Expression is just “using” a gene
A gene is a stretch of DNA that holds instructions for a product, usually a protein. But holding the recipe is not the same as cooking the meal. Gene expression is the act of actually reading that recipe and producing the product. When we say a gene is on, we mean the cell is copying it into messenger RNA by transcription and then often building a protein from that message by translation. When a gene is off, the recipe is still in the book, but nobody is cooking from it.
This is the first half of the central dogma: DNA is copied to RNA, and RNA is used to make protein. Expression is the whole flow from gene to working product. The amount of product can be high, low, or zero, and the cell adjusts it constantly.
Why a cell does not run every gene
A human cell carries roughly 20,000 genes, but at any moment it is only using a fraction of them. Running a gene costs energy and raw materials, and making the wrong protein in the wrong place can be harmful. So cells are choosy: they express what they need, when they need it. This selective use is called gene regulation, and it is the central theme of this whole track.
Some genes, though, are needed by almost every cell almost all the time — the ones that build ribosomes, handle basic metabolism, or copy DNA. These are housekeeping genes, and they stay reliably on. Most other genes are regulated: switched up or down depending on the cell type, the time, and the conditions. A liver cell and a muscle cell share the same genome but express very different sets of regulated genes, which is exactly why they end up so different.
- A signal arrives — a hormone, a nutrient, a developmental cue, or stress.
- The cell decides which genes that signal should turn up or down.
- Transcription of those genes changes, so the amount of mRNA changes.
- Protein levels shift, and the cell behaves differently — the gene was “regulated.”