DNA DNA is a vast chemical information database. It resides in the core, or nucleus, of each of the body's trillions of cells, and it carries the complete set of instructions for making all the proteins a cell will ever need. DNA exists as two long, paired strands spiraled into the famous double helix.
Each strand is made up of millions of chemical building blocks called bases. There are only four different chemical bases in DNA (adenine, thymine, cytosine, and guanine), but they can be arranged and rearranged in countless ways. The order in which the bases occur determines the messages to be conveyed, much as specific letters of the alphabet combine to form words and sentences.
Every human cell (with the exception of mature red blood cells, which have no nuclei) contains the same DNA. Each cell has 46 molecules of double-stranded DNA. Each DNA molecule is made up of 50 to 250 million bases housed in a chromosome.
Gene A gene is a working subunit of a DNA molecule. A gene is any given segment along the DNA carrying a particular set of instructions that allows a cell to produce a specific product--typically, a protein such as an enzyme. There are about 25,000 genes, and every gene is made up of thousands, even hundreds of thousands, of chemical bases.
Building Proteins Building proteins lies at the heart of cell function. For a cell to make a protein, the information from a gene is copied, base by base, from a strand of DNA into a strand of messenger RNA. Messenger RNA travels out of the nucleus into the cytoplasm, to cell organelles called ribosomes. There, messenger RNA directs the assembly of amino acids that fold into a completed protein molecule.
Gene and Its Functions Although each cell contains the genetic potential to make all possible human proteins, cells use genes selectively. Some housekeeping genes encode proteins needed for basic cell functions, and these genes stay active all the time in many cell types. More typically, though, a cell activates just the genes it needs at the moment and actively suppresses the rest. The unique selection of genes used by a cell gives that cell its character--making a brain cell, say, different from a bone cell.
Genetic Diseases A sound body depends on the continuous interplay of thousands of proteins, acting together in just the right amounts and in just the right places--and each properly functioning protein is the product of an intact gene.
Many, if not most, diseases have their roots in our genes. More than 4,000 diseases stem from altered genes inherited from one's mother and/or father. Common disorders such as heart disease and most cancers arise from a complex interplay among multiple genes and between genes and factors in the environment.
Mutation Genes can be altered, or mutated, in many ways. The most common gene change involves a single base mismatch--a misspelling--placing the wrong base in the DNA. At other times, a single base may be dropped or added. And sometimes large pieces of DNA are mistakenly repeated or deleted.
When a gene contains a mutation, the protein encoded by that gene is likely to be abnormal. Sometimes the protein will be able to function, but imperfectly. In other cases, it will be totally disabled. The outcome depends not only on how it alters a protein's function but also on how vital that particular protein is to survival.
Hereditary Mutation Gene mutations can be either inherited from a parent or acquired. Hereditary mutations are carried in the DNA of the reproductive cells. When reproductive cells containing mutations combine to produce offspring, the mutation will be in all of the offspring's body cells. The fact that every cell contains the gene change makes it possible to use cheek cells or a blood sample for gene testing.
Acquired Mutation Acquired mutations are changes in DNA that develop throughout a person's lifetime. Although mistakes occur in DNA all the time, especially during cell division, a cell has the remarkable ability to fix them. But if DNA repair mechanisms fail, mutations can be passed along to future copies of the altered cell.
A Few Examples of Diseases Related to Genes Autism is not just a single disease but can be caused by a range of genetic and environmental factors. The risk factors include female gender, aging, personal history of breast cancer, family history of breast cancer, genetic factors (cancer causing mutations in BRCA1 (BReast CAncer gene 1) and BRCA2 (BReast CAncer gene 2), and hormonal factors (as earlier age at first period, later age at birth of first child, later age at menopause, having no children). Turner syndrome is a disorder of the sex chromosomes.
Gene Therapy Three genes that produce dopamine were inserted into a disabled equine virus that was then injected into the brain. Six human volunteers began clinical trials a year ago, and the results are "extremely encouraging," as measured in control of Parkinson's symptoms and in side effects such as brain inflammation.