Genetics 101

What is DNA?

DNA, or deoxyribonucleic acid, contains all of a person's genetic information (genes) and is found in the body's cells, including blood and saliva.

DNA is the material that makes up each of our genes. It gives the body instructions for how to make proteins and other important molecules that determine how our body develops and functions.

Each person has genes with variations. Some variations don’t cause any known effects. Other variations contribute to unique features such as the color of eyes, hair or skin. The variations that researchers and medical professionals are concerned with are those that alter how a gene works, which may cause risk for disease. These variations are identified through DNA sequencing.

DNA, Genes & Genomes

DNA is the chemical compound that contains the instructions needed to develop and direct the activities of nearly all living organisms. DNA molecules are made of two twisting, paired strands, often referred to as a double helix.

Each DNA strand is made of four chemical units, called nucleotide bases, which comprise the genetic "alphabet." The bases are:

• Adenine (A)
• Thymine (T)
• Guanine (G)
• Cytosine (C)

Bases on opposite strands pair specifically: an A always pairs with a T; a C always pairs with a G. The order of the A’s, T’s, C’s, and G’s determines the meaning of the information encoded in that part of the DNA molecule just as the order of letters determines the meaning of a word.

The Human Genome

An organism's complete set of DNA is called its genome. Virtually every single cell in the body contains a complete copy of the approximately 3 billion DNA base pairs, or letters, that make up the human genome.

With its four-letter language, DNA contains the information needed to build the entire human body. A gene traditionally refers to the unit of DNA that carries the instructions for making a specific protein or set of proteins. Each of the estimated 20,000 to 25,000 genes in the human genome codes for an average of three proteins.

Located on 23 pairs of chromosomes packed into the nucleus of a human cell, genes direct the production of proteins with the assistance of enzymes and messenger molecules. Specifically, an enzyme copies the information in a gene's DNA into a molecule called messenger ribonucleic acid or mRNA. The mRNA travels out of the nucleus and into the cell's cytoplasm, where the mRNA is read by a tiny molecular machine called a ribosome, and the information is used to link together small molecules called amino acids in the right order to form a specific protein.

Proteins make up body structures like organs and tissue, as well as control chemical reactions and carry signals between cells. If a cell's DNA is mutated, an abnormal protein may be produced, which can disrupt the body's usual processes and lead to a disease, such as cancer.

What’s the Benefit of DNA Sequencing?

Sequencing simply means determining the exact order of the bases in a strand of DNA. Researchers can use DNA sequencing to search for genetic variations and/or mutations that may play a role in the development or progression of a disease. The disease-causing change may be as small as the substitution, deletion or addition of a single base pair or as large as a deletion of thousands of bases.

This educational information is provided by the National Human Genome Institute.