This document summarizes the key stages and purpose of meiosis. Meiosis involves two cell divisions that result in four haploid daughter cells, each with half the number of chromosomes as the original cell. This ensures genetic variation in offspring. The stages include prophase I with synapsis and crossing over, metaphase I and II, anaphase I and II, telophase I and II, and cytokinesis. Meiosis reduces the chromosome number through homologous chromosome separation in meiosis I and sister chromatid separation in meiosis II, allowing for genetic recombination and independent assortment of maternal and paternal chromosomes in gametes. Errors can result in conditions like Down syndrome.

1. Meiosis

2. Essential terms Gametogenesis Synapsis Crossing over Disjunction / Non disjunction Chiasmata

3. Key points Mitosis – diploid. Identical. Stability. Meiosis – haploid. Combination. Variation. Sexual reproduction ensures genetic continuity and genetic variety, producing offspring's that often differ greatly from patients. Associated with gametogenesis. Counterbalances fertilization - makes sure # of chromosomes remains constant

4. Meiosis Purpose : production of sex cells (gametes) Outcome 2 divisions 4 daughter cells with 1/2 (haploid) sets of chromosomes

5. 1. Chromosomes found in homologous pairs identical but may have varying gene messages b. Humans have 46 (23 pairs) 22 homologous pairs (autosomes) X and Y (23rd pair) sex chromosomes Females (XX); Males (XY) c. Pairs separate during reproduction Offspring receives info from each parent Exchanging genetic material during crossing over (Prophase I) d. Separated during meiosis (Anaphase I)

6. Homologous chromosomes form pairs in form of tetrads (4 chromatids) Involves 2 divisions = Meiosis I and II

7. Interphase chromosomes replicate Consists of 2 identical sister chromatids attached at their centromeres centriole pairs also replicate into two pairs

8. Prophase I Condenstaion. Chromatin thickens and coils. Become visible . Synapsis occurs Homologous chr. pair up forming tetrads. Sister chromatids are attached at centromeres. Each pair = bivalent. Nonsister chromatids features crossing over resulting in chiasmata. = More than 1 can form = genetic variation) • spindle forms from microtubules • nuclear envelope/nucleoli disperse 5 stages: Leptonema, zygonema, pachynema, diplonema and diakenesis.

9. Metaphase I Chromosomes have thickened. Each tetrad interacts with spindle fibers. Movement to the equitorial plate. Homologues are destined to separate towards opposite poles

10. Anaphase I sister chromatids remain attached while homologues move towards the opposite pole. (Mitosis =sister chromatids are moved apart) One half of each tetrad ( one pair of sister chromatid) is pulled toward each pole of the dividing cell. No attraction of the sister chromatids – reduction in the number of chromosomes.

11. Telophase I + Cytokinesis Very short phase compared to mitosis May not always occurs Each pole now has a haploid set of chromosomes composed of two sister chromatids attached at the centromere cytokinesis occurs producing two daughter cells Nuclear membrane forms. No interphase – No DNA replictaion. Already 2 chromatids.

12. 2nd division (II) Prophase II = spindle apparatus forms Chromosomes move towards the metaphase II plate Metaphase II = chromosomes align on metaphase plate kinetochores of sister chromatids point towards opposite poles

13. Anaphase II = centromeres of sister chromatids separate sister chromatids are pulled towards opposite poles Telophase II = I member of each homologue pair is present at each pole. Each chromosome is referred to as monads.

14. Cytokinesis 4 haploid gametes results. Haploid state has been achieved. If crossing over has occurred = each monad is a combination of maternal and paternal genetic information ie. Receives info from grandparents. = increases genetic variation.

16. Independent assortment Mechanism that allows for genetic variation. Reduction of diploid – diploid means that each gamete will only carry one form of gene for a particular characteristics. Crossing over results in the exchange of genetic info from maternal to paternal chromosomes = possibility of new combination of genes. Random assortment = mixture of maternal and paternal chromosomes = more combination.

17. Nondisjunction and Translocation Too few or too many chromosomes in gametes Translocation: attachment of all or part of chromosome to another chromosome Nondisjunction: failure of chromosomes to separate Down Syndrome 1. Translocation of chromosome #15 to #21, or 2. Nondisjunction with extra #21 Nondisjunction of sex chromosomes 1. XO- Turner Syndrome; sterile female 2. XXY- Kleinfelter Syndrome; sterile male 3. XXX- Metafemale; limited fertility