From an RNA world to modern cells

Chains of nucleotides and amino acids floating in the sea aren’t alive. To become cells, they needed two more things: a container to keep them together and a way to copy themselves.

Stage 3: putting chains in a container

A protobiont is a cluster of prebiotically made molecules surrounded by a boundary, such as a lipid bilayer, that keeps its inside chemically different from the outside. Protobionts are the proposed go-between from chemistry to cells. Scientists picture them gaining four features:

• a boundary separating the inside from the environment
• polymers inside that carry information
• polymers inside that act as catalysts (a catalyst speeds up a chemical reaction without being used up)
• eventually, the ability to copy themselves

Protobionts could divide, but not in the precise way cells reproduce. Over time, their chemistry became more complex and their copying more reliable.

Liposomes as early protobionts

Protobionts may have been liposomes: hollow spheres of water enclosed by a phospholipid bilayer. A phospholipid is a lipid with a water-loving head and two water-avoiding tails. In water, the tails huddle together away from the water while the heads face outward, so these lipids form bilayer spheres on their own.

Liposomes have some lifelike traits. Their bilayer lets only some substances through, and some liposomes can store energy as an electrical difference across the membrane, then release it much as a nerve cell does. Clay speeds up liposome formation, and the liposomes it produces can grow and divide. If RNA is on the clay, the liposomes enclose it, so membranes filled with RNA could have formed from simple chemistry alone.

Stage 4: why RNA probably came first

Most scientists think the first large molecule (macromolecule) in protobionts was RNA, because RNA can do three jobs at once:

1. Store information in its sequence of nucleotides.
2. Self-replicate. Through base pairing, each nucleotide pairs with one specific partner, so an RNA strand can serve as a template for building a matching strand.
3. Catalyze reactions. RNA molecules that speed up chemical reactions are called ribozymes.

The other two candidates fall short: DNA can barely catalyze anything, and no protein is known to copy itself. RNA could be both the genetic material and the worker that copies it. The hypothetical early period when RNA did both jobs, holding life’s information and its catalytic activity, is called the RNA world.

Chemical selection

Chemical selection is the proposed way random RNA molecules became more cell-like. When one molecule in a mixture has a property that lets it increase in number faster than the others, it becomes more common. Over time, the whole population of molecules shifts to a new makeup, a change called chemical evolution. At first, the properties that mattered most were the ability to self-replicate and the ability to catalyze other useful reactions.

Chemical selection resembles natural selection, which acts on living organisms through their survival and reproduction. Here the “population” is a mixture of molecules.

Lab work shows chemical selection can happen. David Bartel and Jack Szostak started with a random pool of RNA molecules and repeatedly kept the ones best at linking nucleotides. After 10 rounds, the pool was about 3 million times more active than at the start.

Handing jobs to DNA and proteins

The RNA world eventually gave way to today’s DNA/RNA/protein world, possibly passing through a DNA/RNA stage or an RNA/protein stage first.

Why DNA took over storage. With DNA holding the information, RNA was free for other jobs, such as binding helper molecules or peptides (short amino acid chains) that boost its catalytic power. DNA is also more stable: its strands break less easily than RNA’s.

DNA itself may have arisen when an ancient RNA gained the ability to build DNA from an RNA template, a process called reverse transcription. Cells like yours still do this: an enzyme called telomerase uses an RNA template to copy the ends of chromosomes.

Why proteins took over most catalysis. Proteins are built from 20 kinds of amino acids with different chemical properties, which gives them far more catalytic range than RNA. Proteins also took on other roles, such as forming the cell’s internal framework and carrying substances into the cell. Still, they haven’t taken over every catalytic job; some RNAs still catalyze reactions.

Clues to how proteins arose come from translation, the process that builds a protein from genetic instructions. RNA can catalyze the formation of peptide bonds (the links between amino acids), and modern translation still runs on RNA:

• messenger RNA (mRNA) carries the instructions for the amino acid sequence
• transfer RNA (tRNA) acts as an adaptor that brings in the amino acids
• ribosomal RNA (rRNA) in the ribosome, the protein-building machine, catalyzes peptide bond formation, so it is a ribozyme

This heavy reliance on RNA fits the idea that RNA helped create proteins in the first place.

Check yourselfWhy do most scientists think RNA came before DNA and proteins?

RNA can store information in its sequence, be copied through base pairing, and catalyze reactions as a ribozyme. DNA can barely catalyze reactions, and no protein is known to copy itself, so only RNA could have done every early job alone.