Chapter 5 Genetics
5.5 RNA
Created by: CK-12/Adapted by Christine Miller
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
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:
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Messenger RNA (mRNA) copies (or transcribes) the genetic instructions from DNA in the nucleus and carries them to the cytoplasm.
- Ribosomal RNA (rRNA) helps form ribosomes, where proteins are assembled. Ribosomes also contain proteins.
- 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
- State the central dogma of molecular biology.
- Drag and drop to compare the structure and function of DNA and RNA:
3.
4.
5.5 Explore More
The RNA Origin of Life, NOVA PBS Official, 2014.
DNA vs RNA (Updated), Amoeba Sisters, 2019.
Attributions
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.
References
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
A central organelle containing hereditary material.
Deoxyribonucleic acid - the molecule carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.
A class of biological molecule consisting of linked monomers of amino acids and which are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes.
A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
Cells which have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles.
A large complex of RNA and protein which acts as the site of RNA translation, building proteins from amino acids using messenger RNA as a template.
The process of creating protein molecules.
A complex organic substance present in living cells, especially DNA or RNA, whose molecules consist of many nucleotides linked in a long chain.
A nucleic acid of which many different kinds are now known, including messenger RNA, transfer RNA and ribosomal RNA.
An explanation of the flow of genetic information within a biological system. It is often stated as "DNA makes RNA and RNA makes protein."
One of the structural components, or building blocks, of DNA and RNA. A nucleotide consists of a base (one of four chemicals: adenine, thymine, guanine, and cytosine) plus a molecule of sugar and one of phosphoric acid.
A simple sugar and carbohydrate with molecular formula C5H10O5.
A sugar derived from ribose by replacing a hydroxyl group with hydrogen.
A large family of RNA molecules that convey genetic information from DNA to the ribosome, where they specify the amino acid sequence of the protein products of gene expression.
A large family of RNA molecules that convey genetic information from DNA to the ribosome, where they specify the amino acid sequence of the protein products of gene expression.
A type of RNA that acts as the primary building block for ribosomes and the assembly line on which protein synthesis occurs in those ribosomes.
A type of RNA that acts as the primary building block for ribosomes and the assembly line on which protein synthesis occurs in those ribosomes.
A small RNA molecule that participates in protein synthesis. Each tRNA molecule has two important areas: an anticodon and a region for attaching a specific amino acid.
The transformation of one molecule to a different molecule inside a cell.