Cell division: Meiosis

Meiosis and sexual life cycles: In the sexual reproduction process, two gametes fuse during fertilization to produce a zygote. Meiosis is a specialized type of cell division which reduces the chromosome number by half (reduces chromosome number from diploid to haploid). This process occurs in animals and plants. Meiosis occurs only in sex gonads to form gametes.

Fertilization restores the diploid condition by combining two haploid sets of chromosomes. Gametes, produced by meiosis, are the only haploid cells. Gametes undergo no divisions themselves, but fuse to form a diploid zygote that divides by mitosis to produce a multicellular organism.

Meiosis and mitosis are preceded by the replication of chromosomes. However, in meiosis, chromosomes replicate once followed by two consecutive cell divisions, meiosis I and meiosis II, which results in four daughter cells. Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice. 1) The first division is known as meiosis I (a reductional division): separates homologous chromosomes. 2) The second is known as meiosis II (an equational division: separates sister chromatids.

The phases of meiotic division: 1) Meiosis I 2) Meiosis II Occurs in two steps Prophase II Metaphase II Anaphase II Telophase II Prophase I Metaphase I Anaphase I Telophase I

The phases of meiosis I Prior to meiosis, chromosome duplication occurs during the S of the interphase stage in order to double the sister chromatid pairs. During prophase I: Homologous chromosomes (tetrad) pair up by the synaptonemal complex in a process called synapsis Each pair of homologous chromosomes is known as a bivalent (tetrad). The pairs of chromosomes consist of four chromatids, with one chromosome coming from each parent. Crossing over between homologous chromosomes occurs and DNA is exchanged between the bivalents in a process called homologous recombination.

Crossing over

2. At metaphase I: homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate. In humans, you would see 23 tetrads. 3. At anaphase I: Homologous chromosomes, not sister chromatids are separated from each other, but the sister chromatids remain attached to each other at the centromeres until anaphase II. The processes during the second meiotic division (meiosis II) are identical to those in mitosis.

Comparison between mitosis and meiosis

Definitions Genes: The units that specify an organism’s inherited characteristic. Chromatin: A DNA-protein complex which is organized into a long thin fiber. Chromosome: The package that formed from a condensed, coiled and folded chromatin. Chromatids: Two sister chromatids formed from each duplicated chromosome. They contain identical copies of the chromosome’s DNA. Centromere: The narrow region which the two sister chromatids are connected together. Mitosis: A type of cell division that occurs in somatic cells and generates two identical daughter cells Meiosis: A type of cell division that occurs in the gonads, and yields four non-identical daughter cells, each with half the chromosomes of the parent.