 Describe the result of meiotic division in terms of sexual reproduction  Discuss the structure of homologous chromosomes  Describe chromosomes in terms of ploidy  Distinguish between sexual and asexual reproduction  Discuss genetic variation

 Chromosomes occur in pairs Each chromosome may contain ~ 1000 genes Diploid cells contain two of each kind of chromosome (2n)  Somatic or body cells (46 chromosomes in humans)  Produced by mitosis Haploid cells contain one of each kind of chromosome (n)  Gametes or sex cells (23 chromosomes in humans)  Produced by meiosis

 Homologous Chromosomes (Homolog's) “Homo” Greek word for the same Represents the two chromosomes of each pair in a diploid cell Each homologous pair has genes for the same trait  Tall or short Homolog's are not always identical

 Why is meiosis needed to produce gametes? Meiosis – form of cell division which produces ½ the number of chromosomes as a somatic cell  Meiosis divided into two phases  Meiosis I begins with one diploid cell  Meiosis II ends with four haploid gametes  Gametes must be haploid in order to continually produce a diploid zygote  Sperm (n) + Egg (n) = Zygote (2n)

 Sexual reproduction Production and fusion of haploid gametes n + n = 2n Genetic information is exchanged  Asexual reproduction Single parent produces one or more identical offspring by dividing into two cells No exchange of genetic material  Binary Fission, Parthenogenesis  Budding, Fragmentation

 Interphase Cell replicates its chromosomes Each chromosome consists of two sister chromatids

 Prophase I DNA coils as the spindle forms Two homologous chromosomes line up gene by gene to form a tetrad Tetrad forms so tightly that crossing over occurs  Exchange of genetic material  Can occur at any location along the chromosome  Results in new combinations of alleles

 Metaphase 1 Centromere of each chromosome becomes attached to a spindle fiber Tetrads move to the equatorial plane of the spindle Unique to Meiosis  Anaphase I Homologous chromosomes separate and move to opposite ends of the cell Centromeres holding sister chromatids together do not split

 Telophase 1 Spindle breaks down Chromosomes uncoil Nucleus Reappears Cytoplasm divides One more cell division is needed b/c each chromosome is still doubled (2 sister chromatids)

 Mitotic division of products of Meiosis I  Prophase 2 Spindle forms in each cell Nucleus disappears Chromatin coils into chromosomes Centrioles migrate to opposite sides of cell

 Metaphase 2 Chromosomes line up on the equatorial plane of the spindle Each centromere connected to two spindle fibers  Anaphase 2 Centromeres split Chromatids separate Chromatids migrate to opposite ends of the cell  Telophase 2 Nuclei reform Spindle breaks down Cytoplasm divides Chromosomes uncoil into chromatin

 End result of Meiosis Four haploid cells formed from one diploid cell Four haploid cells become gametes

 Genetic Recombination – re-assortment of chromosomes and the genetic information they carry Independent segregation and crossing over increase genetic variability and drive evolution

 Genetic Recombination Independent Segregation  Gene combinations vary depending on how each pair of homolog's lines up during Metaphase 1  Random Process  Number of combinations increases as chromosome increases  Each 23 pairs of chromosomes may align independently in a gamete  2 23 = 8 million types of egg or sperm a person can produce  When fertilization occurs 2 23 x 2 23 = 70 trillion possible zygote combinations

 Genetic Recombination Crossing over  May occur at any location when tetrads are formed  Variation is the raw material that forms the basis of evolution

 Nondisjunction Failure of homologous chromosomes to separate properly during meiosis Four basic types  Trisomy  One gamete with an extra chromosome  One gamete missing a chromosome  n + 2n = 3n  Trisomy 21 (down syndrome)  Gamete with an extra chromosome is fertilized by a normal gamete  Resulting zygote has 47 chromosomes

 Nondisjunction Monosomy  Gamete missing a chromosome fuses with a normal gamete  0 + n = n  Most zygotes with monosomy do not survive  Turner syndrome  Human females have only one X chromosome instead of two

 Nondisjunction Tetraploidy  Fusion of gametes, each with a complete set of chromosomes  2n + 2n = 4n  Results from a total lack of separation of homologous chromosomes  Common in plants  Chrysanthemum

 Nondisjunction Polyploidy  Organisms with more than usual number of chromosomes  Rare in animals, usually results in death of zygote  Frequently occurs in plants  Errors in meiosis can be beneficial for agriculture  6n Wheat  3n Apples  Resulting plants are usually larger, healthier and more disease resistant