Genetics

Genetic information is contained in the chromosomes, threadlike structures composed of DNA, and present in the nuclei of all cell types and are passed to daughter cells during cell division. Multicellular organisms contain two types of cells—body cells (or somatic cells) and germ cells (or reproductive cells). Germ cells are the ones that pass on the genetic information to the progeny. In contrast to somatic cells that contain dual copies of chromosomes in each cell, germ cells replicate through a process called meiosis, which ensures that the germ cells have only a single set of chromosomes, a condition called haploidy (designated as n). The somatic cells of humans have 23 pairs of chromosomes (46 chromosomes overall), a condition known as diploid (or 2n). Through the process of meiosis, a new cell, called a haploid gamete, is created with only 23 chromosomes: this is either the sperm cell of the father or the egg cell of the mother. The fusion of egg and sperm restores the diploid chromosome number in the zygote. This cell carries all the genetic information needed to grow into an embryo and eventually a full grown human with the specific traits and attributes passed on by the parents. Offspring of the same parents differ because the sperm cells and egg cells vary in their gene sequences, due to random recombination.

The somatic, or body cells are the primary components of functioning organisms. The genetic information in these cells is passed on through a process of cell division called mitosis. Unlike meiosis, mitosis is designed to transfer the identical number of chromosomes during cell regeneration or renewal. This is how cells grow and are replaced in exact replicas to form specific tissues and organs, such as muscles and nerves. Without mitosis, an organism's cells would not regenerate, resulting not only in cell death, but possible death of the entire organism. (It is important to note that some organisms reproduce asexually by mitosis alone.)