45 5.5 RNA

Created by: CK-12/Adapted by Christine Miller


Image shows a diagram of a basic overview of protein
Figure 5.5.1 Diagram of a basic overview of protein.

A Deceptively Simple Model

This simple model sums up one of the most important ideas in biology, which is called the central dogma of molecular biology (you’ll read more about it below). You probably recognize the spiral-shaped structure in the nucleus. It represents a molecule of DNA, the biochemical molecule that stores genetic information in most living cells. The yellow chain represents a newly formed polypeptide — the beginning stage of creating a protein. Proteins are the class of biochemical molecules that carry out virtually all life processes. What is the structure in the center of the model? It appears to resemble DNA, but it is smaller and simpler. This molecule is the key to the central dogma, and it may have been the first type of biochemical molecule to evolve.

Central Dogma of Molecular Biology

DNA is found in chromosomes. In eukaryotic cells, chromosomes always remain in the nucleus, but proteins are made at ribosomes in the cytoplasm. How do the instructions in DNA get to the site of protein synthesis outside the nucleus?

Another type of nucleic acid is responsible. This nucleic acid is RNA, or ribonucleic acid. RNA is a small molecule that can squeeze through pores in the nuclear membrane. It carries the information from DNA in the nucleus to a ribosome in the cytoplasm and then helps assemble the protein. In short:

DNA RNA  Protein

This expresses in words what the diagram in Figure 5.5.1 shows. The genetic instructions encoded in DNA in the nucleus are transcribed to RNA. Then, RNA carries the instructions to a ribosome in the cytoplasm, where they are translated into a protein. Discovering this sequence of events was a major milestone in molecular biology. It’s called the central dogma of molecular biology.

Introducing RNA

A strand of RNA
Figure 5.5.2 RNA is a single strand of nucleotides, each containing the sugar ribose, a phosphate group, and one of four bases, A, C, G, or U.

DNA alone cannot “tell” your cells how to make proteins. It needs the help of RNA, the other main player in the central dogma of molecular biology. Like DNA, RNA is a nucleic acid, so it consists of repeating nucleotides bonded together to form a polynucleotide chain. RNA differs from DNA in several ways: it exists as a single stranded molecule, contains the sugar ribose (as opposed to deoxyribose) and uses the base uracil instead of thymine.


Functions of RNA

The main function of RNA is to help make proteins. There are three main types of RNA involved in protein synthesis:

  1. Image shows a diagram of the three types of RNA: Messenger RNA, which is a single strand of RNA, Ribosomal RNA, which is an RNA-protein complex with two subunits, and transfer RNA, which is a single strand of RNA enfolded on itself with an anticodon region and a region which can carry a single amino acid.
    Figure 5.5.3 The three types of RNA take very different forms.

    Messenger RNA (mRNA) copies (or transcribes) the genetic instructions from DNA in the nucleus and carries them to the cytoplasm.

  2. Ribosomal RNA (rRNA) helps form ribosomes, where proteins are assembled. Ribosomes also contain proteins.
  3. Transfer RNA (tRNA) brings amino acids to ribosomes, where rRNA catalyzes the formation of chemical bonds between them to form a protein.

In section 5.7 Protein Synthesis, you can read in detail about how these three types of RNA build primary structure of proteins.

RNA is a very versatile molecule which plays multiple roles in living things. In addition to helping to make proteins, for example, there are RNA molecules that regulate the expression of genes, and RNA molecules that catalyze other biochemical reactions needed to sustain life. Because of the diversity of roles that RNA molecules play, they have been called the Swiss Army knives of the cellular world.

It’s an RNA World

The function of DNA is to store genetic information inside cells. It does this job well, but that’s about all it can do. DNA can’t act as an enzyme, for example, to catalyze biochemical reactions that are needed to keep us alive. Proteins are needed for this and many other life functions. Proteins work exceptionally well to keep us alive, but they are unable to store genetic information. Proteins need DNA for that. Without DNA, proteins could not exist. On the other hand, without proteins, DNA could not survive. This poses a chicken-and-egg sort of problem: Which evolved first? DNA or proteins?

Some scientists think that the answer is neither. They speculate instead that RNA was the first biochemical to evolve. The reason? RNA can do more than one job. It can store information as DNA does, but it can also perform various jobs (such as catalysis) to keep cells alive, as proteins do. The idea that RNA was the first biochemical to evolve, predating both DNA and proteins, is called the RNA world hypothesis. According to this hypothesis, billions of years ago, RNA molecules evolved that could both survive and make copies of themselves. According to the hypothesis, early RNA molecules eventually evolved the ability to make proteins, and at some point RNA mutated to form DNA.

Feature: Reliable Sources

The RNA world hypothesis has not gained enough support in the scientific community to be accepted as a scientific theory. In fact, there are probably as many detractors as supporters of the hypothesis. Do a web search to learn more about the RNA world hypothesis and the evidence and arguments for and against it. When weighing the information you gather, consider the likely reliability of the different websites you visit. Based on what you determine are the most reliable sources and the most convincing arguments, form your own opinion about the hypothesis. You may decide to accept or reject the hypothesis. Alternatively, you may decide to reserve judgement until — or if — more evidence or arguments are forthcoming.

5.5 Summary

  • The central dogma of molecular biology can be summed up as: DNA → RNA → Protein. This means that the genetic instructions encoded in DNA are first transcribed to RNA, and then from RNA they are translated into a protein.
  • Like DNA, RNA is a nucleic acid. Unlike DNA, RNA consists of just one polynucleotide chain instead of two, contains the base uracil instead of thymine, and contains the sugar ribose instead of deoxyribose.
  • The main function of RNA is helping to make proteins. There are three main types of RNA involved in protein synthesis: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). RNA has additional functions, including regulating gene expression and catalyzing other biochemical reactions.
  • According to the RNA world hypothesis, RNA was the first type of biochemical molecule to evolve, predating both DNA and proteins. The hypothesis is based mainly on the multiple functions of RNA, which can store genetic information like DNA and carry out life processes (like proteins).

5.5 Review Questions

  1. State the central dogma of molecular biology.
  2. Drag and drop to compare the structure and function of DNA and RNA:



5.5 Explore More

The RNA Origin of Life, NOVA PBS Official, 2014.

DNA vs RNA (Updated), Amoeba Sisters, 2019.



Figure 5.5.1

From DNA to Protein: Transcription through Translation by OpenStax College on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 5.5.2

Molbio-Header by Squidonius  on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.5.2

ARNm-Rasmol by Corentin Le Reun on Wikimedia Commons is is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).ublic domain.


Amoeba Sisters. (2019, August 29). DNA vs RNA (Updated). YouTube. https://www.youtube.com/watch?v=JQByjprj_mA&feature=youtu.be

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.29 From DNA to Protein: Transcription through Translation [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-4-protein-synthesis#fig-ch03_04_05

NOVA PBS Official. (2014, April 23). The RNA origin of life. YouTube. https://www.youtube.com/watch?v=VYQQD0KNOis&feature=youtu.be

Wikipedia contributors. (2020, June 28). RNA world. In Wikipedia. https://en.wikipedia.org/w/index.php?title=RNA_world&oldid=964998696




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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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