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What Is DNA?

Last Updated: Mar 20, 2018

DNA, or deoxyribonucleic acid, is the most important molecule in all of biology. It is the genetic material found in the cells of all living things, from the simplest bacteria to the most complex muscle and nerve cells in your body. DNA contains genetic code necessary to produce mRNA molecules, which in turn produce proteins to control the cell’s functions.

Structure of DNA

The Double Strands

The DNA molecule is an incredibly long string known as a polymer. Polymers are extensive chain-like molecules made of smaller molecular units known as monomers. In the case of DNA, the monomers that make up DNA are called nucleotides.

Each nucleotide consists of a nitrogenous base and a “backbone” made of phosphates and deoxyribose.

  • Deoxyribose is a ring-shaped simple sugar (monosaccharide) molecule that gives DNA its name.
  • Phosphate groups bind the deoxyribose molecules of neighboring nucleotides together to form the backbone chain of the DNA.

DNA is always double-stranded, meaning that one molecule of DNA is always connected to another molecule through weak hydrogen bonds between their nucleotide bases.

  • One of these strands is the coding strand, which stores the genetic code.
  • The other strand, known as the complementary strand, is used to copy DNA and prevent errors in the genetic code.

The two strands of DNA curl around one another, forming a spiral staircase-like structure known as a double helix.

More About Nucleotide Bases

The nucleotide bases are responsible for the genetic information stored in DNA. There are four different bases found in DNA:

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

These bases are the basis for our use of A, C, G and T in what is called the DNA alphabet. Those four letters make up our DNA code.

The bases stick out at a right angle from backbone to meet the base of the other strand like the rungs of a ladder. Hydrogen bonds form between the bases to weakly bind the two DNA strands together.

Function of DNA

A Cell's Library

The order and pattern of the nitrogenous bases in a DNA molecule determines its function and forms the genetic code. While there are only four letters in the DNA alphabet - A, T, C, and G - there are millions of nucleotides in a single DNA molecule and these letters can combine in an almost infinite number of patterns to produce a unique genetic code.

Many regions of the DNA molecule contain genes, or DNA code, that are transcribed to form messenger RNA (mRNA) molecules. mRNA is in turn used as a code to produce proteins, which alter a cell’s structure and function. Thus, DNA acts like the library that contains all the genetic information that a cell needs to function.

Making Copies

When cells reproduce, DNA must be copied so that each cell has its own complete copy of DNA instructions. This process is known as DNA replication, and is performed by the enzymes DNA helicase and DNA polymerase. DNA helicase gets its name from the DNA double helix, and its job is to “unzip” the helix by separating the strands. With the strands separated, a DNA polymerase molecule binds to each of the strands and begin to copy the genetic code. DNA polymerase also checks the copies that it produces for errors in the genetic code.

The four nitrogenous bases bind to bases on the other DNA strand following the base pairing rule: adenine forms two hydrogen bonds with thymine, while cytosine forms three hydrogen bonds with guanine. Thus, adenine is complementary to thymine and cytosine is complementary to guanine. Using the base-pairing rule, DNA polymerase makes a complimentary copy of each DNA strand. The coding strand is used as a template to produce a new complementary strand, while the complementary strand is used to produce a new coding strand. Once all the nucleotides have been copied, two new double-stranded DNA molecules have been produced and return to their double helix structure.


On very rare occasions, errors in the copying of DNA can be made. These errors are called mutations and add variation into the genetic code. Mutations often occur outside of regions that code for genes, producing no noticeable effects.

But when a mutation occurs within a gene, it may alter the mRNA produced from the gene and the protein produced by the mRNA. Sometimes these mutations are harmful to the organism, but sometimes they increase diversity or are beneficial to the organism. Genetic disorders like albinism, cystic fibrosis, and sickle-cell disease along with healthy variations in eye color, hair color, and height are all the results of mutations that occurred in humans over thousands of years. And across geography and time, these mutations are what enable us, through DNA testing, to discover important clues about our ancestry.