2Ramifications of meiosis MOST animals and plants reproduce sexuallyplays a key role in generating the tremendous genetic diversity of evolutionGenetic contributions from 2 cells instead of 1
3History of meiosis 1876- Walther Fleming discovers chromosomes 1879- Cytologist Edouard van Beneden discovers different # of chromosomes in different cell in roundworm1883-proposes idea that gametes contain ½ # of chromosomesThese join to form a zygote
4Overview of MeiosisMeiosis is a form of cell division that leads to the production of gametes.gametes: egg cells and sperm cells-contain half the number of chromosomes of an adult body cellAdult body cells (somatic cells) are diploid, containing 2 sets of chromosomes.Gametes are haploid, containing only 1 set of chromosomes.
5Overview of MeiosisSexual reproduction includes the fusion of gametes (fertilization or syngamy) to produce a diploid zygote.Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages.Some life cycles include longer diploid phases, some include longer haploid phases.
10GERM-LINE CELLSCells that will eventually be gametes are set aside early in embryonic developmentGamete producing germ-line cells areDiploidUndergo meiosis to produce haploid gametes
11Features of MeiosisMeiosis includes two rounds of division – meiosis I and meiosis II.During meiosis I, homologous chromosomes (homologues) become closely associated with each other. This is synapsis.Proteins between the homologues hold them in a synaptonemal complex. Sometimes called a tetrad.
13Features of MeiosisCrossing over: genetic recombination between non-sister chromatids-physical exchange of regions of the chromatidschiasmata: sites of crossing overThe homologues are separated from each other in anaphase I.
14Features of MeiosisMeiosis involves two successive cell divisions with no replication of genetic material in the 2nd, just a separation of the sister chromatids for each homologue.This results in a reduction of the chromosome number from diploid to haploid. The first division is called “reduction division”.
16The Process of Meiosis Prophase I: -chromosomes coil tighter -nuclear envelope dissolves-homologues become closely associated in synapsis-crossing over occurs between non-sister chromatids (different from mitosis)
19The Process of Meiosis Metaphase I: -terminal chiasmata hold homologues together following crossing over-microtubules from opposite poles attach to each homologue, not each sister chromatid-homologues are aligned at the metaphase plate side-by-side-the orientation of each pair of homologues on the spindle is random
23The Process of Meiosis Anaphase I: -microtubules of the spindle shorten-homologues are separated from each other-sister chromatids remain attached to each other at their centromeresGenes on different chromosomes assort differently (independent assortment)
27The Process of Meiosis Meiosis II resembles a mitotic division: -prophase II: nuclear envelopes dissolve and spindle apparatus forms-metaphase II: chromosomes align on metaphase plate-anaphase II: sister chromatids are separated from each other-telophase II: nuclear envelope re-forms; cytokinesis follows
28Final result 4 different haploid cells in animals become gametes Plants, fungi, protists increased number of gametesVarying # of chromosome sets found in some insects and plants – this results in polyploidy- (1 or more entire sets of chromosomes added to a diploid gamete)
33ERRORS IN MEIOSISNondisjunction-Aneuploid gametes-
34Meiosis vs. Mitosis Meiosis is characterized by 4 features: 1. Synapsis and crossing over2. Sister chromatids remain joined at their centromeres throughout meiosis I3. Kinetochores of sister chromatids attach to the same pole in meiosis I4. DNA replication is suppressed between meiosis I and meiosis II.
35Meiosis vs. MitosisMeiosis produces haploid cells that are not identical to each other.Genetic differences in these cells arise from:-crossing over-random alignment of homologues in metaphase I (independent assortment)Mitosis produces 2 cells identical to each other.