The size and form of chromosomes are constant in the nucleus, but highly variable in bacteria, mitochondria and chloroplasts.

From cytological examination, the size and form of the chromosomes in the eukaryotic nucleus are invariant across generations, leading to the expectation that constancy of inheritance likely depends on constancy of the chromosomal DNA molecule conveying the constant phenotype. Indeed, except for rare mutations, major phenotypic traits appear largely without change from generation to generation. Thus, when it was discovered that the inheritance of traits for bacteria, mitochondria and chloroplasts was also constant, it was assumed that chromosomes in those locations were also constant. Such has not turned out to be the case, however; those chromosomes are highly variable in structure. I propose, therefore, that only for the nucleus is there a requirement that a chromosome be "finished" (contain only fully replicated genomes) before it may segregate to daughter cells. This requirement does not apply to the variable chromosomes among chloroplasts, mitochondria and bacteria. (c) 2007 Wiley Periodicals, Inc.