transcription
Meaning of transcription: Copying a gene into RNA is the step where a cell decides which of its instructions to act on and how loudly.
Definition of transcription
Transcription is the process that copies the sequence of a gene from DNA into a matching strand of RNA. An enzyme called RNA polymerase attaches at a start signal on the DNA, unwinds a short stretch, and builds the RNA strand base by base until it reaches a stop signal. The copy may be messenger RNA destined to direct protein assembly, or an RNA with a job of its own.
How does it differ from replication and translation?
| Process | What is copied | What comes out |
|---|---|---|
| DNA replication | DNA | An identical DNA copy, made before a cell divides |
| Transcription | DNA | RNA |
| Translation | Messenger RNA | A chain of amino acids that folds into a protein |
| Reverse transcription | RNA | DNA, made by viruses such as HIV and copied deliberately in the laboratory |
Only one strand of the double helix is read for any given gene, and the RNA produced matches the other strand. The three central processes are easy to confuse because all three copy information from one molecule into another, but each starts and ends with something different, and they happen at different moments and in different parts of the cell.
What happens to the copy afterward?
In cells with a nucleus the freshly made strand is not ready for use. A protective cap is added at one end and a long run of adenine bases at the other, both shielding it from the enzymes that dismantle RNA. Internal stretches that carry no protein code are then cut out and the coding pieces joined together, a process that can be varied so one gene yields several different proteins. Only then does the finished molecule leave the nucleus. Plenty of transcripts are never translated at all: the RNA of the ribosome itself, transfer RNA, and a large population of regulatory RNAs are all end products in their own right.
Why is transcription the main control point?
Because it is cheaper to stop a copy being made than to make it and discard it. Regulatory proteins bind to stretches of DNA near a gene and either help the enzyme settle or block it, and hormones, growth signals and drugs act largely by shifting the balance between them. Measuring how much RNA a gene has produced is therefore the standard way to judge whether that gene is switched on, and profiles built from those measurements are used to classify tumors and predict how they will behave.
Used in a sentence
The laboratory studied how the mutation affected transcription of the gene in question.
Good to know
Transcription produces an RNA copy of a gene and is a separate step from translation, which uses that RNA to build a protein, a distinction commonly confused when gene expression is first taught.
Word origin
Where “transcription” comes from
From Latin transcribere, meaning to copy in writing, from trans-, meaning across, and scribere, meaning to write.