Meiosis
Cells Divide, and Cells Die Your cells are rapidly dying and being replaced. Cell division produces a continuous supply of replacement cells. Cell division also has a role in reproduction, growth, and development. Section 8.1
Cell Division Cells divide to make more cells. While all the other organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA. Mitosis is normal cell division, which goes on throughout life in all parts of the body. Meiosis is the special cell division that creates the sperm and eggs, the gametes.
Two Types of Cell Division Interact in the Sexual Life Cycle Each mature individual produces sex cells by another form of cell division called meiosis. Sex cells combine at fertilization. Section 8.1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 8.1
Two Types of Cell Division Interact in the Sexual Life Cycle A fertilized egg, or zygote, divides by mitosis. The fetus grows and develops into a mature adult consisting of countless cells with identical DNA. Section 8.1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 8.1
Why Meiosis? In order for each generation to have the same number of chromosomes, gametes must have half the number of chromosomes (N) as a body cell (2N). Meiosis a type of cell division that produces four gametes, each with half the number of chromosomes as a body cell.
Homologous Chromosomes Paired chromosomes with genes for the same traits are called homologous. Genes are arranged in the same order on homologous chromosomes.
Human Chromosomes KARYOTYPE P. 128
Meiosis Meiosis occurs in two separate divisions, meiosis I and meiosis II. Meiosis starts with one 2N cell and ends with four N cells. Meiosis is necessary for sexual reproduction.
Meiosis cont.
Meiosis I Crossing Over nonsister chromatids of homologous chromosomes exchange genetic material. Homologous chromosomes are separated into two new cells.
Crossing -Over P. 131
Meiosis II Sister chromatids are separated Results in four cells, each with one copy of each chromosome.
Meiosis Provides Variation Meiosis makes cells that are not identical. Meiosis results in a variety of different gametes. Gametes are randomly fertilized, producing very large numbers of possible unique offspring. Crossing over in Meiosis I adds more variation.
9.2 Phases of Meiosis The same four stages of mitosis occur during meiosis. Prophase Metaphase Anaphase Telophase The four stages occur twice, once during meiosis I and again during meiosis II.
Meiosis I Events Prophase I The spindle appears between the separating chromosomes. Synapsis (tetrad formed) Crossing-over may occur. Metaphase I, tetrads aligned at the spindle equator.
Meiosis I (cont.) Anaphase I- Homologous chromosomes separate. Cell divides to become two haploid (1N) cells. Telophase I, nuclear envelope reforms
P. 132 Meiosis I - Summary
Meiosis II- Second Division Repeats Meiosis I with Sister Chromatids separating to form two more cells- Summary: Single diploid cell undergoes meiosis to produce four haploid gametes.
P. 133 Meiosis II Summary
9.3 Meiosis VS Mitosis Meiosis- two nuclear divisions while Mitosis- one. Meiosis-four daughter nuclei (1N, haploid) Mitosis-only two. (2N, diploid) Both followed by cytokinesis.
9.3 Meiosis VS Mitosis (cont.) Meiosis- Daughter cells genetically dissimilar to parent cell. Mitosis- Daughter cells genetically identical to parent cell.
Meiosis VS Mitosis P. 135
Mistakes in Meiosis Nondisjunction failure of homologous chromosomes to separate. Results in gametes that are missing chromosomes or that have extra chromosomes.
Down syndrome characteristics: Short stature Eyelid folds Stubby fingers Wide gap between the first and second toes. Fissured tongue Round head Palm crease (the Simian line) Mental Retardation Down Syndrome Down syndrome is trisomy (3-copies) of chromosome 21.
Down s Syndrome P. 136
Mistakes in Meiosis cont. Monosomy one chromosome instead of a pair. Triploidy three copies of ALL chromosomes, resulting from complete nondisjunction.