Chapter 10: Meiosis

I.  Meiosis: nuclear division that halves the chromosomal number

            A.  gametes: sex cells, egg and sperm

            B.  sexual reproduction: fusion of gametes to form new life

            C.  zygote: first cell formed after the union of egg and sperm

            D.  diploid (2n): 2 chromosomes of each type

            E.  haploid (1n): 1 chromosome of each pair

II.  homologous chromosomes: matching chromosomes of a pair

III.  Meiosis I: DNA replication during Interphase

            A.  Prophase I

                        1.  synapsis: homologous duplicated chromatids line up (tetrad)

                        2.  crossing-over: exchange of DNA with nonsister chromatids

                                    a.  chiasmata: regions where nonsister chromatids are

                                         attached to each other

                        3.  independent assortment: gametes have different combinations

                             of chromosomes

            B.  Metaphase I: line up at metaphase plate

            C.  Anaphase I: homologues separate and duplicated chromosomes move

                 toward opposite poles

            D.  Telophase I: daughter nuclei are now haploid

            E.  Cytokinesis: division of cytoplasm into 2 cells

            F.  Interkinesis: no DNA duplication

IV.  Meiosis II

            A.  Prophase II: spindle fibers reform

            B.  Metaphase II: duplicated chromosomes line up at metaphase plate

            C.  Anaphase II: chromosomes move to poles

            D.  Telophase II: daughter cells begin to form

            E.  Cytokinesis: division of cytoplasm into 4 cells

V.  human life cycle: meiosis only in sex organs to produce gametes

            A.  spermatogenesis: in the testes, produces sperm, produces 4 sperm for

                 each parent cell

            B.  oogenesis: development of an egg

                        1.  in ovaries: a female is born with all the eggs she will ever have

                        2.  secondary oocyte: receives most of the cytoplasm from division

                                    a.  stops at metaphase II

                                    b.  enters the oviduct, for possible fertilization

                        3.  polar body: other divisions end up in nonfunctional state

VI.  comparison

VII.  variation: factors encouraging diversity

            A.  independent assortment: chromosomes are distributed to daughter

                cells in various combinations

            B.  crossing-over: mixes up DNA on a chromosome

            C.  fertilization: recombination of chromosomes, one from each parent

            D.  mutation: agent of change