Bacteria are simple, tiny organisms, with their DNA contained in one long strand. Still, for a simple organism, a bacterium's DNA is made up of a tremendous number of base pairs, which in turn make up genes. E. coli's DNA strand has 4 million base pairs and about 1,000 genes. A gene can be thought of as a template for a protein. These proteins are commonly enzymes. Each enzyme is responsible for handling a different function. For example, one of E. coli's enzymes may do nothing more than break maltose molecules into two glucose molecules, which another enzyme can then change into usable energy. When a bacterium cell needs a specific enzyme, it copies the enzyme from the DNA strand. A given cell may have lots of one enzyme and very little of another.
Plants and animals also have DNA strands. The strands of plants and animals, however, are stored on chromosomes. Different plants and animals have different numbers of chromosomes. Chromosomes exist in pairs, and each chromosome is made up of a strand of DNA. That means that each plant and animal has two strands of DNA - - which are joined together - - for each trait. The strands can be homozygous (the same) or heterozygous (different).
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