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The sum total of the DNA in an organism is known as its genome. Although DNA was discovered within the nucleus, this is not the only place where it is found in cells. Some cells do not even have a nucleus. As far as molecular biology is concerned the distinction between having a nucleus and not having one turns out to be rather important, and is used for classifying organisms at a cellular level. A daisy and a giraffe have hardly anything in common as far as outward appearance is concerned, but to a molecular biologist they are similar because they are both made of cells that have nuclei. Organisms like this are called eukaryotes. Their DNA is found in the nucleus, but also in cellular structures called mitochondria and chloroplasts. These are the sites of important biochemical activity. Energy production takes place in mitochondria, while the synthesis of glucose from carbon dioxide and water in the presence of sunlight (photosynthesis) happens in the green chloroplasts of plant cells. The fact that mitochondria and chloroplasts have their own DNA suggests that they may have been free-living organisms earlier in evolution (the significance of this is discussed further in Chapter 4). Organisms whose cells do not have a nucleus (or at least not one surrounded by a membrane like the eukaryotic nucleus) are called prokaryotes. Their DNA lies free in the cell, usually in a closed loop. In addition to this, some bacteria contain small circles of DNA called plasmids. [ABSTRACT FROM AUTHOR]

DNA is a database. The information it contains allows the assembly of the all-important protein molecules within the living cell from their component parts, the amino acids. This database is passed on from one cell to another by the DNA molecule's power of self-replication, as we saw in Chapter 1. Now we look at how the protein recipes are extracted from DNA during the lifetime of an individual cell. Information flow in most organisms is a one-way street: from DNA to protein. This rule is known as the Central Dogma of molecular biology. This term was coined by Francis Crick in 1956, many years before the details of the molecular processes involved were actually worked out. The genetic code The concept of the gene as an inheritable factor responsible for an organism's characteristics was first put forward by the Austrian botanist and monk Gregor Mendel in 1865. Mendel looked at how characteristics such as flower colour, height and seed shape were inherited during carefully controlled experiments with pea plants. Peas are normally pure-breeding. They reproduce by self-pollination, in which pollen and ovules (the male and female sex cells) from the same plant unite, to give offspring that are similar to their parent. So a pea with white flowers would normally produce more peas with white flowers, and so on. Mendel removed the stamens (which produce pollen) from his experimental plants, so they could no longer self-pollinate. Then he chose pairs with different characteristics: smooth or wrinkled seeds, red or white flowers, for example. He cross-pollinated these pairs by brushing pollen from one plant onto the stigma (the tip of the female sex organ) of the other. [ABSTRACT FROM AUTHOR]

Take a large onion and chop finely. Place the pieces in a mediumsized casserole dish. Now mix ten tablespoons of washing-up liquid with a tablespoon of salt, and make up to two pints with water. Add about a quarter of this mixture to the onion and cook in a bain-marie in a very cool oven for five minutes, stirring frequently, and liquidise at high speed for just five seconds. Now strain the mixture and add a few drops of fresh pineapple juice to the strained liquid, mixing well. Pour into a long chilled glass and finish off by dribbling ice-cold alcohol (vodka will do) down the side so that it floats on top of the mixture. Wait a few minutes and watch cloudiness form where the two layers meet. Now lower a swizzle stick into the cocktail and carefully hook up the cloudy material. It should collapse into a web of fibres that you can pull out of the glass. This is DNA (short for deoxyribonucleic acid). DNA is the stuff that genes are made of. Genes carry biological information, which is translated into the characteristics of living things and is passed on down the generations. So genes determine the colour of a butterfly's wings, the scent of a rose, and the sex of a baby. DNA is just a chemical – not a more complex entity like a chromosome or a cell – and it is only in a biological context that it acquires its status as the molecular signature of an organism. [ABSTRACT FROM AUTHOR]

A definition of the term "shuttle vector" is presented. It refers to a plasmid constructed to have origins of replication for two hosts, such as Escherichia coli and S. cerevisiae, so that it can be used to carry a foreign sequence in either prokaryotes or eukaryotes.