When calculating the number of chromosomes in a cell, count the number of centromeres—this will tell you how many chromosomes there are."

• Be aware that a chromosome will contain one chromatid before DNA replication and two chromatids after DNA replication.

• Meiosis enables parents to transmit genetic information on to their children.

• Meiosis contributes to this effort by maintaining a steady number of chromosomes in children and promoting genetic variation within the species.

• This chapter discusses the process of meiosis and how it completes these objectives.

This chapter also contrasts meiosis and mitosis.

• Meiosis is a cell division process that occurs during gamete production.

• Meiosis is the process through which haploid (n) gametes are produced from diploid (2n) parent cells.

• This helps to ensure that the offspring have the correct amount of chromosomes.

• The resultant zygote has the right diploid (2n) number of chromosomes when a haploid (n) egg is fertilized by a haploid (n) sperm.

Unlike mitosis, which produces two genetically identical diploid daughter cells, meiosis produces four genetically distinct haploid gametes through two rounds of cell division (meiosis I and meiosis II).

• Meiosis I is the first cycle of cell division in meiosis, and it is also known as a reduction division. Prophase I, metaphase I, anaphase I, and telophase I are the four phases of meiosis I.

• The nuclear membrane breaks down during the prophase stage of mitosis, allowing the chromosomes to condense and become visible.

• This happens at the prophase I stage of meiosis as well.

• In contrast to mitosis, homologous chromosomes can couple up and genetic recombination (also known as crossing-over) can occur during prophase I of meiosis.

• This has far-reaching implications for genetic variety, which will be examined more in this chapter.

Chromosomes line up in a single column along the middle of the cell during the metaphase stage of mitosis.

• However, during the metaphase I stage of meiosis, chromosomes align in homologous pairs down the middle of the cell.

The sister chromatids of each chromosome split and travel to opposite ends of the cell during the anaphase stage of mitosis.

• After this separation, each of these sister chromatids has its own centromere, doubling the number of chromosomes in the cell at the conclusion of the anaphase stage of mitosis.

• Chromosomes condense and become visible again during prophase II of meiosis II.

• During metaphase II, chromosomes align in a single line along the middle of each cell, similar to how chromosomes align during mitosis's metaphase stage.

• The sister chromatids then split and travel to opposing ends of the cell during anaphase II.

• Once this is completed, each sister chromatid will have its own centromere.

• Chromosomes condense and become visible again during prophase II of meiosis II.

• During metaphase II, chromosomes align in a single line along the middle of each cell, similar to how chromosomes align during mitosis's metaphase stage.

• The sister chromatids then split and travel to opposing ends of the cell during anaphase II.

• Once this is completed, each sister chromatid will have its own centromere.

The frequency of genetic recombination between genes on the same chromosome may be used to create genetic maps showing the relative placements of genes on chromosomes.

• Genes that are close together on the same chromosome are more likely to be inherited together (because to the decreased frequency of recombination between them) and are referred to as linked genes.

Genetic recombination between nonhomologous chromosomes can occur at times.

• These occurrences result in mutations known as translocations.

• While translocations can provide novel combinations of genetic material, if they occur in the midst of a gene, they may inactivate it, resulting in an undesirable phenotype.

During the metaphase I stage of meiosis, genetic variety can also be produced.

• Remember that homologous pairs of chromosomes align up during metaphase I?

• Remember that during metaphase I, homologous pairs of chromosomes align at the cell's center.

• Each pair of chromosomes lines up and assorts individually, with distinct pairs having the paternal (male parent) chromosome on one side and the maternal (female parent) chromosome on the other.

• For example, if an organism has two pairs of chromosomes, the two pairs may assort separately each time meiosis occurs, resulting in a distinct mix of genetic material being passed on to the progeny.

During the anaphase I stage of meiosis, pairs of homologous chromosomes will occasionally fail to split and instead travel to the same side of the cell, finally ending up in the same gamete.

• This is known as nondisjunction.

• If the resultant gametes are fertilized, this can result in aneuploidy, or an abnormal number of chromosomes.

• Down syndrome, also known as trisomy 21, is an example of an aneuploidy that may occur when an individual has three copies of chromosome 21 instead of two copies.