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Chapter 26 The Tree of Life: - page 6 / 18





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Under particular conditions, some RNA sequences are more stable and replicate faster and with fewer errors than other sequences.

Occasional copying errors create mutations; selection screens these mutations for the most stable or the best at self-replication.

Beginning with a diversity of RNA molecules that must compete for monomers to replicate, the sequence best suited to the temperature, salt concentration, and other features of the surrounding environment and having the greatest autocatalytic activity will increase in frequency.

Its descendents will be a family of closely related RNA sequences, differing due to copying errors.

Some copying errors will result in molecules that are more stable or more capable of self-replication.

Similar selection events may have occurred on early Earth.

Modern molecular biology may have been preceded by an “RNA world.”

Natural selection could refine protobionts containing hereditary information.

The first RNA molecules may have been short, virus-like sequences, aided in their replication by amino acid polymers with rudimentary catalytic capabilities.

This early replication may have taken place inside protobionts.

RNA-directed protein synthesis may have begun as weak binding of specific amino acids to bases along RNA molecules, which functioned as simple templates holding a few amino acids together long enough for them to be linked.

This is one function of rRNA today in ribosomes.

Some RNA molecules may have synthesized short polypeptides that behaved as enzymes helping RNA replication.

Early chemical dynamics would include molecular cooperation as well as competition.

Other RNA sequences might have become embedded in the protobiont membrane, allowing it to use high-energy inorganic molecules such as hydrogen sulfide to carry out organic reactions.

A protobiont with self-replicating, catalytic RNA would differ from others without RNA or with RNA with fewer capabilities.

If that protobiont could grow, split, and pass its RNA molecules to its daughters, the daughters would have some of the properties of their parent.

The first protobionts must have had limited amounts of genetic information, specifying only a few properties.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-6

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