This View of Life 
The Ultimate Parasite 
What happens to bodies if genes act for themselves? 
by Stephen Jay Gould 
The uncommon good prose of scien- 
tists is more often spare than flowery. In 
my favorite example, James D. Watson 
and Francis Crick used less than a page 
to announce their structure of DNA in 
1953. They began with the sparsest an- 
nouncement: “We wish to suggest a 
structure for the salt of deoxyribose nu- 
cleic acid (D.N.A.). This structure has 
novel features which are of considerable 
biological interest.” And they ended 
with a reminder that they had not over- 
looked a major point just because they 
had chosen to defer its discussion: “It 
has not escaped our notice that the 
specific pairing we have postulated im- 
mediately suggests a possible copying 
mechanism for the genetic material” 
(that is, the two strands of the double 
helix would pull apart and each then act 
as a template for the reconstitution of its 
partner). 
Francis Crick, now a professor at the 
Salk Institute in southern California, 
has continued to generate controversial, 
challenging hypotheses (and he has of- 
ten been right). In late 1981, he will 
publish a book, Life Itself advocating a 
theory of “directed panspermia” — the 
idea that Earth’s original life arrived as 
microorganisms dispatched by intelli- 
gent beings who chose not to make the 
long journey themselves. (Ten will get 
you fifty that he’s wrong this time — but 
only fifty; he’s been right too often.) 
Crick has also not lost his gift for a 
well-turned phrase. In the presentation 
of his latest controversial hypothesis, 
published a year ago in Nature (April 
17, 1980) with Salk colleague Leslie 
Orgel as first author, he outdid the last 
line of his 1953 paper with Watson. 
Orgel and Crick conclude: “The main 
facts are, at first sight, so odd that only a 
somewhat unconventional idea is likely 
to explain them.” Indeed, the facts are 
so interesting, and the wondering about 
them so intense, that the same issue of 
Nature carried an accompanying article 
by Dalhousie University biologists W. 
Ford Doolittle and Carmen Sapienza, 
who had, quite independently, devised 
the same explanation and argued the 
case, in many ways, more forcefully. 
What, then, are these disturbing 
facts? When a younger Crick deter- 
mined the structure of DNA in 1953, 
and others cracked the genetic code a 
few years later, everything seemed mo- 
mentarily to fall into order. The old idea 
of genes as beads on a string (the chro- 
mosome) seemed to gain its vindication 
from the Watson-Crick model. Each 
three nucleotides in DNA codes for an 
amino acid (via an RNA intermediary); 
a string of amino acids makes a protein. 
Perhaps we could simply read down a 
chromosome to find genes lined up, one 
after the other, each ready to begin the 
assembly of its essential part. 
It was not to be so. Is it ever? We now 
know that the genetic material of higher 
organisms is vastly more complex. 
Many genes come in pieces, separated 
in DNA by sequences of nucleotides 
that are not transcribed into RNA. 
Many proteins are coded by partial se- 
quences on two or more chromosomes. 
What controls regulate their assembly? 
(Human globin, the protein component 
of hemoglobin, contains alpha and beta 
chains — and the genes for each chain 
are on separate chromosomes.) 
Even more disturbing (and exhilarat- 
ing) is the discovery, made more than a 
decade ago but gathering intensity ever 
since, that only a small percentage of 
DNA codes for proteins in higher organ- 
isms — and that these are the only bits of 
DNA whose function we may truly un- 
derstand at the moment. In humans, 
somewhat more than 1 percent, but not 
as much as 2 percent, of DNA codes for 
proteins. Much of the rest contains se- 
quences that are repeated over and over 
again — hundreds or thousands of identi- 
cal (or nearly identical) beads, some- 
times following one after the other, 
sometimes dispersed widely over several 
chromosomes. Why so many copies? 
What do they do? The “selfish DNA” 
hypothesis of Doolittle, Sapienza, Orgel, 
and Crick provides an unusual answer to 
the puzzling question of why so much 
DNA exists in repeated copies (but I 
will keep you in suspense for a bit and 
discuss the conventional answers first). 
Higher organisms contain different 
classes of repeated DNA. One type, 
called highly repeated or satellite DNA, 
contains short and simple sequences re- 
peated hundreds of thousands or mil- 
lions of times; 5 percent or so of human 
DNA falls into this class. We hardly 
have a clue about the origin and func- 
tion of satellite DNA; neither the selfish 
DNA hypothesis nor the conventional 
hypotheses can explain it. Satellite 
DNA is, as they say, a “whole ’nother” 
story waiting to be told. 
The current debate over the conven- 
tional and selfish DNA hypotheses cen- 
ters upon the so-called intermediate or 
middle-repetitive DNA, some 15 to 30 
percent of both the human and the fruit 
fly genome. Middle-repetitive DNA ex- 
ists in tens to a few hundred copies per 
sequence; the copies are often widely 
dispersed on several chromosomes. 
I have said nothing, so far, about the 
DNA of simpler organisms — the pro- 
karyotic bacteria and blue-green algae, 
which have no nucleus and carry their 
DNA in a single chromosome. The 
DNA of prokaryote (prenucleate) or- 
ganisms is “better behaved” with refer- 
ence to the original hopes of the Watson- 
Crick model. Most bacterial DNA is 
single copy and protein coding, almost 
those beads on a string after all. But 
even prokaryotes are not immune to 
repetition. A hot topic of late concerns 
the presence in prokaryotes of so-called 
transposons, transposable elements, or 
more colorfully, jumping genes. These 
sequences of DNA, as their various 
names proclaim, can repeat themselves 
and then autonomously move about to 
other positions on the bacterial chromo- 
some. They often exist in about as many 
copies as middle-repetitive DNA in 
eukaryotes (higher organisms with a nu- 
cleus and paired chromosomes). This 
has led many biologists to propose that 
at least some of the middle-repetitive 
DNA in higher organisms amplifies it- 
self by the same mechanism of transpo- 
sition. (The selfish DNA hypothesis as- 
sumes a correspondence between pro- 
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