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Learn about the process of meiosis, the phases involved, and how it leads to gamete formation for genetic diversity in sexual reproduction. Discover the significance of homologous chromosomes, haploid and diploid cells, and the roles of gametogenesis in males and females.
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Meiosis Meiosis – process of reduction division in which the number of chromosomes per cell is cut in ½ by the separation of homologous chromosomes in a diploid cell. -Meiosis refers to sexualreproduction in which genetic info from 2 parents is combined to make genetically different offspring. -Meiosis occurs in sex cells (gametes).
Meiosis – has 2 divisions (Meiosis I & Meiosis II = 4 daughter cells)
A. Chromosome Number -In organisms that reproduce sexually, half of the genetic material comes from the mother & half comes from the father. -These 2 sets of chromosomes from each parent are homologouschromosomes – chromosomes that each have a matching chromosome from the opposite-sex parent. -Cells that contain both sets of chromosomes are diploid or 2N, meaning “two sets”. In humans 2N = 46 chromosomes in a cell. -Cells that contain a single set of chromosomes are haploid or N, meaning, “one set”. In humans N = 23 chromosomes.
B. The Phases of Meiosis *Meiosis involves 2 divisions : Meiosis I & Meiosis II -By the end of meiosis II, the diploid cell has become 4 haploid cells. In meiosis I : 1. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad - structure containing 4 chromatids. 2. Homologous chromosomes begin crossing-over - exchange of portions of their chromatids. -Crossing-over creates genetic variation among organisms. 3. Homologous chromosomes separate & form 2 cells that are ready to divide again.
In Meiosis II : 1. The 2nd meiotic division starts. 2. Chromosomes line up in center of cell, separate, & form 4 daughter cells that are genetically different from the parent cell.
Summarization of Meiosis Interphase I – cells undergo DNA replication, forming duplicate chromosomes. Meiosis I – Stage one of division - Has 4 phases : Prophase I – Each chromosome pairs with its matching homologous chromosome to form a tetrad. Metaphase I – Spindle fibers attach to the chromosomes. Anaphase I – Fibers pull the homologous chromosomes to opposite poles of cell. Telophase I & Cytokinesis – Nuclear membranes form. Cell separates into 2 cells.
Meiosis II – Stage two of division – Has 4 phases : Prophase II – Meiosis I results in 2 haploid (N) daughter cells, each with ½ the number of chromosomes as the original cell. Metaphase II – Chromosomes line up in the center of the cell. Anaphase II – Sister chromatids separate & move to opposite poles. Telophase II & Cytokinesis – Result is 4 haploid (N) daughter cells.
C. Gamete Formation *Meiosis is also called : Gametogenesis – the process by which gametes develop in the gonads (production of gametes). Gametes – haploid (N) sex cells. Sperm – male sex cells & Eggs – female sex cells Gonads – specialized organs used for reproduction. In males they’re testes, which produce sperm & in females they’re ovaries, which produce eggs or ova. -Males undergo spermatogenesis – formation of sperm in the testes. The end result is 4 good sperm cells. -Females undergo oogenesis - formation of eggs in the ovaries. The end result is 1 good egg & 3 polar bodies that aren’t used. *Fertilization restores the diploid (2N) # of chromosomes.
