Bio1151 Chapter 13 Meiosis and Sexual Life Cycles
  1. In          reproduction, one parent produces genetically identical offspring by         fission or by          .

    Budding of a hydra.

    A mass of cells divide by mitosis to form a bud, which develops into a small hydra and detaches from the "parent".

    Such asexual reproduction results in genetically identical individuals.


  2. In         reproduction, two parents produce offspring that have unique combinations of        inherited from each parent.

    Heritable variation. The family of Sissy Spacek and Jack Fisk exhibits genetic variation. Sexual reproduction produces variation in the gametes and unique combinations of genes in the offspring.
  3. Genes are located on chromosomes, which can be visualized under the light microscope to create a            .

    A chromosome contains numerous genes and consists of 2 sister chromatids joined at the centromere.

    Each homologous pair is composed of one chromosome from the maternal set and one from the paternal set.

    A cell that has a full set of chromosomes from both parents is diploid.

    A karyotype is a display of condensed mitotic chromosomes arranged in homologous pairs.

    Human somatic cells have 22 pairs of homologous autosomes (each of the pair inherited from one of the 2 parents), plus one pair of sex chromosomes (X or Y).

    XX individuals are female, while XY are male.

    This karyotype shows the chromosomes from a male.


  4.          are          sex cells that are produced in organs called         by meiosis.

    Sexual life cycle of animals.

    Haploid gametes fuse by fertilization to form a diploid zygote.

    The zygote undergoes many rounds of mitosis to produce the diploid multicellular organism.

    Diploid germ cells undergo meiosis to produce haploid gametes.

    Human life cycle.

    Female gonads called ovaries produce the gamete egg.

    Male gonads called testes produce the gamete sperm.

    The gametes are haploid, consisting of one set (n = 23 in humans) of chromosomes.

    Gametes can fuse by fertilization to form a diploid zygote with two sets (2n = 46) of chromosomes.

    Asexual and Sexual Life Cycles and Meiosis Overview:
  5. Since meiosis reduces the number of chromosome sets from          to          , two cellular divisions are required, preceded by a single             .

    Overview of meiosis. After chromosomes replicate in interphase, the diploid cell divides twice, yielding four unique haploid daughter cells. One chromosome was inherited from the female parent, the other from the male parent.
    • Meiosis I is the chromosome            division.

      Meiosis I separates homologous chromosomes, a reduction from diploid to haploid cells:

      prophase I , metaphase I , anaphase I , telophase I and cytokinesis I.

        In prophase I, homologous chromosomes pair up into tetrads in a process called synapsis.

        Crossing over of DNA segments occurs at chiasmata between non-sister chromatids, and genetic information is exchanged between the homologues.


        In metaphase I, the paired tetrads line up on the metaphase plate.

        Note the alignment of maternal and paternal homologues along the metaphase plate is random.


        In anaphase I, the homologous pairs move toward opposite poles of the cell.

        Notice sister chromatids remain attached, and some of the chromatids have exchanged genetic material.


        At the end of telophase I and cytokinesis I, two haploid cells are produced with two chromatids still attached to each chromosome.


    • Meiosis II then produces       haploid daughter cells.

    Meiosis II is similar to mitosis, except the cell undergoing division is haploid rather than diploid. Meiosis now has produced 4 haploid gametes, each of the chromosomes has one chromatid. Review:
    Review: Meiosis Animation
    Summary: difference between mitosis and meiosis.


  6. One cell division produces two diploid, genetically identical cells.
  7. Functions in growth and repair of somatic cells.


  8. Two cell divisions produce four haploid, genetically different cells.
  9. Synapsis of homologous chromosomes, forming tetrads and resulting in crossing over.
  10. Produces gametes for sexual reproduction with genetic variation in offspring.
  11. Sexual reproduction produces genetic            by several mechanisms: independent             , crossing       , and random fertilization.

    In independent assortment, each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs in metaphase I, giving 2^23 (about 8 million) combinations in humans. Review:

    Crossing over occurs in prophase I.

    Non-sister chromatids of homologous chromosomes exchange portions.

    The recombinant chromosomes carry genes derived from different parents.


    Random fertilization.

    Gametes fuse randomly during fertilization.

    This produces a zygote with 2^23 x 2^23 (over 70 trillion) combinations of chromosomes in humans.

    Review: Genetic Variation