4.3.1 The Units of Life

Cells are the basic units of life in all organisms. They are the smallest entities that are capable of self-reproduction. There are two main types of cells: prokaryotic and eukaryotic cells, named for the types of organisms in which they occur. Prokaryotes are single-celled organisms, such as bacteria and archaea. Eukaryotes are more complex, multicellular organisms, such as plants and animals (including humans). One of the most important components of eukaryotic cells is the enclosed nucleus at the center of the cell; prokaryotic cells do not have this nucleus. The nucleus of a eukaryotic cell houses all of the genetic material, or DNA (deoxyribonucleic acid), that controls cellular function. Normally, the DNA forms a long string within the nucleus.

There are two main types of eukaryotic cells: somatic cells and sex cells (also known as gametes). The somatic cells make up the structural components of a body, such as the tissues, muscles, and organs. The sex cells are specifically involved in reproduction. The function of the sex cells is to unite with a sex cell from another individual to form a fertilized egg, also known as a zygote. In animals, there are two types of sex cells: ova, or eggs, and sperm.

Cell division is the process that results in the production of new cells. However, sex and somatic cells divide differently. The cellular division of somatic cells is known as mitosis, while the cellular division of sex cells is known as meiosis. Mitosis of somatic cells is sometimes referred to as simple cell division because the parent cell divides once to produce two daughter cells that are genetically identical to each other and identical to the original parent cell. During mitosis, the DNA genetic material forms structures known as chromosomes. Each daughter cell inherits an exact copy of all 46 chromosomes found within the parent cell.

Meiosis, or sex cell division, is more complicated. This type of cellular division only occurs in the testes of males and the ovaries of females. Instead of just one division, meiosis results from two cellular divisions that produce four daughter cells. In meiosis, the four daughter cells each receive half of the original genetic material from the parental cell. Thus, each daughter cell only has 23 chromosomes.

It is on the chromosomes that genes are housed. Genes are the fundamental unit of heredity. They are best understood as the sequence or ordering of the DNA material that is housed in the nucleus. The genotype is the genetic material found within an organism's cells and it is the expression of these genes that will produce the phenotype or observable trait. Sometimes, the sequencing of the DNA material produces a variation of a gene, known as an allele. An allele is defined as a similar but slightly different form of the same gene that can activate the expression of a specific trait.