Quotes of interest -- satellite DNA in the news.

I have already made note of some of the coverage of noncoding DNA that appeared in Science during the 1980s, and as a sequel to that earlier installment of the series, I want to talk about the coverage in Nature from the late 1960s and early 1970s. Because SINEs, LINEs, pseudogenes, and introns were all discovered in 1977 or later, this will necessarily focus on satellite DNAs.

As mentioned previously, satellite DNAs were discovered in the early 1960s, and by the late 1960s and early 1970s there was substantial interest in these highly repetitive components of the genome. Nature published several stories about this work in their “News and Views” section, authored by various unnamed correspondents. Of course, one must not take the interpretation of anonymous science writers as definitive (after all, their contemporary counterparts do much to add the the mythology surrounding noncoding DNA), but it supports the overall contention that during this period adaptationist thinking was dominant and thus that it was taken almost as a given that functions would be elucidated for noncoding sequences. You will notice also that many of these stories report on data that contradict proposed functions, yet the expectation remained that some function exists. I am not criticizing the studies of these early authors in any way. Some satellite DNA is functional in chromosomal structure, but the point is that at the time this was an a priori assumption rather than a conclusion, and it is clearly not the case that these elements were dismissed as unimportant.

“Mouse satellite DNA”, Nature 215: 575, August 5, 1967:

What is the function of satellite DNA? It is unlikely to code for protein and yet it forms 10 per cent of the cell’s total DNA. What possible purpose is served by having so many, apparently identical, short sequences within the same genome?

“Satellite DNA”, Nature 222: 327, April 26, 1969:

Unfortunately, the group’s latest data serve only to make ideas about the function of this strange DNA fraction even more obscure.

But if satellite DNA is not transcribed, what is its function? Flamm et al. are impressed by the fact that numerous copies of a nucleotide sequence of 350 bases have been maintained during evolution in the face of the tendency to accumulate random mutations. This implies that satellite DNA has some important function. They suggest that it is required for “housekeeping”, the folding and packing of DNA in the chromosomes. In the absence of any critical data or any way of testing for the function of this DNA, that is as good a suggestion as any.

Like the Edinburgh group, Maio and Schildkraut are convinced that satellite DNA has some vital, albeit unknown, function.

“Hybridization and satellite DNA”, Nature 225: 414, January 31, 1970:

The function of this satellite DNA has always been obscure, reducing investigators to suggest, for example, that it may be involved in chromosome “housekeeping”, but Pardue and Gall claim that it is localized in the centromeres. It may therefore play a role in chromosome pairing, and this may account for the curious properties of satellite DNA, not least its peculiar base sequence.

“Mysterious satellites”, Nature 225: 899-900, March 7, 1970:

Any biologist told that 10 to 12 percent of the total DNA genome of an animal is sequestered in a chemically distinct fraction would find it hard to escape the conclusion that such DNA has some crucial cellular function. That explains why the so-called satellite DNAs are exciting so much interest…

A host of experiments and speculations leap to mind. Perhaps satellite DNA plays some part in the assembly of the mitotic spindle, for example, by influencing polymerization of spindle protein or the attachment of chromosomes to the spindle. Hybrid cells might be useful in studying the specificity of a putative interaction between satellite DNA and components of the mitotic spindle. And the chromosomes of organisms with diffuse centromeres might be useful for further testing the relationship between satellite DNA and centromeres.

The last one is interesting, because it led to a correction by one of the first people to identify satellite DNA, Waclaw Szybalski, in the correspondence segment of the April 4, 1970 issue. Did he complain about the characterization of biologists anticipating functions for satellite DNA? No, he simply noted that the author got the date of discovery wrong (Szybalski 1970; Nature 226: 89-90).

“Satellite DNA and sequence”, Nature 227: 775, August 22, 1970:

What possible function can be served by a DNA which consists of tandem duplication of a sequence of only six base pairs, and why should an animal such as the guinea-pig require some 107 copies of this short sequence in all its cells?

Finally, even though we now know the basic sequence unit of a satellite DNA we are no closer to explaining the function of these specialized DNAs. Since they have no role in coding protein, the most plausible suggestion is that they have some role in maintaining the integrity of the chromosome itself. The localization of satellite DNA in the centromere regions of chromosomes suggests they play a part in the functions conventionally ascribed to the centromere. But for the time being such suggestions remain speculative.

“Satellite DNA and speciation”, Nature 240: 128, November 17, 1972:

The function and evolutionary significance of satellite DNA — DNA which has a reiterated base sequence, is associated with heterochromatin and centromeres and may or may not be transcribed — remain tantalizing mysteries. It seems unlikely that these simple sequences code for any polypeptides and it has, therefore, been suggested that satellite DNA may be involved in processes such as pairing of homologous chromosomes, chromosome movement and chromosome packing, but there is little evidence in support of these speculations.

“The mystery deepens”, Nature 240: 255, December 1, 1972:

But the fact remains that one is still at a loss as to the function of satellite DNA, the chief characteristic of which is its comparatively simple and highly reiterated base sequence, and indeed the more that is learnt about the distribution of satellite DNAs the deeper the enigma of its function becomes.

“DNA dominant at Berkeley, California”, Nature 245: 183-184, September 28, 1973:

The problem of DNA redundancy continues to intrigue several teams, without finally yielding all the secrets of its function or the reason for the wide variation in amount from species to species. Some of the extra DNA is almost certainly present as spacer sequences between cistrons, but this does not account for the large amount of simple sequence DNA, present in millions of copies, in the centromeric heterochromatin. P.M.B. Walker, for whom the Medical Research Council has recently set up a unit in Edinburgh specifically devoted to research on the mammalian genome, reviewed the history of satellite DNA, but said that most investigators would still go no further than suggest that this material, which is not transcribed, has some “housekeeping” function.

Here is the take-home message. From the time it was discovered, satellite DNA was presumed to be functional on the basis of Darwinian adaptationist expectations. This stimulated intensive research on the subject which was considered interesting enough to be reported about regularly in Nature. Some of the proposed functions, such as a structural role for some noncoding sequences, turned out to be correct — which was only shown because the Darwinian assumption prompted researchers to test functional ideas. The claim by creationists that “Darwinism” prevented such research is manifestly and demonstrably inaccurate. The problem, as I have noted, is that a strict focus on adaptive roles for noncoding DNA prevented many researchers from adopting a more balanced approach under which some of it is functional but most of it is not.

____________

Part of the Quotes of interest series.



Leave a Reply

 

 

 

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

*