DNA is a working molecule; it must be replicated (copied) when a cell is ready to divide, and it must be “read” to produce the molecules, such as proteins, to carry out the functions of the cell. For this reason, the DNA is protected and packaged in very specific ways. In addition, DNA molecules can be very long. Stretched end-to-end, the DNA molecules in a single human cell would come to a length of about 2 meters (roughly 6 feet). Thus, the DNA for a cell must be packaged in a very ordered way to fit and function within a structure (the cell) that is not visible to the naked eye.
A cell’s complete complement of DNA is called its genome. In prokaryotes (bacteria), the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle. The region in the cell containing this genetic material is called a nucleoid. The DNA is not located inside a separate structure within the cell. The nucleoid is within the cytoplasm of the cell. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth.
The size of the genome in one of the most well-studied prokaryotes, Escherichia coli, is 4.6 million base pairs, which would extend a distance of about 1.6 mm if stretched out. So how does this fit inside a small bacterial cell? The DNA is twisted beyond the double helix in what is known as supercoiling. Some proteins are known to be involved in the supercoiling; other proteins and enzymes help in maintaining the supercoiled structure.
Eukaryotes, such as animals and plants, have chromosomes that consist of several linear DNA molecules (Figure 5). Chromosomes are thread-like structures located inside the nucleus of eukaryotic cells. Each chromosome is made of protein and a single linear double-helix of DNA. The term chromosome comes from the Greek words for color (chroma) and body (soma). Scientists gave this name to chromosomes because they are cell structures, or bodies, that are strongly stained by some colorful dyes used in research.
Eukaryotes typically have much more DNA than prokaryotes: the human genome is roughly 3 billion base pairs while the E. coli genome is roughly 4 million. For this reason, eukaryotes employ a different type of packing strategy to fit their DNA inside the nucleus (Figure 6). At the most basic level, DNA is wrapped around proteins known as histones. The DNA wrapped around histones wraps and stacks through several additional levels of complexity. DNA and the histones that it is wrapped around is called chromatin. These thicker more compact structures are what you have seen before in pictures labeled “chromosomes”.
Each species of eukaryote has a characteristic number of chromosomes in the nuclei of its cells. Human body cells (somatic cells) have 46 chromosomes. A somatic cell contains two matched sets of chromosomes, one from the male parent and one from the female parent. Eggs and sperm each contain one set of 23 chromosomes.
Chromosomes contain stretches of DNA called genes. A gene is the section of DNA that will be used to produce one protein. Each individual has the same genes located on the same chromosomes, but those genes may be made up of different sequences of DNA. This means that the individuals will make different proteins, which will give them different traits.
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OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:8v2Xzdco@5/The-Structure-of-DNA