What is RNA? How it differs from DNA and what it does

RNA, short for ribonucleic acid, is a molecule found in every living cell that is built much like DNA but does different work. Where DNA is a stable, double-stranded archive, RNA is usually a single strand made for action: it carries copies of genes to the cell's protein factories, forms the core of those factories, ferries in the building blocks and helps decide which genes are used. Some viruses use RNA as their genes.

In short

• RNA differs from DNA in three ways: a different sugar, the letter U instead of T, and usually one strand instead of two.
• The main kinds are messenger RNA (the copy), ribosomal RNA (the factory) and transfer RNA (the delivery), plus many small RNAs that regulate genes.
• RNA can both carry information and speed up reactions, which is why many scientists suspect early life ran on RNA alone.

How is RNA different from DNA?

Both are chains of nucleotides, units made of a sugar, a phosphate group and a base, as what is DNA? explains. The differences are small chemically and large in practice.

	DNA	RNA
Sugar	Deoxyribose	Ribose, which has one extra oxygen atom
Bases	A, T, G, C	A, U, G, C: uracil replaces thymine
Strands	Two, wound into a double helix	Usually one, often folded back on itself
Stability	Very stable; can survive thousands of years in good conditions	Breaks down easily; many messages last minutes to hours
Where it is	Mostly in the nucleus, plus mitochondria	Made in the nucleus, works mostly outside it
Main job	Long-term storage of genetic information	Carrying, using and regulating that information
Length	Whole chromosomes, millions of letters	Usually tens to thousands of letters

The basic chemistry is set out in the U.S. National Human Genome Research Institute's glossary entry on RNA. The reasons behind the differences are what make them worth remembering.

The extra oxygen makes RNA fragile. Ribose carries an extra oxygen-hydrogen group on its sugar ring. That group can attack the RNA's own backbone and snap it, so RNA strands fall apart far faster than DNA. For a working message that is useful: a cell can make a burst of a message, use it, and let it vanish, so protein production stops when it should.

The T in DNA helps with repair. Thymine is uracil with one extra carbon-containing group. A common kind of chemical damage turns the DNA letter C into U. Because DNA never normally contains U, repair enzymes can spot every U in DNA as damage and fix it. If DNA used U as a normal letter, that damage would be invisible.

One strand can fold. A single strand can pair with itself where its letters match, folding into hairpins, loops and compact three-dimensional shapes, much as a protein folds. That is how RNA can do jobs beyond carrying a message.

What does RNA do in a cell?

Most of the RNA in a cell isn't messages at all. Ribosomal RNA makes up about 80% of cellular RNA, according to Wikipedia's article on ribosomal RNA, even though it is never translated into protein.

Kind	What it does	Rough size
Messenger RNA (mRNA)	A working copy of one gene, carried to a ribosome to be read	Hundreds to many thousands of letters
Ribosomal RNA (rRNA)	Forms the core of the ribosome and does its key chemistry	About 120 to 5,000 letters, depending on the strand
Transfer RNA (tRNA)	Brings each amino acid to the ribosome, matching it to a three-letter codon	76 to 90 letters in eukaryotes
MicroRNA (miRNA)	Binds to matching messages and blocks or destroys them	About 22 letters
Other regulatory RNAs	Help cut and splice messages, shape chromosomes, silence genes	Varies

The ribosome is an RNA machine

A ribosome is about 60% RNA and 40% protein. When its structure was solved at the level of atoms, the site that joins amino acids together turned out to be made of RNA, not protein. The 2009 Nobel Prize in Chemistry went to Venkatraman Ramakrishnan, Thomas Steitz and Ada Yonath for that work. Every protein in your body is assembled by a catalyst made of RNA. The discovery that RNA can act as a catalyst at all earned Sidney Altman and Thomas Cech the 1989 Nobel Prize in Chemistry; the Nobel committee called it "a complete surprise to scientists." Such RNA catalysts are called ribozymes, the RNA cousins of protein enzymes.

Small RNAs that control genes

Every cell holds the same genes, so cells differ in which ones they use. One way they choose is with microRNAs, tiny strands that latch onto matching messages and stop them from being made into protein. Victor Ambros and Gary Ruvkun shared the 2024 Nobel Prize in Physiology or Medicine for discovering microRNA and its role in gene regulation, first in a tiny worm.

How do mRNA vaccines use RNA?

An mRNA vaccine delivers a message, not a germ. It carries a stretch of messenger RNA for one viral protein, wrapped in a tiny fat bubble. Cells near the injection read the message, make the protein, and the immune system learns to recognize it; the message itself breaks down, as RNA does. How that trains immunity, and how mRNA vaccines compare with older kinds, is covered in how vaccines work.

The RNA part had one big obstacle. The body treats loose RNA as a sign of viral infection and reacts against it, which made early mRNA both inflammatory and poorly translated. Katalin Karikó and Drew Weissman found that swapping one of RNA's normal building blocks, uridine, for a chemically modified version such as pseudouridine largely solved both problems. Their work won the 2023 Nobel Prize in Physiology or Medicine, for discoveries that the Nobel committee said "enabled the development of effective mRNA vaccines against COVID-19."

Did life start with RNA?

It may have, and this is a hypothesis, not a settled fact. The puzzle is circular: DNA stores genes but needs protein enzymes to copy it, and proteins are built from instructions in DNA. Which came first? RNA can do both jobs, holding information like DNA and catalyzing reactions like a protein. So a world in which RNA molecules copied themselves, before DNA and proteins took over, would break the circle. Alexander Rich first proposed the idea in 1962, and the biologist Walter Gilbert named it the "RNA world" in 1986, as Wikipedia's article on the RNA world recounts.

Still argued

Support for an RNA world comes from the ribosome's RNA core, from ribozymes, and from experiments showing that some RNA building blocks can form under conditions thought to resemble the early Earth, such as a 2009 study that made two of them by a plausible route. The difficulties are real too: RNA is fragile, its building blocks are hard to make and join without help, and no RNA has been found that can copy itself well enough to sustain a living system. Some researchers suggest a simpler molecule came before RNA, or that RNA, proteins and metabolism arose together.

Whatever the answer, RNA sits at the center of how biologists think about the boundary of life: it can store heredity, copy and evolve, which is a large part of what life does.

Check yourself

• Without looking back, name the three chemical differences between RNA and DNA and say why each matters.
• What do messenger, transfer and ribosomal RNA each do when a cell makes a protein?
• Explain why RNA that is both a message and a catalyst would solve a chicken-and-egg problem about life's origin.

Sources

• Ribonucleic acid (RNA) — National Human Genome Research Institute
• The Nobel Prize in Chemistry 1989, press release — Royal Swedish Academy of Sciences, 1989
• The Nobel Prize in Chemistry 2009, press release — Royal Swedish Academy of Sciences, 2009
• The Nobel Prize in Physiology or Medicine 2023, press release — Nobel Assembly at Karolinska Institutet, 2023
• The Nobel Prize in Physiology or Medicine 2024, press release — Nobel Assembly at Karolinska Institutet, 2024
• RNA world — Wikipedia