DNA - DNA is a molecule which encodes genetic information. It is a long, coiled, double-stranded chain of interlocking base-pairs called a double-helix. There are four types of bases in DNA: A (adenine), T (thymine), G (guanine), and C (cytosine). The order of the bases in a DNA strand, called the sequence, creates a code for information: the DNA code 'ATC' has a different meaning than the code 'TCA,' and so on. Each cell is equipped with special machinery used to read the sequence and use the information encoded. All the genetic information in an organism is referred to collectively as a 'genome.' One copy of the human genome is about 3 billion bases long.

Chromosomes - The 3 million bases of the human genome are not all in one continuous strand of DNA. Rather, the human genome is divided into 23 separate pieces of DNA, called chromosomes. Chromosomes are strands of DNA bundled together by proteins. Humans have 22 numbered chromosomes (also called autosomes, and conveniently named 1 to 22) and the X and Y sex chromosomes. A typical cell has 2 copies of each of the numbered chromosomes, one from the mother and one from the father, and two sex chromosomes. Females have two X chromosomes, while males have an X and a Y. This results in a total of 46 chromosomes in each cell.

The collection of chromosomes in an individual is called a karyotype. For example, the typical male karyotype has 22 pairs of autosomes, one X and one Y chromosome. To see photographs of real human karyotypes, link to the Cytogenetics Gallery at the University of Washington.

Cells - Cells are the building blocks which make up the tissues of the body. For example, skin is a multilayered sheet of cells, and blood consists of a variety of cells floating in liquid. Inside each cell are the genes which give the cell information about how it is supposed to function. The chromosomes reside in a compartment of each cell called the nucleus.

RNA - Because DNA is stored in the nucleus of the cell, the first step in interpreting the information encoded by a gene is to make a copy of the information that can travel outside the nucleus. The process of making this copy is called transcription. The copy is called a 'messenger RNA' or 'mRNA.' It is called a messenger, because it carries the information in the gene to other parts of the cell, and RNA is an information encoding molecule that is very similar to DNA, except that it is a single strand, and it uses a base called uracil (U) in the place of the thymine (T) in DNA.

Proteins In the case of a few genes, the RNA copy of the gene has a function on its own, usually by folding into a distinctive shape. However, the majority of genes encode instructions for making specific proteins. Proteins are complex chemicals which perform most of the work in a cell (and therefore most of the work in the body). For example, proteins called enzymes direct the building of cell structures, allow our bodies to get energy from the food we eat, and perform many other functions. To make the protein encoded by a gene, the mRNA attaches to special machinery in the cell called a ribosome that can read the genetic code and make a protein, a process called translation.

The progression from gene to protein is often referred to as 'the Central Dogma' of molecular biology.