SEXUAL REPRODUCTION MEIOSIS SPERMATOGENESIS & OOGENESIS 2/6/2011. Asexual Reproduction:

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1 Asexual Reproduction: SEXUAL REPRODUCTON & MEOSS Many single-celled organisms reproduce by splitting, budding, parthenogenesis. Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent). SEXUAL REPRODUCTON Other organisms go through sexual reproduction and produce offpsrings by combining a male s sex cell with a female s sex cell (fertilization). Meiosis is the process by which sex cells (gametes) are formed. n humans, meiosis takes place in: males testes to create sperm (spermatogenesis) and in females ovaries to create ova/egg (oogenesis). SPERMATOGENESS & OOGENESS MEOSS Similar in many ways to mitosis, but with several differences: nvolves 2 cell divisions (Meiosis and Meiosis ) Results in 4 cells with ½ the normal genetic information (haploid) The 4 cells are different from each other, as well as different from the parent cells. VOCABULARY FOR MEOSS Diploid (2n) - Normal amount of genetic material (2 sets of chromosomes). n humans, autosome (body cells) contain 46 Haploid (n) - ½ the genetic material (1 set of chromosomes). n humans, gametes (sex cells) contain 23 1

2 Ex: Spermatogenesis VOCABULARY FOR MEOSS sperm Homologous chromosomes: paired chromosomes (1 from father and 1 from mother) that are similar in shape, size, gene arrangement and gene information. 2n=46 diploid (2n) meiosis meiosis haploid (n) From mother From father MEOSS = REDUCTON REDUCTON DVSON Prior to cell division, DNA replication occurs. Meiosis is divided into 4 stages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Replicated chromosomes condense, nuclear membrane dissolve, centrioles moves to opposite poles, spindle fibres formed Homologous chromosomes come together (synapsis) and form a tetrad. Crossing over occurs at chiasmata. Chromosomes then attach to the spindle fibers. CONTNUES CROSSNG OVER Diagram Actual Segments of homologous chromosomes break and reform at similar locations. Chiasmata Results in new genetic combinations of offspring. This is the main advantage of sexual reproduction 2

3 METAPHASE HOMOLOGOUS chromosomes line-up at the equatorial plate. ANAPHASE During anaphase, each homologous chromosome is pulled to opposite sides of the cell. Unlike mitosis, the centromeres do not break up. Nucleus division completed. Nuclear membrane starts to form for each nucleus. Each chromosome is still composed of sister chromatids. CONTNUES Cytoplasm divides. Unlike mitosis, each daughter cells do NOT contain identical homologous MEOSS DNA does not get replicated again prior to meiosis. Meiosis also composed of 4 stages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Centrioles move to opposite poles. Spindle fibers form. Chromosomes attach to spindle fibers. 3

4 METAPHASE Chromosomes will randomly line-up along metaphase plate, just like in regular mitosis. ANAPHASE During anaphase, centromeres break, allowing sister chromatids to separate and be pulled to opposite sides of the cell. Nuclear membrane begins to form. Chromatids now referred to as 2 nd division of cytoplasm occurs End up with 4 haploid daughter cells. Chromosome number has been reduced by half (going from 2n to n) The 4 daughter cells are different from each other and from the parent. Number of combinations = 2 n (n = # of chromosome pairs) n= 2, therefore 2 n = 2 2 = 4 possible combinations 4

5 n humans, 23 pairs of chromosomes n humans, with random fertilization, we get 8 million possible combinations for the mother s egg cell and 8 million possible combinations for the father s sperm cell. n= 23, therefore 2 n = 2 23 = 8,388,608 possible combinations A human child born represents 1 genetic possibility in over 64 trillion different possible genetic combinations! 5