1. 10.2 MEIOSIS Martin 2022

2. GENES, CHROMOSOMES, AND NUMBERS A. Dipolid and Haploid Cells: ▫Diploid Cells: cell that have two of each kind of chromosome  2n  Found in all “body” cells ▫Haploid Cells: only have 1 of each kind of chromosome  n  Found ONLY IN SEX CELLS (sperm and egg)  Sex cells also known as gametes

3. GENES, CHROMOSOMES, AND NUMBERS B. Homologous Chromosomes: ▫Two chromosomes in each pair of a diploid cell ▫Each pair has genes for the same trait  Genes arranged in the same order, but may be different in their alleles (dominant or recessive)

5. GENES, CHROMOSOMES, AND NUMBERS Why Meiosis?: ▫Meiosis: a type of cell division that allows gametes (sex cells) to form, so that after fertilization, offspring have the same number of chromosomes as their parents  Sperm: male gametes  Egg: female gametes

6. GENES, CHROMOSOMES, AND NUMBERS Meiosis is used to create cells for sexual reproduction: ▫Male gamete (23) fertilizes female gamete (23) and produces a zygote (offspring with 46 chromosomes) Remember, this is different from mitosis ▫Mitosis = 2n  2n ▫Meiosis = 2n  n

8. THE PHASES OF MEIOSIS PMAT I, then PMAT II ▫Has 2 cell divisions that lead to 4 haploid cells During PROPHASE I ▫2 homologous chromosomes pair so tightly that crossing over can occur  Where non-sister chromatids can break off and exchange genetic information  Can occur at ANY LOCATION on a chromosome, at several locations, and at the same time

9. Crossing Over

10. THE PHASES OF MEIOSIS METAPHASE I ▫Homologous chromosomes line up along the midline/equator as a pair (tetrads) ANAPHASE I ▫Homologous chromosomes separate METAPHASE II ▫Chromosomes line up independently/randomly along equator ANAPHASE II ▫Sister chromatids are separated

11. MEIOSIS PROVIDES FOR GENETIC VARIATION This variation can be because of crossing over during metaphase I Can also be due to genetic recombination ▫the reassortment of chromosomes and the genetic information they carry by crossing over or independent segregation of homologous chromsomes

12. NONDISJUNCTION Sometimes there’s an error in meiosis Nondisjunction: ▫The failure of homologous chromsomes to separate during meiosis ▫Leaves 1 gamete with an extra chromosome, and 1 gamete with 1 missing chromosome (22, 24) ▫Trisomy 21 = Down Syndrome