CH. 7 Section 1 Meiosis

CH. 7 Section 1 Meiosis

Meiosis a form of cell division that halves the number of chromosomes when forming reproductive cells, such as gametes (egg & sperm cells) Stages of Meiosis: *Before meiosis begins, DNA in the original cell is replicated. In meiosis, the cell undergoes two cell divisions: Meiosis I and Meiosis II.

The cell then goes through two cell divisions: Meiosis I and Meiosis II. Meiosis I: Meiosis I: Homologous chromosomes separate; This stage consists of prophase I, metaphase I, anaphase I, and telophase I.

Meiosis I Diploid 2n Crossing Over Independent Assortment Haploid n

Haploid n Prophase I: chromosomes condense; nuclear envelope breaks down; homologous chromosomes pair up

forming a tetrad (4 chromatids); crossing over can occur here Crossing Over Occurs when portions of a chromatid on one homologous chromosome is broken and exchanged with the corresponding chromatid portion of the other homologous chromosome. This leads to genetic variation.

Metaphase I homologous chromosomes line up in the middle of the cell independent assortment occurs Anaphase I

homologous pairs separate Telophase I nuclear membrane reforms two cells are produced each containing 1 set of

the homologous pairs cytokinesis begins (cytoplasm divides) Meiosis II: Meiosis II: The 2 chromatids (referred to as sister chromatids) separate; This stage consists of prophase

II, metaphase II, anaphase II, and telophase II. Meiosis II Haploid n * All 4 cells are genetically different!! Haploid

n Haploid n Haploid n Haploid n

Prophase II a new spindle fibers form around the chromosomes Metaphase II sister chromatids line up along the

equator independent assortment occurs Anaphase II centromeres divide sister chromatids separate

Telophase II nuclear membrane forms around each set of chromosomes cytokinesis begins (cytoplasm divides) four haploid cells are produced

Diploid (2n) Meiosis: -reduces the centromere Crossing-over occurs in Prophase I.

Haploid (n) Haploid (n) number of chromosomes by half to form gamete cells Three Mechanisms that Contribute

to Genetic Variation: 1. crossing over when portions of a chromatid on one homologous chromosome is broken and exchanged with the corresponding chromatid portion of the other homologous chromosome 2. independent assortment the random distribution of homologous chromosomes during meiosis 3. random fertilization the random joining of 2 gametes

Importance of Genetic Variation: It allows organisms to change so they can better adapt to the changing environment !

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