Overview of Genes, DNA, and Chromosomes

The science community's knowledge of genetics increases every day, making medical discoveries and treatments more likely with each passing day. If you or someone you love has been diagnosed with a genetic condition, you may be finding it difficult to keep up with all of these genetics-based terms. Here are some of the most common terms and what you need to know about them.

DNA (deoxyribonucleic acid) carries the genetic information in your body’s cells. DNA is made up of four similar chemicals—adenine, thymine, cytosine, and guanine—which are called bases and abbreviated as A, T, C, and G. These bases are repeated over and over in pairs to make up your DNA.

A gene is a distinct portion of your cell’s DNA. Genes are coded instructions for making everything your body needs, especially proteins. You have about 25,000 genes. Researchers have yet to determine what that majority of our genes do, however, some of our genes can be associated with disorders such as cystic fibrosis or Huntington’s disease.

Proteins are chains of chemical building blocks called amino acids. A protein could contain just a few amino acids in its chain or it could have several thousand. Proteins form the basis for most of what your body does such as digestion, making energy, and growing.

Genes are packaged in bundles called chromosomes. Humans have 23 pairs of chromosomes, resulting in 46 individual chromosomes. Of those pairs, one pair, the x and y chromosome, determines whether you are male or female, plus some other body characteristics. Females have an XX pair of chromosomes while men have a pair of XY chromosomes. The other 22 pairs are autosomal chromosomes, which determine the rest of your body’s makeup.

The human genome is a complete copy of the entire set of human gene instructions. The Human Genome Project, completed in 2003, identified all the human genes in DNA and stored the information in databases so all researchers everywhere could use it.

The particular order of the pairs of As, Ts, Cs, and Gs is extremely important in your DNA. Sometimes there is a mistake—one of the pairs gets switched, dropped, or repeated. This changes the coding for one or more genes and is called a genetic mutation. Some mutations are harmless, while other mutations can cause diseases or lead to nonviable pregnancies.

Another way your DNA code could be changed is by errors in your chromosomes. Parts of a chromosome could break off, switch with part of another chromosome, or be swapped within the same chromosome. If any of these or other mistakes occur then changes, also known as mutations, happen within the coding of your genes. You can also have three copies of a chromosome, known as a trisomy, or only one chromosome, instead of the normal pair. Down syndrome, also called trisomy 21, occurs when there are three copies of chromosome 21.