CASE 1 THE FIRST CELL: LIFE’S ORIGINS

During transcription and translation, proteins and nucleic acids work together to convert the information stored in DNA into proteins. If we think about how such a system might have originated, however, we immediately confront a chicken-and-egg problem: Cells need nucleic acids to make proteins, but proteins are required to make nucleic acids. Which came first? In Chapter 3, we discussed the special features that make RNA an attractive candidate for both information storage and catalysis in early life. Early in evolutionary history, then, proteins had to be added to the mix. No one fully understands how they were incorporated, but researchers are looking closely at tRNA, the molecule involved in the “translating” step of translation.

In modern cells, tRNA shuttles amino acids to the ribosome, but an innovative hypothesis suggests that in early life tRNA-like molecules might have served a different function. This proposal holds that the early precursors of the ribosome were RNA molecules that facilitated the replication of other RNAs, not proteins. In this version of an RNA world, precursors to tRNA would have shuttled nucleotides to growing RNA strands. Researchers hypothesize that tRNAs bound to amino acids may have acted as simple catalysts, facilitating more accurate RNA synthesis. Through time, amino acids brought into close proximity in the process of building RNA molecules might have polymerized to form polypeptide chains. From there, natural selection would favor the formation of polypeptides that enhanced replication of RNA molecules, bringing proteins into the chemistry of life.

All of the steps in gene expression, including transcription and translation, are summarized in Fig. 4.19.

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