The Organization of Genes in Chromosomes in Some Ciliated Protozoa
Eukaryotic species appear to contain much more DNA in their genomes than can be reasonably accounted for in genetic terms. Mammals, for example, commonly contain DNA sequence complexities of ∿109 base pairs (BP). Assuming about 103 BP are required to code for an average size polypeptide, then such an organism could contain ∿106 genes. This would be at least twenty times more coding capacity than the highest estimates for RNA complexities would require. However, in addition to the amino acid coding capacity many eukaryotic genes have been shown to have one or more transcribed intervening sequences (introns). The gene for ovalbumin, for example, would require only 1,872 nucleotides to code for the structural gene, but the presence of seven intervening sequences expand the “gene” to 7.6 kilobases (KB). The gene for dihydrofolate reductase has been shown to span 42 KB. Thus, at least some of this extra DNA could be accounted for by introns.
KeywordsPolytene Chromosome Inverted Terminal Repeat Vesicle Stage Inverted Terminal Repeat Sequence Nontranscribed Spacer
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