosome Comparison of Asexual and Sexual Reproduction Asexual reproduction single individual is sole parent and passes copies of all its genes to its offspring without the fusion of gametes Reproduce by mitotic cell division DNA is copied allocated equally to to daughter cells Clone genetically identical individuals Sexual reproduction two parents give rise to offspring that have unique combinations of genes inherited from the two parents Genetic variations Fertilization and Meiosis Alternate in Sexual Life Cycles Life Cycle generation to generation sequence of stages in the reproductive history of an organism from conception to production of its own offspring Sets of Chromosomes in Human Cells Somatic cells have 46 chromosomes 23 pairs of chromosomes Karyotype resulting ordered display of the chromosomes pairing up Homologous chromosomes homologs two chromosomes with same length centromere position and staining pattern create karyotype Carry genes controlling same inherited characters Sex chromosomes X Y chromosomes break homologous rule Autosomes all other chromosomes 46 chromosomes is formed from a maternal set of 23 and a paternal set of 23 Fertilization and Meiosis Alternate in Sexual Life Cycles Sets of Chromosomes in Human Cells cont n represents number of chromosomes Diploid cell any cell with two chromosome sets 2n somatic cells After DNA synthesis all chromosomes are replicated and consist of two identical sister chromatids associated closely at centromere and along the arms Haploid cell cells with a single set of chromosome n gametes Behavior of Chromosome Sets in the Human Life Cycle Fertilization union of gametes from male sperm and female egg culminating in fusion of their nuclei Zygote fertilized egg diploid from containing the two Mitosis of zygote generates all somatic cells of body Gametes are only cells not produced by mitosis develop from germ cells Meiosis reduces number of chromosomes from two to one in the gametes to counterbalance the doubling that occurs in fertilization The Variety of Sexual Life Cycles Human Cells Meiosis occurs in germ cells during the production of gametes After fertilization the diploid zygote divides by mitosis producing a diploid multicellular organism Alternation of Generations includes diploid and haploid stages that are multicellular plants Sporophyte multicellular diploid stage Meiosis produces haploid spores Doesn t fuse with another cell but divides mitotically Gametophyte haploid stage generated by spore s mitotic division Give rise to gametes by mitosis Fusion of two haploid gametes at fertilization results in a diploid zygote Sporophyte produces a gametophyte as its offspring which then produces the sporophyte generation Fungi and protists After gametes fuse to form a diploid zygote meiosis occurs without a multicellular diploid offspring developing Meiosis produces haploid cells that divide by mitosis and give rise to either unicellular descendents or haploid multicellular adult organism Haploid organism carries out further mitosis producing cells that develop into gametes All cells can undergo mitosis only diploid cells undergo meiosis Meiosis Reduces the Number of Chromosome sets from Diploid to Haploid page 254 255 Meiosis I and Meiosis II result in four daughter cells with only half as many chromosomes as parent cell The Stages of Meiosis Fig 13 7 The two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents Homologs may have different versions of a gene called alleles at corresponding loci Reduces number of sets of chromosomes from 2 to 1 with each daughter cell receiving one set of chromosomes A Comparison of Mitosis and Meiosis Fig 13 9 Meiosis reduces the number of chromosome sets from two diploid to one haploid Mitosis conserves the number of chromosome sets Meiosis produces cells that differ genetically from their parent cell and from each other Mitosis produces daughter cells genetically identical to parent cells Three unique events occur in Meiosis I 1 Synapsis and crossing over Duplicated homologs pair up during prophase and synaptonemal complex holds them together Crossing over occurs 2 Homologous pairs at the metaphase plate Chromosomes line up as pairs of homologs instead of as individual chromosomes 3 Separation of homologs At anaphase I the duplicated chromosomes of each homologous pair move toward opposite poles Sister chromatids of each duplicated chromosome remain attached Sister chromatids stay together through meiosis I because of protein complexes cohesins but separate from each other in meiosis II and mitosis Attachment in mitosis lasts until the end of metaphase In meiosis sister chromatid ends in anaphase I and anaphase II In metaphase I homologs are held together by cohesion between sister chromatid arms in regions beyond points of crossing over Combo of crossing over sister chromatid cohesion along arms results in chiasma Chiasmata hold homologs together as spindles At anaphase I release of cohesion along arms allows separation At anaphase II release of sister chromatid cohesion at the centromeres allows sister chromatids to separate Meiosis I reductional division halves number of chromosomes reduction from diploid to haploid Meiosis II equational division produces haploid daughter cells Genetic Variation Production in Sexual Life Cycles Contributes to Evolution Changes in organism s DNA creates different versions of genes known as alleles