What is DNA?
Deoxyribonucleic acid (DNA) is the cell’s hereditary material and contains instructions for development, growth and reproduction. DNA is passed from generation to generation in humans and many other organisms. The same DNA is located in nearly every cell of the human body. DNA is mostly located within chromosomes in the nucleus, but some DNA is also found in the mitochondria. Chromosomes consist of DNA coiled around histones (alkaline proteins). If extended, the DNA would measure approximately 2 meters. For more information on chromosomes, see the section “What are Chromosomes?”
DNA is made up of four chemical (nitrogen) bases:
Purines
adenine
guanine
Pyrimidines
cytosine
thymine
The side-by-side arrangement of bases in a particular sequence, unique to each human (or other organism), spells out the exact instructions needed to create that person and also gives the person his or her unique phenotype or traits (distinguishing characteristics).
Genes, the basic physical and functional units of heredity, are composed of DNA sequences. Genes serve as the blueprint for the production of proteins. Only about 2% of the entire human genome (complete set of genetic information for a person) consists of genes. For details on the human genome, see section “What is a Genome?”
DNA composition and structure
Source: U.S. National Library of Medicine
Adenine + Thymine
Cytosine + Guanine
These pairs are called base pairs. The base pairs are held together by a hydrogen bond. One base plus a deoxyribose sugar molecule and a phosphate group create a nucleotide.
Nucleotides are arranged in two long strands and are held together by the sugar-phosphate backbone. The nucleotide strands form a spiral double helix that looks similar to a ladder. The sides of the ladder consist of sugar and phosphate molecules. The middle rungs are formed by the base pairs (and the hydrogen bond). Nucleotides are the building blocks of nucleic acids.
Types of DNA in the cell
There are two types of DNA in the cell – autosomal DNA and mitochondrial DNA. Autosomal DNA (also called nuclear DNA) is packaged into 22 paired chromosomes. In each pair of autosomes, one was inherited from the mother and one was inherited from the father. Autosomal DNA is passed down from both the mother and the father and provides clues to a person’s ancestry. For more information on chromosomes, see section “What are Chromosomes?"
As noted in the section “What are Eukaryotic Cells?”, mitochondria are organelles that are responsible for the cell’s energy production. Mitochondria contain their own DNA called mitochondrial DNA. Mitochondrial DNA has one chromosome that codes for the specific proteins needed for the metabolic processes that mitochondria perform. Mitochondrial DNA replicates separately from the rest of the cell and is passed down only from the mother.
Noncoding and coding DNA
Noncoding DNA is sometimes referred to as junk DNA. However, noncoding DNA does have a purpose. Noncoding DNA typically refers to any DNA that does not code for a protein. This type of DNA is now referred to as regulatory DNA. The regulatory function of this DNA is to determine when and where some genes are transcribed. Noncoding DNA also provides chromosomal structure and binding sites for regulatory proteins. Much research is being conducted on noncoding DNA.
Coding DNA is responsible for harboring the specific DNA sequences that encode instructions for making proteins.
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