A cell has thousands of genes and only needs a few hundred at any moment. So it needs switches. Most of those switches sit in the stretches of DNA that never get made into protein at all — and the proteins that flick them are called transcription factors.
📚 What you need to know
Coding sequences are the parts of DNA that code for polypeptides. Most DNA is non-coding.
Non-coding DNA still does useful work: it makes functional RNA such as tRNA, and it contains control regions like promoters and enhancers.
A promoter sits right next to the gene it controls. An enhancer does the same job from further away.
Promoters and enhancers are not transcribed themselves.
Transcription factors are proteins that bind to promoters or enhancers and help RNA polymerase attach, increasing the rate of transcription.
This is the most common way a cell controls gene expression: the right genes, in the right cells, at the right time, in the right amount.
Translation is controlled separately, mainly by how long an mRNA molecule survives before nucleases break it down.
Coding and non-coding DNA
Only a small slice of your DNA actually spells out a protein. Those stretches are the coding sequences. For a long time the rest was dismissed as junk, which was a bad guess — a lot of it is doing a job.
Some non-coding DNA is transcribed into RNA that never becomes protein but is useful as it is, such as transfer RNA (tRNA). Other non-coding stretches are never transcribed at all; they exist purely as landing sites that control whether a nearby gene is read. Promoters and enhancers are exactly this kind of sequence.
If a question says “suggest a function of a non-coding region”, you have two safe answers: it makes functional RNA such as tRNA, or it regulates the expression of a nearby gene.
Promoters, enhancers and transcription factors
The promoter is a short non-coding sequence just upstream of a gene. It is the spot RNA polymerase has to attach to before it can start copying. An enhancer does a similar job but sits much further away along the DNA.
Transcription factors are proteins that bind to these regions. Once a transcription factor is in place, RNA polymerase attaches to the promoter far more easily, so the gene is transcribed at a higher rate. Different cell types contain different transcription factors, which is precisely why they express different genes.
The chain of events
transcription factor binds → RNA polymerase attaches to promoter → more mRNA → more protein
Notice how much of the picture is control and how little is actual gene. That ratio is roughly true of real genomes too.
Why this matters so much. Because transcription factors are themselves proteins coded by genes, one gene can control many others. That is how a single signal — a hormone, say — can change the behaviour of a whole cell at once.
Regulating translation
Getting the mRNA made is only half of it. Once an mRNA molecule has been produced, modified and sent out to the ribosomes, the cell still has a say in how much protein comes from it.
The main control is how long the mRNA is allowed to live. When translation has happened, the mRNA stays in the cytoplasm until enzymes called nucleases chop it up. In human cells this can be anything from a few minutes to a couple of days.
Follow the logic through: a short-lived mRNA is translated only a few times, so little protein is made; a long-lived one is translated over and over, so a lot is made. Breaking mRNA down also clears out messages the cell has finished with, so proteins are only made when they are actually needed.
Degrading mRNA is a control step, not damage. It is how a cell stops making a protein it has finished with.
🧩 Answering “how is gene expression controlled?” in order
Name the level. Transcription is the usual answer unless the question points at mRNA or ribosomes.
Name the region. Promoter next to the gene, or enhancer further away.
Name the protein. A transcription factor binds there.
Give the effect. RNA polymerase attaches more easily, so the rate of transcription rises.
Finish on the outcome. More mRNA, so more of that protein is made in that cell.
Worked examples
WORKED EXAMPLE
Explain how a transcription factor can increase the rate of transcription of a gene. [3]
Step 1 — where it binds
The transcription factor is a protein that binds to the promoter or enhancer sequence of the gene.
Step 2 — what that does
This helps RNA polymerase attach to the promoter region.
Step 3 — the result
More mRNA is transcribed from that gene, so more of the protein is produced.
3 marksSay “binds to the promoter”, not “binds to the gene” — they are different sequences.
WORKED EXAMPLE
The mRNA for one protein is broken down after about 20 minutes, while the mRNA for another lasts two days. Suggest what this means for the amounts of the two proteins. [2]
Step 1 — connect lifespan to translation
The longer-lasting mRNA can be translated at ribosomes many more times before nucleases destroy it.
Step 2 — state the outcome
So much more of the second protein is made, while the first is only produced in a short burst.
2 marksA short-lived mRNA is useful when a protein is only needed briefly, e.g. during a fast response.
💡 Exam tip
Promoter = near, enhancer = far. That single contrast is worth a mark on its own.
State clearly that promoters and enhancers are not transcribed. Students often assume everything on the DNA gets copied.
Transcription factors are proteins, so they are themselves the products of gene expression. Saying so shows the examiner you understand the loop.
Link every regulation answer to a rate: more binding means a faster rate of transcription, not just “it happens”.
For translation questions, the key words are nucleases, degradation and cytoplasm.
Enzyme names ending in -ase tell you the substrate. Nucleases digest nucleic acids, so they act on mRNA.
⚠ Common mix-up
Non-coding does not mean useless. It means it does not code for a polypeptide.
Transcription factors do not build the mRNA. RNA polymerase does that; the factor makes it easier to start.
The promoter is not part of the gene’s coding sequence. It sits upstream of it.
Nucleases are not the same as proteases. Nucleases break nucleic acids; proteases break proteins.
mRNA degradation is normal. It is deliberate regulation, not a fault or a mutation.
Do not say the gene “disappears” when it is not expressed. Only the transcription stops.
Up next: Environment & Gene Expression — how tags added on top of the DNA let your surroundings change which genes are read, without altering a single base.
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