DNA, Genes and Genomics

A Brief Guide to Genomics



View largerDeoxyribonucleic acid (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


Where is DNA Found?

In organisms called eukaryotes, DNA is found inside a special area of the celle called the nucleus. Because the cell is very small, and because organisms have many DNA molecules per cell, each DNA molecule must be tightly packaged. This package is called a chromosome.

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), and 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 As, Ts, Cs and Gs 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.

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 (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.

Besides the DNA located in the nucleus, humans other complex organisms also have a small amount of DNA in cell structures known as mitochondria. Mitochondria generates the energy the cell needs to function properly.


The nucleus is a highly specialized organelle that serves as the information processing and administrative center of the cell. This organelle has two major functions: it stores the cell’s hereditary material, or DNA, and it coordinates the cell’s activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division).

In sexual reproduction, organisms inherit half of their nuclear DNA from the male parent and half from the female parent. However, organisms inherit all of their mitochondrial DNA from the female parent. This occurs because only the egg cells, and not the sperm cells, keep their mitochondria during fertilization.

Sources: National Institute of Health, Education Digital Library, DNA,  11/20/2016

National Institute of Health, Genome.gov., Digital Library, “A Brief Guide to Genomics”, 11/20/2016

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