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MEIOSIS Chapter 10.2 Chromosomes 2 Normal human body cells each contain 46 chromosomes. Body Cells are produced by mitosis Produces daughter cells that are virtually identical to the parent cell. Contain two copies of each chromosome Gametes are produced by meiosis Eggs & Sperm Contain one copy of each chromosome Diploid & Haploid Cells 3 Diploid (2n)= a cell with two of each kind of chromosome Normal body cells are diploid Haploid (n)= a cell with one of each kind of chromosome Gametes are haploid Homologous Chromosomes 4 Homologous chromosomes = paired chromosomes in diploid cells Each chromosome is made up of two sister chromatids Meiosis 5 When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. Meiosis produces daughter cells containing half the number of chromosomes (haploid cells) as the parent cell – gametes Female gametes = eggs Male gametes = sperm Sexual reproduction: When a sperm cell fertilizes the egg, the resulting cell is called a zygote. Zygotes are diploid because they contain all of the chromosomes from the egg and the sperm. Process of Meiosis 6 Meiosis is made up of two cell divisions Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Prophase 2 Metaphase 2 Anaphase 2 Telophase 2 Prophase 1 7 Similar to Prophase in mitosis Chromosomes and spindle form, nuclear membrane disappears Homologous chromosomes pair up & form tetrads Crossing over occurs – homologous chromosomes exchange genetic information Metaphase 1 8 Chromosome centromeres are attached to the spindle Homologous chromosomes are lined up in the middle of the cell as tetrads Anaphase 1 9 Homologous chromosomes are separated and pulled to opposite ends of the cell This step ensures that each new cell will receive only one chromosome from each homologous pair. Telophase 1 10 The spindle is broken down Chromosomes uncoil Cytoplasm divides Each new cell has half of the genetic information of the original cell Prophase 2 11 Chromosomes coil A spindle forms in each new cell & spindle fibers attach to chromosomes Metaphase 2 12 Chromosomes are lined up in the center of the cell Looks similar to metaphase in mitosis Anaphase 2 13 The centromere of each chromosome splits Sister chromatids are separated & pulled to opposite ends of the cell Telophase 2 14 Cytoplasm divides, nuclear envelope reforms, spindle breaks down Similar to mitosis & telophase 1 After Meiosis 15 Four haploid gametes have formed from the one original diploid cell Each daughter cell contains 1 chromosome from each homologous pair Gamete formation is different in males & females Males – 4 sperm are produced Females – 1 egg and 3 polar bodies are produced Cytoplasm is divided unevenly Genetic Variation 16 Meiosis helps to produce new genetic combinations. Crossing over Random separation of homologous chromosomes The number of different genetic combinations any egg or sperm can produce is more than 8 million When fertilization occurs, 70 trillion different zygotes are possible.