Biology Chapter: Metaphase I

Overview of Meiosis I

• Meiosis I is a reductional division that decreases chromosome number from diploid (2n) to haploid (n).
• Homologous chromosomes are separated into different cells, generating four genetically distinct daughter cells from one parental cell.
• Essential for sexual reproduction in all sexually reproducing organisms, including animals, plants, and fungi.
• Comprises stages: Prophase I, Metaphase I, Anaphase I, and Telophase I.

Prophase I

• Longest and most complex phase, further divided into five stages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
• Leptotene: Chromosomes condense and become visible. Each chromosome consists of two sister chromatids.
• Zygotene: Homologous chromosomes pair through synapsis, forming tetrads (bivalents) with four chromatids.
• Pachytene: Crossing over occurs; homologous chromosomes exchange genetic material at chiasmata, increasing genetic diversity.
• Diplotene: Synaptonemal complex breaks down; homologous chromosomes begin to separate but remain linked at chiasmata.
• Diakinesis: Chromosomes condense further, nuclear envelope disintegrates, spindle apparatus forms, and kinetochores attach to spindle fibers.

Metaphase I

• Tetrads align at the metaphase plate in the center of the cell.
• Orientation of tetrads is random, facilitating independent assortment and contributing to genetic variability.
• Spindle fibers attach to kinetochores of homologous chromosomes from opposite cell poles.

Anaphase I

• Homologous chromosomes are pulled apart to opposite poles by spindle fibers.
• Sister chromatids remain attached at centromeres and do not separate during this phase.
• Ensures each daughter cell receives a haploid set of chromosomes.

Telophase I

• Chromosomes reach opposite poles, and the nuclear membrane may reform around each chromosome set.
• Chromosomes may slightly de-condense but may remain condensed in some species.
• Followed by cytokinesis, which divides the cytoplasm and forms two haploid daughter cells.

Cytokinesis

• Accompanies Telophase I, resulting in two haploid cells.
• Each daughter cell contains half the original chromosome number, but each chromosome still has two sister chromatids.

Outcome of Meiosis I

• Produces two non-identical haploid daughter cells, each containing half the number of chromosomes as the original diploid cell.
• These haploid cells proceed to Meiosis II, leading to the formation of four genetically distinct haploid gametes.

Significance of Meiosis I

• Critical for sexual reproduction, ensuring the chromosome number is halved.
• Guarantees that upon fertilization, the resulting offspring have the correct diploid chromosome number.