How did ribosomes work without proteins?

Grade Level: 
11
Answered by: 
Adam Orr

Hi, Jason.

This is a great question that has sparked a lot of interest:

If a ribosome is required to make proteins, but the ribosome itself is made of proteins, how did proteins first form?

While this seems puzzling, the mystery started to unravel when scientists began to look at how ribosomes work.

Ribosomes use RNA to make protein

Structure of ribosome subunit showing RNA, protein, and active site

The structure of the large subunit of a ribosome, responsible for making proteins. Click for more detail.

They focused on the part of the ribosome that helps amino acids to join (making proteins). This part of the ribosome is made of RNA. The other parts of the ribosome, that are made of protein, only play a structural role and help keep the molecule stable.

Enzymes are molecules that help make chemical reactions happen. While many enzymes are proteins, there are also many enzymes made of only RNA; these are called ribozymes. Because the ribosome is a ribozyme, that means that an RNA-only molecule could help with protein formation.

The RNA World Hypothesis

Thus, it's very likely that the first ribosomes were only made of RNA. The protein parts of the molecule were likely added later, and helped prevent it from breaking down. This is part of a wider body of work focused on the very beginning of life on Earth—the RNA World Hypothesis.

The hypothesis considers the observations that:

  1. RNA can be used to store information in its sequence
  2. RNA can perform functional roles and catalyze (help or speed up) reactions

It suggests that the first life forms used RNA for both these functions.

DNA and proteins evolved later and are more stable. But there was likely a time when all organisms had RNA genomes and cellular functions depended on RNA-based enzymes.

No proteins required!


Additional images via Wikimedia Commons.

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