RNA

Ribonucleic acid, or RNA, is a very important molecule for life. It works with another molecule called deoxyribonucleic acid, known as DNA. Together, they are part of a group called nucleic acids, which are necessary for all living things. RNA is made from smaller parts called nucleotides.

A hairpin loop from a pre-mRNA. Highlighted are the nucleobases (green) and the ribose-phosphate backbone (blue). This is a single strand of RNA that folds back upon itself.

RNA helps cells make proteins, which are tiny workers inside our bodies. There are different kinds of RNA. One type, called messenger RNA or mRNA, carries instructions from DNA that tell cells which proteins to build. Another type, called transfer RNA or tRNA, brings the building blocks of proteins, called amino acids, to a structure in the cell called a ribosome. There, ribosomal RNA or rRNA helps link the amino acids together to form the proteins.

Scientists believe that long ago, before DNA existed, RNA might have been the main way that life's instructions were stored. This idea is called the "RNA world." Even today, some viruses use RNA instead of DNA to hold their genetic information. RNA also helps control many important jobs inside cells, showing just how vital it is for life.

Chemical structure of RNA

Main article: Nucleic acid structure

Watson-Crick base pairs in a siRNA. Hydrogen atoms are not shown.

Each small piece of RNA, called a nucleotide, is made of three parts: a sugar called ribose, a small molecule called a base, and a piece called a phosphate. The bases can be adenine, cytosine, guanine, or uracil. These pieces link together in a chain to form RNA.

RNA is very similar to DNA, but there are three big differences. First, RNA is usually a single, loose strand, while DNA is usually a pair of strands twisted together. Second, RNA uses ribose sugar, which has an extra piece that DNA’s sugar does not have. Third, RNA uses uracil instead of a base called thymine that DNA uses. These differences make RNA behave in special ways that help cells work.

Types of RNA

RNAs involved in protein synthesis

Messenger RNA (mRNA) brings the protein-making instructions to the ribosomes. In cells with a nucleus, mRNA is prepared and then moves out to help make proteins. In other cells, mRNA can start making proteins right away.

Transfer RNA (tRNA) is a small RNA that carries a specific building block to the growing protein chain.

Structure of a hammerhead ribozyme, a ribozyme that cuts RNA

Ribosomal RNA (rRNA) is a key part of ribosomes, where proteins are made. Most RNA in a cell is rRNA.

Regulatory RNA

RNAs can also help control which genes are active. For example, microRNAs can stop certain messages from being used to make proteins.

MicroRNA (miRNA) and small interfering RNA (siRNA)

Long non-coding RNAs

Enhancer RNAs

Small RNA in prokaryotes

Small RNA

Small RNAs also exist in bacteria and help control genes, especially during stress.

CRISPR RNA

Archaea use CRISPR RNA to protect against viruses.

RNA synthesis and processing

RNA is made inside the cell using a special helper called RNA polymerase. This helper reads a copy of DNA to build a matching RNA piece. After making the basic RNA copy, extra parts are often added or removed to finish it properly.

Many RNAs help change other RNAs by cutting out extra bits or changing some letters. These changes help RNAs do their jobs better in the cell.

RNA in genetics

Like DNA, RNA can carry genetic information. Some viruses have RNA as their genetic material, and this RNA helps make proteins and protect the virus as it moves to new cells. There are also tiny pathogens called viroids that are made only of RNA and do not make any proteins themselves.

Some viruses make DNA copies from their RNA, and these DNA copies are then used to make new RNA. Certain elements in cells and a special enzyme also use RNA to build parts of chromosomes.

Double-stranded RNA

Double-stranded RNA has two matching strands, like DNA, but with a different building block. It serves as genetic material for some viruses and can help activate the body’s natural defenses against infections.

Circular RNA is a special form of RNA that forms a ring. It was discovered in the late 1970s and is found in both animals and plants, but its exact functions are still being studied.
Main article: Double-stranded RNA
Main article: Circular RNA

Key discoveries in RNA biology

Further information: History of RNA biology

Research on RNA has led to many important discoveries and several Nobel Prizes. In 1868, Friedrich Miescher discovered nucleic acids, calling them 'nuclein' because they were found in the nucleus of cells. Later, it was found that even cells without a nucleus, called prokaryotic cells, also contain these important molecules. In 1959, Severo Ochoa won a Nobel Prize for discovering an enzyme that could create RNA in a lab, though later it was found this enzyme actually breaks down RNA.

Robert W. Holley, left, poses with his research team.

Important milestones include the discovery of the exact sequence of a yeast molecule in 1965 by Robert W. Holley, earning him a Nobel Prize in 1968. In the 1970s, scientists found that certain viruses could copy RNA into DNA, a process that goes against the usual flow of genetic information. This work earned a Nobel Prize in 1975. Over the years, many more discoveries about RNA have been made, including finding that RNA can act like enzymes and play a role in controlling genes. These discoveries have led to new medical treatments and a better understanding of how life works at the molecular level.

In 1968, a scientist named Carl Woese suggested that RNA might have been important in the very first forms of life. In 2022, scientists found that RNA can form naturally on certain types of rock that would have been common on the early Earth. Research also shows that the building blocks of RNA can form under conditions similar to those found in space, suggesting that these molecules might have played a key role in the origin of life.

Medical applications

RNA used to be thought too unstable for medical use, but new ways to keep it stable have made it very useful. RNA can shape itself in special ways and connect with other tiny parts of cells, which helps it work as a medicine. RNA-based vaccines are easier to make than older types of vaccines because they don’t need to grow live germs first. Some medicines now use RNA to treat different diseases.

Special short pieces of RNA, called siRNAs, help our bodies fight viruses and can be used to turn off certain genes in studies and medicine. The most famous use of RNA in vaccines was during the COVID-19 pandemic, where mRNA vaccines helped protect many people.