Coding Regions Are Often Interrupted by Intervening Sequences

The protein coding regions of DNA, the transcripts of which ultimately appear in the cytoplasm as single mRNA molecules, are usually interrupted in the eu-karyotic genome by large intervening sequences of

Table 36-2. The packing ratios of each of the orders of DNA structure.

Chromatin Form

Packing Ratio

Naked double-helical DNA

~1.0

10-nm fibril of nucleosomes

7-10

25- to 30-nm chromatin fiber of superheli-

40-60

cal nucleosomes

Condensed metaphase chromosome of

8000

loops

nonprotein coding DNA. Accordingly, the primary transcripts of DNA (mRNA precursors, originally termed hnRNA because this species of RNA was quite heterogeneous in size [length] and mostly restricted to the nucleus), contain noncoding intervening sequences of RNA that must be removed in a process which also joins together the appropriate coding segments to form the mature mRNA. Most coding sequences for a single mRNA are interrupted in the genome (and thus in the primary transcript) by at least one—and in some cases as many as 50—noncoding intervening sequences (in-trons). In most cases, the introns are much longer than the continuous coding regions (exons). The processing of the primary transcript, which involves removal of introns and splicing of adjacent exons, is described in detail in Chapter 37.

The function of the intervening sequences, or in-trons, is not clear. They may serve to separate functional domains (exons) of coding information in a form that permits genetic rearrangement by recombination to occur more rapidly than if all coding regions for a given genetic function were contiguous. Such an enhanced rate of genetic rearrangement of functional domains might allow more rapid evolution of biologic function. The relationships among chromosomal DNA, gene clusters on the chromosome, the exon-intron structure of genes, and the final mRNA product are illustrated in Figure 36-7.

Figure 36-6. A human karyotype (of a man with a normal 46,XY constitution), in which the metaphase chromosomes have been stained by the Giemsa method and aligned according to the Paris Convention. (Courtesy of H Lawce and F Conte.)
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