DNA, otherwise known as deoxyribonucleic acid, is a molecule in a form of a double helix that encodes the genetic instructions for the proteins that form the physical structure and anatomical characteristics of living organisms. All living creatures contain DNA, and the DNA of two creatures' offspring is a combination of its parents' DNA.
Subunits of DNA called genes serve as the instructions for the body to carry out its functions and give rise to traits such as eye color, hair color, etc. Genes are packaged into chromosomes. Each individual has 23 pairs of chromosomes, where one set is inherited from his/her mother and the other set is inherited from his/her father.
DNA changes account for variability in a population. However, they can also have clinical significance.
DNA is transcribed into RNA, which is in turn being translated into Amino Acids that that are used to form proteins which are the building blocks of all living creatures. RNA can also be reverse transcribed back into DNA, which is the called Complimentary DNA or cDNA.