The two versions, which are very similar but not identical in DNA sequence, are called homologs, and in most cells they maintain a completely separate existence as independent chromosomes.Īfter a chromosome is duplicated by DNA replication, the twin copies of the fully replicated chromosome at first remain tightly linked along their length and are called sister chromatids. For each of the autosomal chromosome pairs, one member was initially inherited from the male parent (a paternal chromosome) and the other was initially inherited from the female parent (a maternal chromosome). A diploid nucleus contains two closely similar versions of each chromosome. The set of chromosomes of a typical sexually-reproducing organism consists of autosomes, which are common to all members of the species, and sex chromosomes, which are differently allocated according to the sex of the individual. More recent genetic and molecular studies have begun to identify the meiosis-specific proteins that cause the chromosomes to behave in a special way and mediate the genetic recombination events that occur in meiosis.ĭuplicated Homologous Chromosomes Pair During Meiosis Consequently, it was not until the early 1930s, as a result of painstaking cytological and genetic studies, that the essential events of meiosis were finally established. (Mitosis, which refers to the nuclear division that occurs during an ordinary mitotic cell division (discussed in Chapter 18), is from the Greek word mitos, meaning “a thread.” The term refers to the threadlike appearance of the chromosomes as they condense during nuclear division-a process that occurs in both meiotic and mitotic divisions.) The behavior of the chromosomes during meiosis turned out to be considerably more complex than expected. This type of division is called meiosis, from the Greek, meaning diminution. The finding also implied that germ cells must be formed by a special kind of nuclear division in which the chromosome complement is precisely halved. The chromosome theory of heredity therefore explained the long-standing paradox that the maternal and paternal contributions to the character of the progeny seem to be equal, despite the enormous difference in size between the egg and sperm (see Figure 20-4). In 1883, it was discovered that, whereas the fertilized egg of a roundworm contains four chromosomes, the nucleus of the egg and that of the sperm each contain only two chromosomes. The realization that gametes are haploid, and must therefore be produced by a special type of cell division, came from an observation that was also among the first to suggest that chromosomes carry genetic information.
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