DNA (also called Deoxyribonucleic acid) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. DNA, along with the instructions it contains, is passed from adult organisms to their offspring during reproduction. Nearly every cell in a person’s body has the same DNA.
The main role of DNA molecules is to store information. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 % of those bases are the same among the entire human race. The order, or sequence of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.
The DNA segments that carry genetic information are called genes. Other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.
Where is DNA found?
Most DNA is located in the cell nucleus and is called nuclear DNA. However, a small amount of DNA is found in the mitochondria and is called mitochondrial DNA or mtDNA. Because the cell is very small, and organisms have many DNA molecules per cell, each DNA molecule must be tightly packaged. This packaged form of DNA is called a chromosome.
In sexual reproduction, organisms inherit half of their nuclear DNA from the male parent and half from the female parent. However, organisms inherit their entire mitochondrial DNA from the female parent. This occurs because only egg cells keep their mitochondria during fertilization.
What is DNA made of?
DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group, and one of four types of nitrogen bases. To form a strand of DNA, nucleotides are linked into chains, with the phosphate and sugar groups alternating.
The complete DNA instruction book, or genome, for a human contains about 3 billion nitrogen bases and about 20,000 genes on 23 pairs of chromosomes.
What does DNA do?
DNA contains the instructions needed for an organism to develop, survive, and reproduce. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies.
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