Meiosis: Cell Division and Genetic Variation

• Meiosis is a cell division process where one gamete-producing cell divides twice to create four gametes or sex cells.
• Gamete-producing cells are diploid (2n), containing a full set of chromosomes.
• Gamete daughter cells are haploid (n), containing half the number of chromosomes as the parent cell.
• Mitosis involves one cell division, resulting in two identical diploid cells, and contributes to growth and repair.
• Meiosis produces gametes (sex cells): ova/egg cells in females, and sperm cells in males.
• Meiosis involves two cell divisions, producing gametes containing half the DNA of the parent cell.
• Human gametes have 23 chromosomes containing mixed genetic information from each parent.
• Alleles are varying options for genes, and crossing-over produces new combinations of alleles.
• Tetrads form when homologous pairs of chromosomes come together and crossing over occurs during prophase I.
• Meiosis creates genetically unique sex cells through tetrad formation and crossing over.
• Gametes are produced during meiosis, with resulting gametes being unique, contributing to variations in offspring.

Steps of Meiosis

• Meiosis begins after interphase, involving two meiotic cell divisions: meiosis I and meiosis II.
• Meiosis I produces two diploid (2n) daughter cells.
• Meiosis II produces four haploid (n) cells.
• Each meiotic cell division involves prophase, metaphase, anaphase, and telophase (P.M.A.T.).
• Cytokinesis, the division of a cell's cytoplasm, occurs after each P.M.A.T.
• A gamete producing cell will go through interphase before proceeding to the steps of meiosis I and II

Interphase

• Interphase is the period before meiosis where a cell prepares for cell division.
• During interphase, the cell grows to have enough material to produce four cells.
• DNA synthesis occurs and chromosomes replicate during interphase.
• Interphase results in a diploid cell with double the chromosomes.
• Humans have 46 chromosomes after interphase, consisting of 23 paired homologous chromosomes from each parent.
• Homologous chromosomes are chromosomes paired with a corresponding chromosome from the opposite parent.
• Meiosis I and Meiosis II result in four genetically unique haploid gamete cells.
• Meiosis produces gametes through meiotic cell division.

Meiosis I

Prophase I

• The cell's nuclear membrane dissolves.
• Spindle fibers form from centrioles, which migrate to opposite ends of the cell.
• Replicated chromosomes pair with homologous chromosomes, forming a tetrad.
• A tetrad contains two pairs of chromosomes or four chromatids.
• Crossing-over occurs as tetrads form, with chromatids exchanging alleles.

Metaphase I

• Spindle fibers attach to each tetrad structure.
• Paired homologous chromosomes (tetrads) line up in the middle of the cell.

Anaphase I

• Tetrads separate into two pairs of homologous chromosomes.
• Spindle fibers pull each chromosome pair towards opposite ends of the cell.

Telophase I and Cytokinesis

• Chromosome pairs cluster on opposite ends of the cell.
• Nuclear membranes form around the two clusters of chromosomes, forming separate nuclei.
• The cell's cytoplasm begins to pinch in, initiating cytokinesis.
• Cytokinesis results in two haploid cells with new combinations of alleles.

Meiosis II

• Two genetically unique haploid cells from meiosis I enter their own cell division process.

Prophase II

• Each cell's nuclear membrane dissolves, and chromosome pairs become visible.
• Spindle fibers form from centrioles, which migrate to opposite ends of the cell.
• Crossing-over doesn't occur, and chromosome pairs do not replicate again.

Metaphase II

• Each cell's chromosome pairs line up in the middle of each cell.
• Spindle fibers attach to opposite sides of individual chromosome pairs.

Anaphase II

• Each cell's chromosome pairs separate into chromatids.
• Spindle fibers pull chromatids toward opposite ends of each cell.

Telophase II and Cytokinesis

• Each cell's separated chromatids cluster on opposite ends.
• Two nuclear membranes form around the clustered chromatids within each cell, resulting in four haploid nuclei.
• Cytokinesis begins, with each cell's cytoplasm pinching along their middles.
• Cytokinesis results in four haploid gamete cells, each with a genetically unique set of chromatids.
• In humans, meiosis II results in four gametes, each with 23 chromosomes (as 23 chromatids).

Explore meiosis, the cell division process that creates genetically unique gametes. Understand how diploid cells divide twice to form haploid sex cells with half the number of chromosomes. Learn about tetrad formation, crossing over, and the production of diverse allele combinations in gametes.