Meiosis and Sexual Life Cycles Fred and George Weasley… Identical Twins

Heredity Heredity: the transmission of traits from one generation to the next Asexual reproduction: clones Sexual reproduction: variation A budding hydra

Karyotyping Chromosomes are photographed, paired, and arranged into a karyotype Homologous chromosomes- same length, centromere position, and staining pattern Carry same types of genes Arranged from longest to shortest 1-22 = autosomes #23 = sex chromosomes (X and Y)

Human Life Cycle 23 pairs of homologous chromosomes (46) 1 pair of sex and 22 pairs of autosomes Gametes are haploid (1N); all other cells are diploid (2N) Fertilization results in a zygote Meiosis: cell division to produce haploid gametes

Alternate Life Cycles

Plants and Some Algae Alternation of generations: 2N sporophyte produces 1N spores by meiosis Spore can divide by mitosis to create a 1N gametophyte Gametophyte makes gametes by mitosis which fertilize to become a 2N sporophyte

Most fungi and some protists Meiosis produces 1N cells that divide by mitosis to produce 1N adults Adults undergo mitosis to produce gametes Fertilized gametes become a 2N zygote

Meiosis After chromosome replication 2 cell divisions (Meiosis I & Meiosis II) Produces 4 daughter cells 1/2 chromosome number (1N) Variation in gametes that are produced

Meiosis I Metaphase I Interphase Prophase I Anaphase I Tetrads line up along the metaphase plate Both sister chromatids of a chromosome are controlled by a single kinetochore Anaphase I Homologous chromosomes are pulled apart Sister chromatids remain intact Telophase I and Cytokinesis Haploid set of chromosomes Clevage furrow/Cell plate forms In some species, chromosomes uncoil and nuclear envelope forms Interphase Chromosomes and centrosomes replicate Prophase I Chromosomes condense Homologous chromosomes pair up and crossing over occurs Synaptonemal complex (protein) forms between homologues, forming a tetrad Centrosomes move to opposite ends of the cell Spindle fibers attach to kinetochores and begin moving tetrads to the metaphase plate

Meiosis II Prophase II MetaphaseII Anaphase II Mitotic spindle forms again Spindle fibers attach to kinetochores and chromosomes begin moving toward the metaphase plate MetaphaseII Chromosomes line up on metaphase plate as in mitosis Anaphase II Chromosomes separate at the centromere Telophase II and Cytokinesis Nuclei form Chromosomes decondense Each of the 4 daughtercells are genetically distinct from the parent cell

Meiosis vs. Mitosis Synapsis/tetrad/chiasmata (prophase I) Homologous vs. individual chromosomes (metaphase I) Sister chromatids do not separate (anaphase I) Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

Genetic Variation Independent assortment: homologous chromosomes position and orient randomly (metaphase I) In humans, about 8 million different combinations Recombinant chromosomes (from crossing over) chromatids are no longer identical More possibilities for genetic variation

Crossing Over Synapsis- homologous chromosomes come together Synaptonemal complex holds them together Form a tetrad Arms of homologues overlap and form a chiasma Arms of homologues exchange at gene lines May occur multiple times Produces recombinant chromosomes

No wonder siblings can be so different! Random fertilization: 1 sperm (1 of 8 million possible combinations) x 1 ovum (1 of 8 million possibilities) = 64 trillion diploid combinations! With the variation from crossing over, the number of possibilities is astronomical Prince Harry and Prince William George W and Jeb Bush Venus and Serena Williams The Jonas Brothers