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Transcript
Mitosis and Meiosis Mitosis and Meiosis • Mitosis: -division of somatic (body) cells • Meiosis -division of gametes (sex cells) Mitosis 1) Interphase 2)Cell division • Prophase • Metaphase • Anaphase • Telophase 3) Cytokinesis 1) Interphase • Interesting things happen! 1. Cell preparing to divide 2. Genetic material doubles 2. Cell division Prophase • 1. Chromosome pair up! Chromosomes thicken and shorten -become visible -2 chromatids joined by a centromere 2. Centrioles move to the opposite sides of the nucleus 3. Nucleolus disappears 4. Nuclear membrane disintegrate Metaphase • Chromosomes meet in the middle! 1. Chromosomes arrange at equator of cell 2. Become attached to spindle fibres by centromeres 3. Homologous chromosomes do not associate Anaphase • Chromosomes get pulled apart 1. Spindle fibres contract pulling chromatids to the opposite poles of the cell Telophase • 1. 2. 3. 4. 5. Now there are two! Chromosomes uncoil Spindle fibres disintegrate Centrioles replicate Nucleur membrane forms Cell divides 3) Cytokinesis • Division of cytoplasm • Division of cell into two daughter cells Meiosis 1) Interphase 2)Cell division • Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I) • Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II) 3) Cytokinesis Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n) Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die. 1) Interphase • Similar to interphase. mitosis Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I Prophase I • Longest and most complex phase. • 90% of the meiotic process is spent in Prophase I • Chromosomes condense. Prophase I spindle fiber centrioles Metaphase I • Shortest phase • Chromosomes align on middle of spindle fibers. the Metaphase I OR metaphase plate metaphase plate Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres. Anaphase I Telophase I • Each pole now has haploid set of chromosomes. Telophase I 3) Cytokinesis Cytokinesis occurs and two haploid daughter cells are formed. Meiosis II • No interphase II (no more DNA replication) • Remember: Meiosis similar to mitosis II is Prophase II • same as prophase in mitosis Metaphase II • same as metaphase in mitosis metaphase plate metaphase plate Anaphase II • same as anaphase in mitosis • sister chromatids separate Telophase II • Same as mitosis. • Nuclei form. telophase in Telophase II 3) Cytokinesis • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg Meiosis and sex chromosomes Sex chromosomes carry genes that determine sex (gender) In humans, females have two X chromosomes. But human males have one X chromosome and one Y chromosome During meiosis, one of each of the chromosome pairs ends up in a sex cell. Females have two X chromosomes in each body cell. When meiosis produces the egg cells, each egg gets one X chromosome Males have both an X chromosome and a Y chromosome in each body cell. Meiosis produces sperm with either an X or a Y chromosome An egg fertilized by a sperm with an X chromosome will produce a female. If the sperm contains a Y chromosome, the offspring will be male. Meiosis and sex chromosomes Sex linked disorders The Y chromosome does not carry all of the genes of an X chromosome Females have two X chromosomes, so they carry two copies of each gene found on the X chromosome. This makes a backup gene available if one becomes damaged. Males have only one copy of each gene on their one X chromosome. The genes for certain disorders, such as color blindness, are carried on the X chromosome. These disorders are called sex-linked disorders. Because the gene for such disorders is recessive, men are more likely to have sex-linked disorders