Chromosomes are cell organelles specialized for the storage and transmission of genetic material. Genes are located on chromosomes and travel with them during cell division and gamete formation.
In sexually reproducing organisms, somatic cells carry a precise number of homologous pairs of chromosomes, which is characteristic of the species. One chromosome of each pair is of maternal origin; the other, paternal.
During the first division of meiosis, homologous chromosomes in germ cells segregate from each other. As a result, each gamete receives one member of each matching pair, as predicted by Mendel's first law.
Also during the first meiotic division, the independent alignment of each pair of homologous chromosomes at the cellular midplane results in the independent assortment of genes carried on different chromosomes, as predicted by Mendel's second law.
Crossing-over and the independent alignment of homologs during the first meiotic division generate diversity.
The second meiotic division generates gametes with a haploid number of chromosomes (n).
Fertilization--the union of egg and sperm--restores the diploid number of chromosomes (2n) to the zygote.
Mitosis underlies the growth and development of the individual. Through mitosis, diploid cells produce identical diploid progeny cells. During mitosis, the sister chromatids of every chromosome separate to each of two daughter cells; before the next cell division, the chromosomes again duplicate to form sister chromatids.
