MEIOSIS Cellular Reproduction of Reproductive Cells

Vocab you should know…  Meiosis: cell division where one body cell produces four gametes, each containing half the number of chromosomes as a parent’s body cell  Homologous Chromosomes: paired chromosomes w/ genes for the same traits arranged in the same order  Gametogenesis: process by which male & female gametes are formed  Spermatogenesis: process by which male gametes form

More Vocab…  Oogenesis: production, growth & maturation of an egg (ovum)  Zygote: diploid cell formed when a sperm fertilizes an egg  Crossing Over: exchange of genetic material b/t nonsister chromatids from homologous chromosomes during prophase I of meiosis; results in new allele combinations

More Vocab…  Genetic Recombination: major source of genetic variation among organisms caused by reassortment or crossing over during meiosis  Nondisjunction: failure of homologous chromosomes to separate properly during meiosis; results in gametes with too many or too few chromosomes  Fertilization: fusion of male & female gametes  Independent Assortment: random distribution of the pairs of genes on different chromosomes to the gametes

More Vocab…  Gamete: male & female sex cells; sperm & eggs  Tetrad: 4 chromatids in a pair of homologous chromosomes that come together as a result of synapsis during meiosis  Polar Body: a short-lived product of the formation of gametes by meiosis  Gene: short segment of DNA containing instructions for a single trait

More Vocab…  Synapsis:the pairing of homologous chromosomes during meiosis  Homologue:something homologous  Ovum:a mature egg cell (female reproductive cell)  Spermatid:immature sperm cell that has almost completed development  Spermatazoa: mature male reproductive cell

Cell Cycle of a Reproductive Cell  Interphase – long first stage of cell grow & DNA duplication (same as in mitosis)  Meiosis I – similar to mitosis, results in production of 2 2n (diploid) cells  Interkinesis or Cytokinesis I – division of cytoplasm to create 2 separate haploid cells  Meiosis II – similar to mitosis also, except that 2 cells are undergoing it at the same time, resulting in 4 n (haploid) cells  Cytokinesis II – division of cytoplasm, similar to mitosis, except results in 4 cells

Cell Cycle for Reproductive Cells  Interphase  Meiosis I:prophase I, metaphase I, ananphase I, telophase I  Cytokinsise I / interkinesis  Meiosis II:prophase II, metaphaseII, ananphase II, telophase II  Cytokinesis / Cytokinesis II

Interphase  Just as in mitosis, prior to division, the cell must duplicate all of its contents, including its DNA and must grow in size in order to be able to accommodate all of the duplicate material  Humans = 46 chromosomes, after interphase = 46 x 2 = 92

Meiosis I  There are four stages to meiosis I Prophase I Metaphase I Anaphase I Telophase I

Prophase I  DNA coils into chromatids  Sister chromatids pair up  Spindle fibers appear  Nucleolus & nuclear membrane disappear  **synapsis occurs: each pair of sister chromatids lines up next to its homologue forming a tetrad (the two pairs of sister chromatids join together instead of a pair of just a pair of sister chromatids)

Prophase I

Prophase I cont’d  In the tetrad, chromosomes are aligned lengthwise, so genes of one chromosomes are adjacent to the same genes on the other chromosome

Prophase I – crossing over  During synapsis, chromosomes twist around each other  Sometimes they twist so tightly they exchange genetic material = crossing over  In humans this happens 2-3 times for every pair of chromosomes  Crossing over can result in new traits w/in the organism = genetic recombination  After prophase I = 92 chromosomes

Crossing Over

Metaphase I  Tetrads line up randomly at the equator  Spindle fibers from the poles (centrioles) attach to the centromere of each pair of the homologous chromosomes  After metaphase I = 92 chromosome s

Metaphase I

Anaphase I  Homologous chromosomes (sister chromatids) move to the poles, each pair of the tetrad to the opposite pole  Independent assortment = the random separation of homologous chromosomes resulting in genetic variation (this is what is responsible for the differences in individuals)  After anaphase I = 92 chromosomes

Anaphase I

Independent Assortment

Telophase I  Homologous chromosomes reach the opposite poles  Spindle fibers break down  Nuclear envelopes forms around each set of chromosomes, nuclei reappear  After telophase I = 92 chromosomes, 46 in each new nucleus

Telophase I

Interkinesis or Cytokinesis I  Interkinesis, like cytokinesis, divides the cells cytoplasm resulting in 2 2n (diploid) cells, each having a complete set of DNA  Meiosis I starts w/ 1 2n cells, at the end of Meiosis I, we have 2 2n cells  Each new cell has 46 chromosomes

Interkinesis

Meiosis II  After Meiosis I is complete, the two cells go directly into Meiosis II  ***No Interphase*** this means no duplication of chromosomes  Meiosis II has four phases Prophase II Metaphase II Anaphase II Telophase II  Cytokinesis II – division of cytoplasm of both cells to create 4 separate cells

Prophase II  Nuclear membrane breaks down, nucleoli disappears  Spindle fibers form  Chromatin condenses  Sister chromatids pair up  After prophase II = 46 chromosomes in each cell

Prophase II

Metaphase II  Pairs of sister chromatids move to equator in each cell  After metaphase II = 46 chromosomes in each cell

Metaphase II

Anaphase II  Chromatids separate & move towards opposite poles in each cell  After anaphase II = 46 chromosomes in each cell

Anaphase II

Telophase II  Nuclear membranes form around all 4 sets of chromosomes  Spindle fibers break down in all cells  Nucleoli appear in the nucleus of all 4 cells  Chromatids uncoil into chromatin  After telophase II = 46 chromosomes in each cell, 23 in each new nucleues

Telophase II

Cytokinesis II  Cleavage furrow deepens until it pinches off each cell, creating 4 new cells  All 4 cells are haploid (n) (contain 1 set of chromosomes)  After cytokinesis II = 23 chromosomes in each new cell

Cytokinesis II

Gametes & Gametogenesis  Meiosis results in the production of gametes  In humans: Females: oogenesis Males: spermatogenesis

Oogenesis  Diploid reproductive cell divides meiotically to form 1 mature ovum (egg)  But…meiosis results in the formation of 4 cells…how is it that we only get 1 mature ovum?  Ova are very large, so when cytokinesis occurs, one cells get the majority of the cytoplasm, this becomes the ovum  the other 3 cells do not receive enough cytoplasm to survive, they are called polar bodies & will degenerate (disappear)

Oogenesis

Spermatogenesis  Diploid reproductive cell divides meiotically to form 4 haploid spermatids  Spermatids develop into mature spermatozoa

Spermatogenesis

Meiosis Quiz 1.What happens to the DNA of a reproductive cell during interphase? 2.Synapsis occurs during which phase of Meiosis (be sure to note I or II)? 3.What does synapsis result in? 4.During what phase does crossing over occur in?

Meiosis Quiz cont’d 5.What is crossing over? 6.The random separation of homologous chromosomes resulting in genetic variation is ____ _____? 7.Interkinesis/Cytokinesis I results in the formation of 2 ________ cells. 8.Cytokinesis II results in the formation of 4 _______ cells.

Meiosis Quiz Cont’d 9.Oogenesis results in the formation of _____ mature egg cells & 3 ______ ______. 10.Spermatogenesis results in the formation of ________ spermatids.

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