An illustration of a protocell, composed of a fatty acid membrane encapsulating RNA ribozymes.

The theoretical protocell shown in the image on the right is made up of only two molecular components, a RNA replicase and a fatty acid membrane.  An extremely pared down and simple version of a cell, the protocell is nonetheless capable of growth, replication, and evolution.  Although a working version of a protocell has not yet been achieved in a laboratory setting, the goal appears well within reach.

The animation below illustrates the protocell life cycle.  The protocell includes two or more RNA replicases which are able to make copies of each other. Concurrent with RNA replication, the vesicle membrane grows through the addition of fatty acids from micelle collisions.  This causes the surface area of the protocell to increase while the volume remains constant, resulting in the elongation and increased instability of the protocell membrane.  The membrane eventually divides, forming two daughter protocells, with the RNA replicases randomly divided between them.

Every once in a while, a replicase will make a mistake, and a mutant replicase RNA is produced.  Usually, this mutation will result in a poorer replicase, if catalytic activity is retained at all.  Rarely, however, a better replicase could be formed – a replicase that might be able to copy RNAs faster, for example. Even more rarely, a RNA will be introduced to a protocell (by mutations or some other means) that has a new, different catalytic activity, such as the ability to catalyze the formation of fatty acids.  These protocells with faster replicases or new functional ribozymes will have an advantage over other protocells by being able to grow and divide faster.  Since all protocells in a population will be competing for resources (such as RNA nucleotides and fatty acids), those that can grow and divide more quickly will use up more of the resources, causing the “extinction” of slower-dividing protocell species.

Go back : Explore a timeline of the evolution of Life on Earth.

To learn more about protocells, check out Links to Learn More.

To download any of the illustrations or animations seen here, visit the Resources for Educators section.