Proteins are powerful enzymes that perform several tasks necessary to sustain life. One of these tasks is the production of DNA, a double-stranded molecule found in cells that carries the genetic instructions for all living organisms. Interestingly, DNA is required for the production of protein, so proteins cannot exist without DNA to specify their construction, and DNA cannot be produced or replicated without proteins. It would be impossible for cells to exist without both components, yet it is highly unlikely that they evolved simultaneously.
Some biologists considering this classic chicken-and-egg paradox have offered a solution known as the RNA World Hypothesis. This hypothesis is based on the knowledge that RNA, a single-stranded molecule similar to DNA, can also carry genetic information, and RNA can perform some of the same chemical reactions performed by proteins. The RNA World Hypothesis posits that RNA was originally the sole performer of both genetic and enzymatic functions in early cells. DNA and protein evolved later to take over these roles. However, this hypothesis depends on ribozymes (RNA molecules with enzymatic properties) being able to catalyze a greater variety of chemical reactions than any currently known natural ribozyme can.
Searches for new ribozymes with a wider range of enzymatic properties have had some success. Intriguingly, these new ribozymes were unable to match the speed with which proteins can perform their catalyst roles. This may suggest that RNA is relatively ineffective at carrying out these reactions, which could explain why proteins ultimately evolved to perform them instead. Then again, perhaps the best possible ribozymes for these reactions have simply not been found yet.
1. The author is primarily concerned withA. reporting on the results of recent experiments to isolate new ribozymes
B. discussing a possible solution to a vexing puzzle in cellular evolution
C. examining the evidence that ribozymes evolved prior to DNA and proteins
D. refuting the argument that ribozymes are not versatile enough to provide the enzymatic and genetic components of early cells
E. explaining how DNA and proteins interact to provide cell function
2. The author uses the term "chicken-and-egg" in order toA. illustrate the cyclical nature of the replication of DNA and the construction of proteins
B. demonstrate that the mechanisms for producing DNA and proteins are fundamentally enigmatic
C. point out the apparent contradiction that DNA could not exist without proteins having already evolved, and vice versa
D. show that an apparent conundrum in the evolution of the DNA molecule can be resolved if one also considers the simpler RNA molecule
E. provide a metaphor for the relationship between DNA and RNA
3. All of the following can be inferred from information in the passage EXCEPT:A. Scientists have yet to find ribozymes in nature that could perform enough to satisfy the RNA World Hypothesis.
B. In the RNA World Hypothesis, RNA would outperform DNA in its function of carrying genetic information.
C. Known ribozymes are not maximally effective at carrying out all of the known functions of proteins in modern cells.
D. A catalyst can be more effective when it performs its role more quickly.
E. DNA is not the only molecule that can carry an organism's genetic information.