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Genomic DNA is the genetic information that makes up the genome, or the complete set of an organism's genetic information. DNA, or deoxyribonucleic acid, is a molecular chain made up of four different nucleotide bases called adenine, thymine, guanine, and cytosine. The sequence of these bases in strands of DNA contains coded "instructions" that define most of the processes that occur during the development and day-to-day function of an organism. Genomic DNA stores the entirety of this coded "instruction manual." It usually exists in cells in the form of chromosomes, which are large, compact complexes made up of DNA and a variety of regulatory proteins.
Segments of both coding DNA, which contains information used to produce proteins and other functional units, and noncoding DNA, which does not produce a functional end product, exist in genomic DNA. Coding DNA segments are generally transcribed to ribonucleic acid, or RNA, and translated to proteins. Proteins are highly abundant functional units in the body that are involved in some way in almost all of the body's biochemical processes. The function of most noncoding genomic DNA, much of which is spaced between some segments of coding DNA, is not well understood. Some of it does function in various structural and regulatory roles, but scientists have been unable to attribute a precise function to most of it.
Genomic DNA has a variety of functions in heredity. Small points of variance in the genomes of different people result in individuals with different traits, such as height and eye color. When parents reproduce, their offspring receive some genomic DNA from the father and some from the mother, and their traits depend on genetic information received from each parent. This is beneficial from an evolutionary perspective, as it introduces greater diversity into the genome, ensuring that at least some subset of a population will be genetically able to handle situations which may be unfavorable to survival.
Different types of organisms and infectious agents possess different types of genomic DNA. A bacterium, for instance, stores its DNA in a single circular chromosome while human DNA is stored on 23 paired chromosomes. Viruses in particular show a great deal of variation in their genomic DNA. A virus's genome may be composed of single or double stranded DNA and may be linear or circular. Viruses tend to inject their DNA into host cells in order to take over the "machinery" of those cells to produce copies of themselves, enabling them to spread.