Chapter 13: Meiosis & Sexual Life Cycles Genetics & Inheritance Genes- hereditary units program specific traits in the offspring Most program cells to synthesize specific enzymes & proteins Cumulative action of proteins produces traits Specific gene location on a chromosome = locus Gametes- reproductive cells which transmit genes between generations

Asexual vs. Sexual Reproduction Asexual Reproduction All offspring’s genes originate from one parent New offspring produced through mitosis Sexual Reproduction Offspring’s genes originate from two parents Unique combinations of genes can be made so that each offspring varies from its siblings (exception- identical twins)

Fertilization & Meiosis alternate life cycles Generation-to generation sequence of stages in an organism’s life from conception to reproduction Example organism: Humans Somatic cell (body cell) = 46 chromosomes Homologous chromosomes = carry genes controlling the same inherited trait 22 pairs of autosomes 1 pair of sex chromosomes Diploid cells = have all chromosome sets (2n) Haploid cells = have one chromosome from each set (n); egg or sperm cells

Fertilization & Meiosis alternate life cycles Fusion of egg & sperm nuclei = zygote (2n) Mitosis Used to develop zygote into sexually mature adult; Forms all somatic cells from zygote Meiosis Occurs in gonads Reduces diploid number to half Creates sperm & egg Maintains diploid number for organisms; when fertilization occurs the diploid number is restored

Sexual Life Cycles Alternation of fertilization & meiosis is characteristic of all sexually reproducing organisms Timing of fertilization & meiosis in the life cycle varies between different species

Three types of sexual life cycles: Animals Gametes are only n cells Gametes do not divide before fertilization 2n zygote divides by mitosis after fertilization

Three types of sexual life cycles Alternation of generations Plants & some algae Has n & 2n multi-cellular stages 2n multi-cellular stage = sporophyte Formed by ferilization of gametes Makes spores by meiosis n multi-cellular stage = gametophyte Formed by mitosis of spores Makes gametes by mitosis

Three types of sexual life cycles Most fungi & some protists Diploid zygote forms & immediately goes through meiosis forming haploid cells Haploid cells formed during meiosis divide by mitosis to form a multi-cellular n adult organism n adult forms gametes through mitosis Only 2n cell is the zygote

Concept Check How does the karyotype of a human male differ from that of a human female? How does the alternation of meiosis & fertilization in life cycles maintain the normal chromosome number for that species? What process mitosis or meiosis is more directly involved in gamete production: In animals? In plants & most fungi?

Meiosis Has two consecutive cell divisions Results in 4 haploid daughter cells

Stages of Meiosis: Meiosis I Prophase I 90% of meiosis spent in this phase Chromosomes condense Crossing over may occur between homologous chromosomes Synapsis occurs holding homologues together Tetrads form Nucleoli disperse; nuclear envelope fragments; formation of spindles Metaphase I Tetrads arrange on metaphase plate

Stages of Meiosis: Meiosis I Anaphase I Chromosomes move toward poles (sister chromatids still attached) Telophase I & cytokinesis In some species nuclear envelope & nucleoli reform Cleavage furrow (animals) or cell plate (plants) forms

Stages of Meiosis: Meiosis II No replication of chromosomes between meiosis I & meiosis II Prophase II Spindle apparatus forms Metaphase II Chromosomes position of metaphase plate Anaphase II Sister chromatids move toward opposite poles Telophase II & Cytokinesis Nuclei form; chromosomes de-condense Cleavage furrow or cell plate forms

Comparison of Mitosis & Meiosis Meiosis reduces chromosome number by half; mitosis conserves chromosome number Synapsis, crossing over, & tetrad formation occur in prophase I of meiosis Tetrads take position on metaphase plate in metaphase I of meiosis Homologues separate during anaphase I of meiosis; sister chromatids remain attached during meiosis I

Concept Check Explain how mitosis conserves chromosome number, while meiosis reduces the chromosome number by half. Compare the chromosomes present in metaphase of mitosis to those in metaphase II of meiosis.

Sexual Reproduction & evolution Sexual reproduction produces genetic variation which can contribute to evolution Genetic variation Mutations Independent assortment Crossing over Random fertilization

Genetic Variation: independent assortment Random orientation of homologues during metaphase I leads to unique combinations of maternal & paternal chromosomes in the egg & sperm

Genetic variation: crossing over Allows genes to exchanged between maternal & paternal chromosomes creating unique chromosomes

Genetic Variation: random fertilization Each egg & sperm cell is unique Which sperm fertilizes which egg contributes to extreme variation among offspring 2n = # of possible combinations per gamete

Evolutionary Significance of Genetic Variations Populations evolve as a result of differential reproductive success among its variant members Individuals with variations best suited to the environment reproduce more thus transmitting more of their genes Success of variations dependant on environment

Concept Check In fruit flies 2n= 8, while in honeybees 2n=32. Assuming no crossing over, is genetic variation higher among fruit flies or honeybees? Explain. When would crossing over not contribute to increased genetic variation? If a human cell had 22 autosomes & a Y chromosome what kind of cell is it? What is the immediate product of meiosis in a plant? What feature is unique to a plant life cycle versus an animal life cycle?