Meiosis

Autosomes

Chromosome pairs 1 to 22 are designated as ____, which are chromosomes associated with genes not directly related to the sex of an organism.

Sex Chromosomes

The twenty-third pair is a special one called ______, which are directly linked to the development of sexual characteristics of an organism.

Meiosis 1

focuses on the division of homologous chromosomes to produce two haploid cells with duplicated chromosomes.

Prophase 1

is characterized by the breaking down of nuclear envelope. Spindle fibers begin co assemble. Duplicated chromosomes condense, while the homologous chromosomes pair and line up by gene precisely in its entire length. Aside from pairing along their length, they cross over at a point called chiasma. At each chiasma, crossing-over happens when portions of a chromatid on one homologous chromosome breaks, exchanges, and rejoins genetic material with the corresponding portions on one of the chromatids of the other homologous chromosome. This is a way of trading genes between homologous chromosomes.

Metaphase 1

The pairs of homologous chromosomes are randomly moved by the spindle fiber to the equator of the cell. This results in the lining up of 23 chromosomes alongside each other in the middle of the cell. This process is important, as this arrangement mixes the chromosomes, helping to create genetic diversity and variation. Having 23 chromosomes, meiosis can result in about 223 or 8 388 608 combinations of chromosomes.

Anaphase 1

Homologous chromosomes separate from each other. As in mitosis, chromosomes of each pair are pulled to the opposite sides of the cell by the action of spindle fibers. Sister chromatids remain together and do not separate at their centromeres throughout meiosis I. The genetic material, however recombines.

Telophase

The individual chromosomes that have been pulled in opposite directions now gather at each pole. Both poles contain one chromosome from each pair of the homologous chromosomes. There is also a reappearance of the nuclear membrane and disassembly of spindle fibers. The cytoplasm divides through cytokinesis, forming two new cells each having a combination of 23 chromosomes from parent cells.

Meiosis II

is to divide sister chromatids, resulting in sex cells with only half the chromosome number.

Prophase II

New spindle forms around the chromosome. The nuclear envelope breaks down with chromosomes pulled at opposite sides of the cell by the spindle fibers.

Metaphase II

Chromosomes line up along the equator through the spindle fiber. At this stage, each
chromosome has sister chromatids still attached to the centromere.

Anaphase II

Centromeres divide and sister chromatids are individually pulled apart, then move to
opposite poles of the cell.

Telophase II

Nuclear envelope forms around each set of chromosomes at opposite ends of the cell. The spindle fiber breaks down and the cell undergoes cytokinesis. The result of meiosis is four haploid cells with a recombination of the chromosomes both from the mother and father.

Independent Assortment

The random distribution of homologous chromosomes during meiosis is called _______.

Apoptosis

The polar body may divide again, but its cells will not survive and the polar bodies will die in a process called _____.