Meiosis
Driving Question – What is meiosis? How is it important for sexual reproduction?
Vocabulary:
Homologous Chromosomes – 2 chromosomes with the same function, but are not necessarily the same; one chromosome comes from each parent. Ex – eye color, hair color, etc
Diploid – number of chromosomes in a body cell; twice the number of chromosomes in a haploid cell; abbreviated 2N. In humans, 2N = 46
Haploid – number of chromosomes in a sex cell; half the number of chromosomes in a diploid cell; abbreviated N. In humans, N = 23
Gamete – sex cell; sperm and eggs (ova)
Somatic Cell – Body cell; any cell that is not a gamete
Zygote – single cell produced when a sperm fertilizes an egg; grows in to an organism through Mitosis
Crossing-Over – process by which homologous chromosomes exchange chromatids (pieces of the chromosome) to produce new genetic combinations
Meiosis – Process by which gametes are produced; divided into 2 stages – Meiosis I and Meiosis II
Process of Meiosis
Meiosis I is nearly identical to Mitosis: a single diploid cell duplicates its chromosomes and produces 2 diploid cells. In Meiosis II, each of the 2 new cells undergoes another cell division without duplicating chromosomes, resulting in a total of 4 haploid cells (gametes).
Meiosis I
Interphase – Cell is growing, replicating chromosomes, and producing organelles for cell division (G1, S, and G2 phases of Cell Cycle)
Prophase I – Each copied chromosome pairs up with its homologous chromosome, forming a tetrad, made of 4 chromatids. Crossing-over may occur, in which parts of the tetrads touch and exchange genetic material.
Metaphase I and Anaphase I – Tetrads line up in the center of the cell; spindles form and attach to each chromosome in the tetrad; homologous chromosomes separate.
Telophase I and Cytokinesis – Nuclear membrane forms around each cluster of chromosomes; cell splits into 2 new cells; each new cell has homologous, but not identical, chromosomes.

Meiosis II
Prophase II – Chromosome pairs in each cell are made of identical chromatids
Metaphase II and Anaphase II – Chromosome pairs line up at center of the cell; spindles attach to, and separate, identical chromatids.
Telophase II and Cytokinesis – Nuclear membrane forms are each set of genetic material, and each cell separates into 2 haploid cells (total of 4 haploid cells from one diploid cell)