RNA
Meaning of RNA: Best known as the messenger between gene and protein, RNA also stores the genes of many viruses and regulates the cell directly.
Definition of RNA
RNA is a nucleic acid built from a chain of nucleotides, usually single stranded, that carries out the day to day work of using the information held in DNA. Its best known job is copying a gene and delivering that copy to the machinery that builds proteins, but other forms assemble proteins directly or control which genes are switched on.
How does a gene become a protein?
The sequence of a gene is first copied into messenger RNA, a step called transcription. In cells with a nucleus the raw copy is then edited, with intervening stretches cut out and the remaining pieces joined, and the finished message leaves the nucleus. A ribosome reads it three bases at a time, and each triplet either names a building block or marks the end. Transfer RNA molecules deliver those building blocks in the order the message specifies, and the chain that results folds into a protein. A single message can be read many times before it is destroyed, so the amount of protein a cell makes depends on how many copies exist and how long each survives. The full chemical name for the molecule is ribonucleic acid.
Which viruses use RNA as their genes?
| Group | How it copies itself |
|---|---|
| Positive sense | The viral RNA is read directly as a message, so the host cell starts making viral proteins at once |
| Negative sense | The viral RNA must first be copied into a readable form by an enzyme the virus carries in with it |
| Double stranded | Paired strands are copied inside a protective shell rather than out in the cell |
| Retroviruses | The RNA is copied into DNA, which is then inserted into the cell's own genome |
The enzymes that copy RNA proofread poorly, so RNA viruses change far faster than DNA viruses do. That single fact explains why some of them need a reformulated vaccine at intervals, why drug resistance emerges quickly, and why sequencing is used to track how an outbreak spreads.
What is RNA used for in medicine?
Detecting it is the basis of many virus tests, which convert viral RNA into DNA and then amplify it until enough exists to measure. The same approach counts how much virus is present in blood. Measuring which messages a tissue is producing shows which genes are active, and that pattern is used to classify tumors and to estimate how a cancer is likely to behave. RNA is also a treatment in its own right. Vaccines can supply a message that instructs cells to make one protein from a pathogen, and other drugs are short RNA sequences designed to bind a chosen message and stop it being read or cause it to be destroyed. A message given this way is read, then broken down like any other, and it does not enter the nucleus or become part of a person's own DNA. That is the difference from a virus, which uses the cell it enters to replicate its genome and go on to infect others.
Also known as
Used in a sentence
Laboratory analysis measured RNA expression levels within the tumor sample.
Good to know
Most RNA is single stranded, and the messenger form is degraded quickly so that output stops when copying stops, though ribosomal and transfer RNA are stable and account for most of the RNA present in a cell.
Word origin
Where “RNA” comes from
Formed from ribose, the five-carbon sugar that forms part of its structure, and nucleic acid, referring to the acidic molecules first isolated from the nucleus of the cell.