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Meiosis Genetic Variation Homologous Chromosomes • homologous chromosomes- a pair of chromosomes that code for the same trait (one from mom one form dad) • 23 pairs in humans Karyotype • A karyotype - is an ordered display of the pairs of homologous chromosomes from a cell • • • • Chromosomes Types sex chromosomes - are called X and Y Human females (XX) homologous pair Human males (XY)homologous pair autosomes -The 22 pairs of chromosomes that do not determine sex (determine everything else) XY XX Meiosis • Meiosis – creates four haploid (n) gametes (egg or sperm) (two divisions) • Egg and sperm unite to form a diploid (2n) zygote. (develops into an embryo) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-4 Interphase • The same as mitosis Prophase I • Longest phase (two main things happen in prohase one) • Synapsis • Crossing over Synapsis • Synapsis – homologous chromosomes pair up • Sister chromatids connnect to other homologous sister chromatids to form a tetrad. Tetrad (Synapsis) Crossing Over • Crossing over – homologous chromosomes exchange DNA while connected as tetrads. • Chiasma -the point at which they connect to exchange DNA • ** Metaphase I • In metaphase I, tetrads line up at the metaphase plate • Independent assortment – homologous chromosomes line up randomly on any side (ex. All of mom’s chromosomes don’t line up on one side) Anaphase I • In anaphase I, tetrads separate (homologous chromosomes are separated) • Sister chromatids remain attached at the centromere and move as one unit toward the pole Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Telephase I • The cell is divided into two halves with sister chromatids in each half. They are no longer identical because of crossing over. • Cytokinesis usually occurs simultaneously, forming two daughter cells (not identical). 2 cells are now present. Cleavage Furrow Meiosis II (repeat) • Meiosis II is very similar to mitosis: • Prophase II – spindle fibers and kinetichores form • Metaphase II – chromatids line up • Anaphase II – chromatids are separated into individual chromosomes • Telophase II and cytokinesis – each cell splits, and nuclei reform (4 unidentical haploid cells are now present) • ** Fig. 13-8d Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming Mitosis vs. Meiosis Creating Variation • The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation • Three mechanisms contribute to genetic variation: – Independent assortment of chromosomes – Crossing over – Random fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Independent Assortment • The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number Crossing Over • Crossing over produces recombinant chromosomes (new combinations of chromosomes), which combines genes inherited from each parent Random Fertilization • Random fertilization adds to genetic variation because any sperm can fuse with any egg • An egg and sperm can make about 70 trillion diploid combinations • ** You should now be able to: 1. Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid 2. Describe the events that characterize each phase of meiosis 3. Describe three events that occur during meiosis I but not mitosis 4. Name and explain the three events that contribute to genetic variation in sexually reproducing organisms Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings