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AP Biology Notes Outline Chapter 13: Meiosis and the Sexual Life Cycle SEXUAL REPRODUCTION ADVANTAGE: results in greater genetic variation! Two parents give rise to offspring that have unique combonations of genes inherited from both parents. Sexual reproduction involves two different types of gametes (sex cells): oocytes and spermatozoa. HOMOLOGOUS CHROMOSOMES REVIEW Figure 13.4 in your textbook. Homologous chromosomes are chromosome pairs of the same length, centromere position, staining pattern, and gene possession for the same characters: One homologous chromosome is inherited from the organism’s father and the other from the mother (see diagram below). These can easily be seen on a human karyotype (see karyotype of human male below). DIPLOID – means “two sets”. This represents the cells in which the chromosomes are paired up and have a partner in size and shape. A cell containing TWO sets of chromosomes (2n), one set inherited from each parent is referred to as a DIPLOID CELL. Meiosis Overview Meiosis is the process by which the number of chromosomes per sex cell is cut in half through the separation of homologous chromosomes in a diploid cell. In other words, meiosis is the division of sex cells that results in the production of eggs (in females) and sperm (in males). The GOAL of meiosis is to reduce the number of chromosomes from a diploid number to a haploid number. The new cells that result are called gametes, and can join together with their counterparts to produce a new diploid organism (egg with sperm, sperm with egg). HAPLOID – means “one set”; this describes the sex cells or gametes (EGG AND SPERM) – these contain only a SINGLE set of chromosomes. So, for a human, body cells are diploid and have a chromosome number of 46 (23 pairs). But, sex cells are haploid, and have a chromosome number of JUST 23. AP Biology Notes Outline Chapter 13: Meiosis and the Sexual Life Cycle Phases of Meiosis: REVIEW Figure 13.7 in your textbook. Meiosis is divided into two distinct stages: Meiosis I Meiosis II Meiosis I accomplishes separation of homologous chromosomes (see diagram to right). Meiosis I is very similar to mitosis, except that the duplicated chromosomes do not separate the chromatids; instead, the homologous chromosomes line up and form a TETRAD and the homologues separate. Meiosis II accomplishes separation of sister chromatids (see diagram to right). Meiosis II begins WITHOUT a copying of chromosomes; thus, when division occurs, there are less chromosomes in the resulting daughter cells that were in the parent cell – this accomplishes reduction division. NOTE: DNA replications ONLY ONCE during meiosis (during meiosis I only)!!! REVIEW Figure 13.8 in your textbook and watch the following USEFUL ANIMATION http://www.sumanasinc.com/webcontent/animations/content/meiosis.html Crossing Over: As homologous chromosomes pair up and form tetrads in meiosis I, they may exchange portions of their chromatids. This event is called CROSSING OVER. This results in an exchange of genetic material that gives new gene combinations. The site at which the exchange occurs is called the chiasma. During prophase I of meiosis, the duplicated chromosomes pair with their homologues, in a process called synapsis. During this process, a protein “zipper” called the synaptonemal complex holds the homologous chromosomes tightly together all along their lengths. When this complex disappears in late prophase, the four closely associated chromatids of a homologous pair are visible as a tetrad. Crossing Over (exchanging portions of homologous chromosomes) occurs during PROPHASE 1 OF MEIOSIS I. Crossing over gives rise to recombinant chromosomes, individual chromosomes that have some combination of DNA originally derived from 2 different parents. AP Biology Notes Outline Chapter 13: Meiosis and the Sexual Life Cycle Comparison of Meiosis in Males & Females: In males, meiosis results in four equal-sized gametes – sperm. In females, only one large egg is made. The other three cells, called polar bodies, are reabsorbed by the organism and the materials recycled. The reason for this in the female is the need for more space and organelles in order to support the development of a new organism. The sperm will join with the egg, but it is in the egg where continuation and development of the new organism will actually occur. AP Biology Notes Outline Chapter 13: Meiosis and the Sexual Life Cycle Mistakes in Meiosis REVIEW the following animation: http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.html Mistakes in meiosis I and II can cause nondisjunction (not coming apart) of chromosomes - leads to abnormal number of chromosomes in gametes -- trisomy 21 is the most common. In trisomy 21, individuals have 3 copies of chromosome 21 (down syndrome). Child will exhibit facial characteristics of Down syndrome. If the nondisjunction occurs during Meiosis I, then FOUR ABNORMAL gametes will be formed. If the nondisjunction occurs during Meiosis II, then two NORMAL and two ABNORMAL GAMETES will be formed. Parthenogensis & Cloning: Both bypass typical meiotic processes! Parthenogenesis is a type of reproduction, occurring in some insects and flowers, in which the unfertilized ovum develops directly into a new individual - Does not necessarily produce identical clones of the parent. Rare in plants! Cloning occurs when an organism is descended asexually from a single ancestor, such as a plant produced by layering or a polyp produced by budding. Offspring is IDENTICAL to parent! AP Biology Notes Outline Comparing Mitosis & Meiosis BE SURE YOU CAN DO THIS!!! Chapter 13: Meiosis and the Sexual Life Cycle