A gene is a unit of genetic information that codes for a single biological function or product. Genes are found on chromosomes and are made up of nucleic acid (deoxyribonucleic acid [ DNA ] for most organisms) and proteins.
|COMPARISON OF GENE NUMBERS AND DNA FROM DIFFERENT SPECIES|
|Organism||Number of genes||Number of base pairs (millions)|
|SOURCE: Bork, Peer, and Copely, Richard (2001). "The Draft Sequence: Filling in the Gaps." Nature 409:818–820.|
|Caenorhabditis elegans (worm)||19,099||97|
The information is contained in the sequence of the four nucleic acid components much like the way written information is contained in the sequence of letters in a sentence.
DNA is measured in base pairs (bp) since it occurs as a double helix . The sum total of all genetic information in an organism is its genome. Different organisms have different sized genomes with different numbers of chromosomes, genes, and base pairs. Table 1 shows the values for several organisms, including the preliminary results for the human genome from the Human Genome Project (HGP) report. One surprising finding of the HGP was that only 30,000 to 40,000 genes were found. The human genome has almost 3 billion base pairs and many different gene products, so scientists were expecting over 100,000 genes. Results from the HGP suggest that about 75 percent of DNA is "nongene." This DNA is often referred to as "junk" or "selfish" DNA, but some portions do have important functions in maintaining the structure of chromosomes.
Genes "tell" a cell which molecules to synthesize based on the genetic code that those genes contain. The amount of code needed varies widely. Genes vary greatly in size, from those that code for small transfer RNA molecules (tRNAs) and have 73 base pairs, to those that code for very large proteins and have 250,000 base pairs (Maulik and Patel, p. 26). A eukaryotic gene for a large protein may be much larger than its coding region due to intervening noncoding sequences (introns). Each gene has two or more coding regions (exons) separated by introns. The introns are "cut out" of the genetic information during expression and do not show up in the final gene product. Some genes have as much as 90 percent intron DNA. Sometimes the exons are cut and pasted from the same gene in different ways, creating two or more different gene products issuing from the same gene. This added flexibility opens the door to even more debate about how genes and gene products are controlled. Although each gene codes for one or sometimes a few gene products (due to splicing variations), biological functions often require many genes working together or in sequence. The proper interplay of genes produces healthy cells. Some forms of cancer occur when certain genes, called oncogenes, become uncontrolled.
Berg, Paul, and Singer, Maxine (1992). Dealing with Genes: The Language of Heredity. Mill Valley, CA: University Science Books.
International Human Genome Sequencing Consortium (2001). "Initial Sequencing and Analysis of the Human Genome." Nature 409:860–921.
Maulik, Sunil, and Patel, Salil (1997). Molecular Biotechnology: Therapeutic Applications and Strategies. New York: Wiley-Liss.
McCarty, MacLyn (1985). The Transforming Principle: Discovering that Genes Are Made of DNA. New York: Norton.
Singer, Maxine, and Berg, Paul (1991). Genes and Genomes: A Changing Perspective. Mill Valley, CA: University Science Books.