The adult human body contains some 50 trillion cells of many different types. Who you are and what you are is basically determined by the DNA that makes up the 46 chromosomes found in every nucleus of each cell (except for sex cells which contain only 23 chromosomes).
DNA consists of long chains of organic molecules called nucleotides. Each person is unique in that he/she has a specific order of nucleotide units that make up their DNA and is termed their genome. There are only four different nucleotide units found in DNA, and they are designated by the letters, A, T, G and C that identifies the different nitrogenous base each contains.
DNA is made up of two twisting, paired strands. If you know the nucleotide sequence of one strand, you will know the sequence on the opposite strand since the bases always have A pairing with T and C pairing with G.
Each persons genome is the exact sequence of the 3.2 billion base pairs that make up the DNA of the 46 chromosomes as well as a small DNA molecule found within each of the thousands of mitochondria found in every cell of the body (where energy for cell activity is produced).
Modern technology can now readily determine the sequence of the nucleotide units that makes up one's genome. The DNA is double stranded, but because of the specific base pairing, the identity of one of the bases in the pair determines the other member of the pair. Therefore, it is only necessary to report the base sequence for one strand.
Every human genome is different, and a book containing your personal genome, using just the four letters (identifying the bases) for one strand one after another, would require 1.5 million pages or 66 times the size of the complete Encyclopedia Britannica. It should be noted that those companies currently offering personal DNA analysis to determine your possible ancestry and/or the identity of certain genes that we know could affect ones health in adverse ways is only analyzing a very tiny part of your entire genome.
Because the time and cost of determining a complete human genome is steadily decreasing, scientists are determining the genome of many individuals from various ethnic groups in hopes of discovering information that can be used to benefit humankind in a multitude of ways, such as helping your doctor deal with your health issues. Sometime in the future a person's genome or portion of the genome may serve as his/her identification in conjunction with or in place of a social security number.
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