Where is DNA found?
DNA is found inside a special area of the cell called the nucleus. Because the cell is very small, and because organisms have many DNA molecules per cell, each DNA molecule must be tightly packaged. This packaged form of the DNA is called a chromosome. DNA spends a lot of time in its chromosome form. But during cell division, DNA unwinds so it can be copied and the copies transferred to new cells. DNA also unwinds so that its instructions can be used to make proteins and for other biological processes. Researchers refer to DNA found in the cell’s nucleus as nuclear DNA. An organism’s complete set of nuclear DNA is called its genome. Besides the DNA located in the nucleus, humans and other complex organisms also have a small amount of DNA in cell structures known as mitochondria. Mitochondria generate the energy the cell needs to function properly. In sexual reproduction, organisms inherit half of their nuclear DNA from the male parent and half from the female parent. However, organisms
Throughout the body – in cells… Our bodies are formed from between 50 and 100 trillion cells (a trillion is a thousand billion, or a thousand, thousand million). These cells are organized into tissues, such as skin, muscle, and bone. Each cell contains all of the organism’s genetic instructions stored as DNA. However, each cell uses only the instructions from part of the DNA. For example, a muscle cell uses the DNA that specifies the muscle apparatus, whereas a nerve cell uses DNA that specifies the nervous system. It is as if each cell reads only that part of a book of instructions that it needs. Within the cell – in chromosomes… Each very long DNA molecule is tightly wound and packaged as a chromosome. Humans have two sets of 23 chromosomes in every cell, one set inherited from each parent. A human cell therefore contains 46 of these chromosomal DNA molecules. Within each chromosome – in genes… Each DNA molecule that forms a chromosome can be viewed as a set of shorter DNA sequ
