Meiosis Facts

Meiosis The form of cell division to produce gametes- cells with half the number of chromosomes Diploid (2n)  haploid (n) Meiosis is sexual reproduction Two divisions (meiosis I and meiosis II)

Meiosis Sex cells divide to produce gametes (sperm or egg) Meiosis-similar to mitosis with some differences Requires 2 cell divisions Makes 4 haploid daughter cells

Meiosis Occurs only in gonads (testes or ovaries) Male: Spermatogenesis Female: Oogenesis

Spermatogenesis n=23 n=23 2n=46 Human sex cell Sperm Haploid (n) Meiosis II 2n=46 Human sex cell Diploid (2n) n=23 Meiosis I

Interphase I Similar to mitosis interphase Chromosomes replicate (S phase) Duplicated chromosomes have two identical sister chromatids attached at the centromeres Centriole pair also replicates.

Interphase I Nucleus and nucleolus visible. Chromatin Nuclear membrane Cell membrane Nucleolus

Meiosis I

Stages of Meiosis I Meiosis I- Cell division to reduce chromosome number by one-half (reduction division) Four phases: a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I

Prophase I Longest and most complex phase (90%) Chromosomes condense Synapsis occurs: homologous chromosomes come together forming the tetrad Tetrad- two replicated chromosomes or 4 chromatids (sister and nonsister)

Prophase I - Synapsis Homologous chromosomes Sister chromatids Tetrad

Homologous Chromosomes Pair of chromosomes (maternal and paternal) similar in shape size, & genes carried Homologous pairs (tetrads) carry genes controlling same inherited traits Each locus (position of a gene) holds same position on both homologues

Homologous Chromosomes Humans- 23 pairs of homologous chromosomes First 22 pairs- called autosomes, look alike Last pair- called sex chromosomes XX female or XY male (Y much smaller than X)

Homologous Chromosomes eye color locus hair color Paternal Maternal

Crossing Over Synapsis- Homologues join together Crossing over (variation) can occur btwn nonsister chromatids segments of nonsister chromatids break and reattach to the other chromatid Chiasmata (chiasma)- sites of crossing over

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

Sex Chromosomes XX chromosome - female XY chromosome - male

Prophase I Centrioles Spindle fiber Aster fibers

Metaphase I Shortest phase Tetrads align on the metaphase plate Independent assortment occurs Chromosomes separate randomly to the poles of the cells

Metaphase I Independent assortment causes variation in the forming cells Orientation of homologous pair to poles is random Formula for determining variation: 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 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 1n = 23 (human) Cytokinesis occurs and two haploid daughter cells are formed

Telophase I

Meiosis II

Meiosis II Short or No interphase II DNA NOT replicated again Remember: Meiosis II is similar to mitosis Prophase I, Metaphase II, Anaphase II, and Telophase II

Prophase II Same as prophase in mitosis Nuclear envelope breaks down and the spindle apparatus forms

Metaphase II Same as metaphase in mitosis Chromatids lined up at equator Metaphase plate

Anaphase II Same as anaphase in mitosis Sister chromatids separate

Telophase II Same as telophase in mitosis Nuclei reform Cytokinesis occurs Remember: Four haploid daughter cells produced Gametes = sperm or egg

Telophase II

Meiosis n=2 n=2 2n=4 Sperm Haploid (n) Meiosis II Meiosis I sex cell diploid (2n)

Genetic Variation

Variation Important to the survival of populations Aids in natural selection Strongest individuals are able to survive and reproduce

Question What are the three sources of genetic variation in sexual reproduction?

Answer: Crossing Over (Prophase I) 2. Independent Assortment (Metaphase I) 3. Random Fertilization (sperm joins with egg)

Remember: Variation is helpful to the survival of a species!

Question: A diploid cell containing 20 chromosomes (2n = 20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes (haploid) 1n = 10

Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type.

Fertilization n=23 egg 2n=46 zygote The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

Question: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: Four cells with 10 chromosomes each