Meiosis I: Prophase I

• Meiosis is a specialized type of cell division halving the chromosome number and yielding four genetically distinct haploid cells from a single diploid parent cell
• It is essential for sexual reproduction in eukaryotes, ensuring genetic diversity in offspring
• Meiosis involves two successive nuclear divisions: meiosis I and meiosis II

Meiosis I

• Meiosis I separates homologous chromosomes
• It starts with a diploid cell that has undergone DNA replication during interphase

Prophase I

• Prophase I is the longest phase of meiosis
• Chromatin condenses during prophase I, and homologous chromosomes pair up via synapsis, forming tetrads (bivalents)
• Non-sister chromatids within a tetrad exchange genetic material via crossing over
• Crossing over leads to genetic recombination, increasing genetic diversity
• Chiasmata, the physical manifestations of crossing over, become visible
• The nuclear envelope breaks down and spindle microtubules attach to the kinetochores of the chromosomes as prophase I progresses; this transition is sometimes called pro-metaphase I

Metaphase I

• Tetrads align along the metaphase plate, with each chromosome attached to spindle microtubules from opposite poles
• The orientation of each tetrad is random, contributing to independent assortment

Anaphase I

• Homologous chromosomes separate and move toward opposite poles
• Sister chromatids remain attached at the centromere

Telophase I and Cytokinesis

• Homologous chromosomes arrive at opposite poles
• Nuclear envelopes may reform
• Cytokinesis typically occurs simultaneously, resulting in two haploid cells
• Each cell contains one chromosome from each homologous pair

Meiosis II

• Meiosis II separates sister chromatids, similar to mitosis
• It starts with the two haploid cells created in meiosis I

Prophase II

• Chromatin condenses and the nuclear envelope breaks down if it has reformed
• Spindle microtubules attach to kinetochores of sister chromatids

Metaphase II

• Sister chromatids align along the metaphase plate
• Each sister chromatid is attached to spindle microtubules from opposite poles

Anaphase II

• Sister chromatids separate and move toward opposite poles
• These separated sister chromatids are now individual chromosomes

Telophase II and Cytokinesis

• Chromosomes arrive at opposite poles
• Nuclear envelopes reform
• Cytokinesis occurs, resulting in four haploid cells

Genetic Variation

• Meiosis generates genetic variation through several mechanisms

Crossing Over

• During prophase I, crossing over between non-sister chromatids results in the exchange of genetic material
• Results in new combinations of alleles on the same chromosome

Independent Assortment

• During metaphase I, the random orientation of tetrads leads to independent assortment of chromosomes
• Results in different combinations of maternal and paternal chromosomes in each daughter cell

Random Fertilization

• The fusion of gametes (sperm and egg) during fertilization is a random process, further increasing genetic variation
• Any sperm can fuse with any egg

Comparison with Mitosis

• Meiosis and mitosis are both forms of cell division, but they have different purposes and outcomes
• Mitosis results in two diploid cells that are genetically identical to the parent cell, while meiosis results in four haploid cells that are genetically distinct from the parent cell
• Mitosis is involved in growth, repair, and asexual reproduction, while meiosis is involved in sexual reproduction

Errors in Meiosis

• Nondisjunction occurs when chromosomes fail to separate properly during meiosis I or meiosis II
• Results in gametes with an abnormal number of chromosomes (aneuploidy)
• Can lead to genetic disorders such as Down syndrome (trisomy 21)

Significance of Meiosis

• Meiosis is essential for sexual reproduction
• Maintains a constant chromosome number from generation to generation
• Generates genetic variation
• Provides the raw material for natural selection and evolution