Chapter 10.1

process that REDUCES the amount of genetic material contained in the DNA and the chromosomes by half Meiosis, like mitosis, must be followed by cytokinesis in order for cell division to be complete.

 follows a period of interphase (cell growth and DNA replication)  produces sex cells/gametes/germ cells that are genetically different from one another (to be used in sexual reproduction)  involves the pairing and separation of HOMOLOGOUS CHROMOSOMES  involves SYNAPSIS and CROSSING OVER  involves 2 nuclear divisions – Meiosis 1 and Meiosis II  begins with 1 diploid cell and produces 4 haploid cells

are PAIRS OF CHROMOSOMES found in organisms that will be used to reproduce sexually are PAIRS OF CHROMOSOMES that are alike ( BUT NOT IDENTICAL) in (1) size, (2) shape, (3) type of information they carry (genes) Where did this PAIR of chromosomes come from? 1 from the mother + 1 from the father

DIPLOID CELLS  contain 2 CHROMOSOMES in each pair  somatic (body/autosomal) cells  represented 2n (n= # of chromosome pairs) HAPLOID CELLS  contain 1 CHROMOSOME in each pair  sex cells/gametes/germ cells  represented n or 1n (n= # of chromosome pairs) So fertilization is …Haploid (n) + Haploid (n) = Diploid (2n)

How many chromosomes do humans have in….  their diploid cells?  their sex cells?  their haploid cells? How many chromosome pairs do humans have in …  their skin cells?  their diploid cells?  their sex cells?

MEIOSIS I - first nuclear division  pairing and division of homologous chromosomes  produces 2 cells  4 stages plus cytokinesis prophase I, metaphase I, anaphase I, telophase I MEIOSIS II – second nuclear division  division of the chromosomes  produces 4 cells  4 stages plus cytokinesis prophase II, metaphase II, anaphase II, telophase II

Prophase I Metaphase I Anaphase I Telophase I And Cytokinesis Meiosis I and meiosis II Meiosis I is here Meiosis I

Prophase I – chromosomes condense, centrioles move to poles and spindle fibers form, nucleolus and nuclear membrane break down *SYNAPSIS occurs - the pairing of homologous chromosomes and the formation of tetrads *CROSSING-OVER occurs - the exchange of genetic material between homologous chromosomes Metaphase I - spindle fibers align pairs of homologous chromosomes (tetrads) in the center of the cell Anaphase I - pairs of homologous chromosomes (tetrads) separate and chromosomes move toward opposite poles of the cell * INDEPENDENT ASSORTMENT is the term for the random separation. Telophase I (and Cytokinesis) chromosomes unwind, 2 nuclei are formed, spindles disappear, the cytoplasm and its contents divide into 2 cells

*DNA is NOT replicated as Interphase is not performed*  Prophase II – same as mitotic prophase  Metaphase II - chromosomes are at the equator (midline) of the cells  Anaphase II - chromatids separate at the centromeres and move to opposite poles  Telophase II (and Cytokinesis) – same as mitotic telophase; each cell divides resulting in four haploid cells (each containing a single-stranded chromosome)

Asexual Reproduction  1 parent (set of genes)  no union of gametes  offspring is genetically identical to the parent  I cell  2 identical cells  Example: binary fission; budding; runners Sexual Reproduction  2 parents (sets of genes)  fusion of haploid (1n) gametes  offspring is genetically different from the parents  2 different cells  1 unique zygote  Example: conjugation

OCCURS DURING CROSSING OVER IN PROPHASE I OF MEIOSIS allows for variety among organisms THE MORE VARIETY WITHIN A SPECIES THE MORE LIKELY THAT SPECIES WILL SURVIVE

Video

 Interactive Table A Interactive Table A  Interactive Table B Interactive Table B