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

Interphase The same as mitosis Fig. 13-4 Figure 13.4 Describing chromosomes

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) **

Telophase II and Cytokinesis Fig. 13-8d Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Figure 13.8 The meiotic division of an animal cell 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: Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid Describe the events that characterize each phase of meiosis Describe three events that occur during meiosis I but not mitosis 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