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IV. Meiosis
A.
B.
Definition1.
nuclear division in sexual organisms
2.
reduces the number of original chromosomes in new cells by ½
3.
Humans produce gametes or sex cells (sperm or egg)
4.
23 chromosomes (1N-haploid)
5.
Fusion of a sperm and egg reestablish the 46 (2N-diploid)
chromosomes.
Stages
1.
Interphase- G1; S; G2
2.
Meiosis I
a. Prophase 1
 Chromosomes coil around histone proteins
 Spindle fibers appear, centriols begin to move
 Organelles disappear
 Synapsis occurs- Homologous chromosomes (XX) line up
next to each other (XX XX).
o Each homologous pair (XX XX) is called a tetrad
o Crossing over may occur between the tetrad
 Paternal and Maternal chromosomes
exchange genetic material
 Genetic Recombination creates diversity
b. Metaphase 1
 Tetrads (XX XX) line up randomly in the middle of the
cell
 Spindal fibers attach to the centromere of each
chromosome
c. Anaphase 1
 Spindal fibers pull the tetrad apart. Each chromosome
moves toward opposite poles
 Independent assortment: random separation of the
homologous chromosomes
d. Telophase 1 and Cytokinesis 1
 Chromosomes reach opposite ends of the cell and
cytoplasm divides
 Two new cells contain a haploid number of chromosomes
3. Summary of Meiosis 1
 Original cell produces two new cells
 New cells contain a homologous chromosome from the tetrad
 New cells contain ½ the number of original chromosomes (1n)
 New cells contain 2 chromatid strands in each chromosome
4.
Meiosis II (Telophase 1  Prophase II, no need for Interphase)
a. Prophase II
 Spindle fibers form, centriols begin to move
B.
 Chromosomes move to center of cell
b. Metaphase II
 Chromosomes line up in center of cell
 Centromeres replicate
c. Anaphase II
 Chromatids separate
 Chromosomes move to opposite poles
d. Telophase II and Cytokinesis II
 Nucleus reforms
 Organelles reappear
 Four new cells are formed which are haploid (1n)
Gamete Production in Males vs. Females (human)
1.
Male
a.
Meiosis occurs in the Testes (4 new cells are spermatids)
b.
Spermatids (immature sperm) develop into mature sperm
cells
c.
Spermatogenesis: production of sperm cells
2.
Female
a.
Meiosis occurs in the Ovaries (1 ovum, 3 polar bodies)
b.
Polar bodies (smaller than ovum) do not receive enough
cytoplasm to support a zygote and are reabsorbed.
c.
Oogenesis: production of egg cell