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DNA
Deoxyribonucleic acid (DNA) is the hereditary material in most organisms, including human beings. DNA is a code consisting of four chemical bases: thymine, guanine, cytosine and adenine. More than 99 percent of these bases are identical in all human beings. These bases form bonded pairs: adenine paired with thymine and cytosine with guanine. The order of these bases determines the make-up of the organism itself. The chemical base is combined with a sugar and a phosphate to form a nucleotide. The nucleotides form a spiral shape known as a double helix. This strand of DNA replicates itself. When cells divide, it is critical that the new cell contain an identical copy of the old cell's DNA. Each strand of the DNA serves as a pattern for the sequence of the chemical bases in a duplicate cell.
Gene Expression
The goal of most DNA study is to determine how and why genes are turned on, also known as gene expression. An understanding of how genes are expressed under normal circumstances could help us further our understanding of how genes behave under other conditions, such as disease. How the genes are expressed depends upon two primary factors: the type of cell and the function of the cell at the time. For example, the same DNA sequence would be expressed differently in a liver cell than in a skin cell. Genes are also expressed differently by cells that are dividing or performing other complex tasks.
Gene Identification
To gain this understanding, scientists must determine which proteins are coded for by which genes. Only 1.5 percent of all DNA actually codes for proteins, which are the genes that code for traits in humans. Therefore, gene identification is extremely complicated under natural conditions.
EST
Expressed sequence tags ()are short DNA sequences made up of two to 500 nucleotides, that scientists use to identify genes expressed in a particular cell at a given time. To create these ESTs, scientists separate messenger RNA of a particular type of cell. They then use a process known as reverse transcriptase with the mRNA to create complementary DNA to reformulate a DNA sequence in which the gene would be expressed. ESTs, therefore, represent the part of a DNA sequence that results in an expressed gene. These sequences can be used to identify disease-related gene sequences. The National Center for Biotechnology Information has created a database of expressed sequence tags in order to provide organization and access to those ESTs that have been discovered from a variety of organisms.