Genetics & Public Policy Center
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Genetic material is composed of DNA (deoxyribonucleic acid). DNA is made up of four chemical subunits, or nucleotides, abbreviated as A, T, C, and G, that are strung together in long chains. If one chain reads A-G-G-T-C, it will pair with a string reading T-C-C-A-G. Two complementary chains of nucleotides run in parallel and twist like a spiral staircase to create DNA’s famous double helix. The full letter sequence of DNA in an individual is called his or her genome.

The beauty of the double helix structure, first described by in 1953, is that it allows DNA to be easily copied. This in turn enables cells to replicate. During replication, the double-stranded chain unzips and each unzipped single strand becomes a template to build a new, complementary strand for a new cell.

In humans, fewer than two percent of the three billion nucleotide pairs are organized into genes. The stretches of DNA that make up the genes provide the instruction for building proteins, which are the molecules that do most of the work in the body, such as converting sugars into energy or acting as hormones to regulate body functions. Humans have between 20,000 and 30,000 genes. The other 98 percent are non-coding DNA and its purpose is as yet unknown. Some non-coding DNA may regulate genes (turn them on or off).

Each strand of DNA in the cell usually is tightly coiled and packaged into a chromosome. There are 46 chromosomes. Chromosomes occur in pairs, one from the mother and one from the father. The only exceptions to this rule are the sex cells, eggs and sperm, which contain only one set so that when they fuse at conception they create a normal cell with two sets of DNA. In this way, one copy of each chromosome (and the genes it carries) comes from each parent. Twenty-two of the chromosome pairs are called autosomes; the 23rd pair are the sex chromosomes. The sex chromosomes are designated either XX, for a girl, or XY, for a boy.


Last updated 8/2008 by Graham Watson