MEIOSIS Objective: Students know that meiosis is an early step of sexual reproduction (2a) Students know that gametes are produced during meiosis

Interphase I Chromosomes replicate (S phase).

Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

Prophase I Chromosomes condense. Synapsis occurs: homologous chromosomes come together to form a tetrad (or a pair).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Crossing Over Crossing over (variation) may occur between nonsister chromatids Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

Crossing Over - variation Tetrad nonsister chromatids chiasmata: site of crossing over variation

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Sex Chromosomes XX chromosome - female XY chromosome - male

Prophase I centrioles spindle fiber aster fibers

Metaphase I Tetrads align in the middle. INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

Metaphase I metaphase plate OR metaphase plate

Question: In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

Answer 223 = ~8 million combinations Formula: 2n Human chromosomes: 2n = 46 n = 23 223 = ~8 million combinations

Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.

Anaphase I

Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis

Prophase II same as prophase in mitosis Spindle and centrioles form

Metaphase II same as metaphase in mitosis Chromosomes line up in a single line along the equator metaphase plate

Anaphase II same as anaphase in mitosis sister chromatids separate

Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced. gametes = sperm by spermatogenesis or egg (ova) by oogenesis

Telophase II

Meiosis n=2 n=2 2n=4 sperm haploid (n) meiosis II meiosis I sex cell diploid (2n)

Variation Important to population as the raw material for natural selection. Question: What are the three sexual sources of genetic variation?

Answer: Remember: variation is good! 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!

Question: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes (haploid)

Sources www.ursulinehs.org/powerpoint/meiosis.ppt www.biology4teachers.com/Cell%20Division/MEIOSIS.ppt http://www.iteachbio.com/Life%20Science/LifeFunctionsandTheCell/Meiosis.ppt