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Chapter 26 The Tree of Life: - page 5 / 18

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The necessary conditions may have been provided by protobionts, aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure.

Protobionts exhibit some of the properties associated with life, including reproduction and metabolism, and can maintain an internal chemical environment different from their surroundings.

Laboratory experiments show the spontaneous formation of protobionts from abiotically produced organic compounds.

For example, droplets of abiotically produced organic compounds called liposomes form when lipids and other organic molecules are added to water.

The lipids form a molecular bilayer at the droplet surface, much like the lipid bilayer of a membrane.

These droplets can undergo osmotic swelling or shrinking in different salt concentrations.

Some liposomes store energy in the form of a membrane potential.

Liposomes behave dynamically, growing by engulfing smaller liposomes or “giving birth” to smaller liposomes.

If similar droplets forming in ponds on early Earth incorporate random polymers of linked amino acids into their membranes, and if some of these polymers made the membranes permeable to molecules, then those droplets could have selectively taken up organic molecules from their environment.

RNA may have been the first genetic material.

The first genetic material was probably RNA, not DNA.

Thomas Cech and Sidney Altman found that RNA molecules not only play a central role in protein synthesis, but also are important catalysts in modern cells.

RNA catalysts, called ribozymes, remove their own introns and modify tRNA molecules to make them fully functional.

Ribozymes also help catalyze the synthesis of new RNA polymers.

Ribozyme-catalyzed reactions are slow, but the proteins normally associated with ribozymes can increase the reaction rate more than a thousandfold.

Laboratory experiments have demonstrated that RNA sequences can evolve under abiotic conditions.

Unlike double-stranded DNA, single-stranded RNA molecules can assume a variety of 3-D shapes specified by their nucleotide sequences.

RNA molecules have both a genotype (nucleotide sequence) and a phenotype (three-dimensional shape) that interacts with surrounding molecules.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-5

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