Reproduction is so fundamental to life that it is one of the characteristics by which we define life. As well as the generation of new life in the short term, in the long term, reproduction provides an opportunity for heritable change of various kinds, and hence for evolution. A key view of 20th century biology was that life is basically a nucleic acid information phenomenon, in the dual senses that biological information flows from DNA to RNA to protein (the Central Dogma), but not the other way, both in an individual and across generations. Thus an important primary form of reproduction, replication, is molecular copying, of RNA for individual genes and of DNA for chromosomes. Replication of other molecules of life, such as amino acids, steroids, lipids, carbohydrates, and nucleic acid components themselves and reproduction of organisms follow as a result. We've seen in earlier chapters that this view can be tempered by other ways in which interited change can come about, but they all involve reproduction in some fashion, and an important element is that in each case the heritable aspect of the new iteration is sequestered from direct modification by its parent. A new generation has a life of its own.
We usually associate the term "reproduction" with the production of new organisms from a parental generation. But this same process is essentially the way a single cell differentiates and divides into a multicelled organism. Among those cells, genetic information flow is also a unidirectional evolutionary microcosm. This includes somatic mutation that induces gene structure change and the various mechanisms that affect gene expression within each, partially sequestered, lineage of cells. There are many interesting subtleties in the processes of reproduction, which we discuss in this chapter.
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