Molecular Evolution

Once formed, how would the first RNA chains cause copies of themselves to be created? Replication is the process by which DNA or RNA makes copies of specific base sequences. However, the "copy" is not identical to the original, called a template. Rather, it is analogous to a photographic negative, with predicable differences from the original. In the case of RNA, the new strand has a different pattern of bases, determined by the specific interactions (hydrogen bonding) between the old strand and the new one. Thus, adenine bonds to uracil (causing it to become part of the copy), and guanine "directs" the incorporation of cytosine in the same way. As an example, a template sequence abbreviated as AACCAA would be replicated as UUGGUU (the letters stand for each of the four bases). Of course, the faithful transfer of genetic information is a much more complex process, involving an array of complicated protein enzymes and requiring special "activated" precursors for each of the bases.

Leslie Orgel and his associates have carried out extensive studies of replication since 1980. Their goals were, first, to establish whether this process could occur without the help of enzymes (which would not have been present in the environment of early Earth); second, to analyze the accuracy of the copies; third, to explore the limits on the types of sequences that could be copied; and fourth, to determine if the copies could then serve as templates for self-perpetuating replication. These aspects have met with different degrees of success, as discussed below.

Orgel's first breakthrough came with the reaction of activated guanine derivatives (again using phosphorimidazolides) on a template consisting of repeating cytosines, which was thus very similar to a small piece of RNA, except that it had only one type of base. In the presence of a zinc catalyst, the guanine derivatives bound to the template and formed chains with more than thirty guanines linked to one another. (Without the template, the only products were those with two or three bases.) Equally striking was that the guanine chain contained mainly the same type of phosphate-ribose backbone as in native RNA, and the template preferentially bound the "correct" base more than 99 percent of the time. Even if the activated derivatives of uracil, adenine, and cytosine were present, they become incorporated into the product with less than 1 percent efficiency. These early experiments demonstrated that templates could accurately form long chains with the appropriate bond between the sugar and phosphate.

The copying of other sequences using this approach has been more difficult, however, partly because of the unreactive structures that the templates often form. For example, RNA chains containing only adenine mixed with other chains of uracil tend to form aggregates that hinder replication. Guanine is even more unusual, in that it organizes into arrays with four chains locked together, which also prevents replication, although this problem may be overcome by employing very dilute reaction conditions (more relevant to early Earth). The greatest success has been achieved with mixed, cytosine-rich templates that contain adenine, guanine, or uracil as isolated bases separated by at least three cytosines.

A further difficulty is that none of the systems studied by Orgel is capable of replication beyond the first stage: the copies can never serve as templates themselves, because they remain tightly bound to the original template. However, Gunter von Kiedrowski made an important advance in 1994, when he showed that sets of three bases containing guanine and cytosine on a DNA backbone could assemble on a template six units long and then separate. For example, two fragments of GGC could link together on a CCGCCG framework, and then break apart to form a GGCGGC template, available for further replication. The reaction conditions were quite

The phosphorimidazole activates the nucleotide, promoting the linking of multiple units. This synthetic compound may mimic compounds present in prebiotic environments. different from what might have existed on early Earth, especially the chemical used to form the bond between the GGC units, but it supports the concept that true replication of this type might be possible.