10.2 MEIOSISMartin 2022

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

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)

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

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

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

Crossing Over

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

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

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