Randomness in biology
Thermal fluctuations in chemical reactions lead to many kinds of microscopic randomness in biological systems, sometimes amplified when organisms grow. For example, small-scale randomness in embryos can affect large-scale pigmentation patterns in adult organisms, as discussed on page 1013. Random changes in single DNA molecules can have global effects on the development of an organism. Standard mitotic cell division normally produces identical copies of DNA—with random errors potentially leading for example to cancers. But in sexual reproduction genetic material is rearranged in ways normally assumed by classical genetics to be perfectly random. One reason is that which sperm fertilizes a given egg is determined by random details of sperm and fluid motion. Another reason is that egg and sperm cells get half the genetic material of an organism, somewhat at random. In most cells, say in humans, there are two versions of all 23 chromosomes—one from the father and one from the mother. But when meiosis forms egg and sperm cells they get only one version of each. There is also exchange of DNA between paternal and maternal chromosomes, typically with a few crossovers per chromosome, at positions that seem more or less randomly distributed among many possibilities (the details affect regions of repeating DNA used for example in DNA fingerprinting).
In the immune system blocks of DNA—and joins between them—are selected at random by microscopic chemical processes when antibodies are formed.
Most animal behavior is ultimately controlled by electrical activity in nerve cells—and this can be affected by details of sensory input, as well as by microscopic chemical processes in individual cells and synapses (see page 1011).
Flagellated microorganisms can show random changes in direction as a result of tumbling when their flagella counter-rotate and the filaments in them flail around.
(See also page 1011.)