siRNA (small interfering RNA) is a short RNA molecule that base-pairs with a complementary target mRNA and triggers its degradation, lowering the protein made from that gene. It's a form of post-transcriptional regulation in eukaryotes (AP Bio Topic 6.5).
siRNA stands for small interfering RNA. It's a short double-stranded RNA that cells (and scientists) use to shut down specific genes. Here's the basic move: the siRNA finds an mRNA with a matching, complementary sequence, binds to it, and flags it for destruction. No intact mRNA means no protein gets made from that gene. This whole process is called RNA interference (RNAi).
The key thing for AP Bio is when this happens. siRNA doesn't stop the gene from being transcribed. The mRNA is already made. siRNA works after transcription by targeting the mRNA itself. That makes it a layer of regulation that sits on top of all the transcription-level controls (regulatory sequences, transcription factors) you study in Topic 6.5. A protein called AGO (Argonaute) does the actual cutting once the siRNA points it at the right mRNA.
siRNA lives in Unit 6: Gene Expression and Regulation, specifically Topic 6.5. It supports learning objective [AP Bio 6.5.A], which asks you to describe the types of interactions that regulate gene expression. Most of 6.5 focuses on controlling transcription (regulatory sequences, transcription factors, epigenetic changes). siRNA is the reminder that regulation also happens after the mRNA exists. Knowing this lets you answer a question that hinges on the difference between blocking transcription and degrading an existing transcript, which is exactly the kind of distinction the CED wants you to make about "the levels at which genes are expressed" (EK 6.5.A.3).
Keep studying AP® Biology Unit 6
Regulatory Sequences and Transcription Factors (Unit 6)
Both control how much protein a cell makes, but at different stages. Regulatory sequences and transcription factors decide whether mRNA gets made in the first place; siRNA destroys mRNA that's already been made. Two valves on the same pipe, one upstream and one downstream.
Epigenetic Changes (Unit 6)
Epigenetic modifications (DNA methylation, histone changes) and siRNA both turn gene expression down without changing the DNA sequence. They're reversible, cell-specific switches, which is why both fall under EK 6.5.A about interactions that regulate expression rather than mutate the gene.
Cell Differentiation (Unit 6)
Different cell types express different genes, and silencing tools like siRNA help shut off the genes a particular cell doesn't need. This connects siRNA to EK 6.5.A.3.i, where tissue-specific protein expression produces observable differentiation.
Inducible System (Unit 6)
An inducible system turns a gene ON when a signal shows up. siRNA is the opposite vibe, a way to turn expression DOWN. Comparing them helps you see that regulation runs in both directions and at multiple steps.
On multiple choice, siRNA shows up as the answer to questions about RNA interference and gene silencing. Expect a stem like "Which molecule silences gene expression through RNA interference?" or a model showing a small RNA binding a complementary mRNA strand and asking which regulatory mechanism it demonstrates. You may also get a data question: scientists add siRNA targeting a gene, protein levels drop to 20% of control with a p-value of 0.005 (α = 0.05), and you decide whether to reject the null hypothesis (yes, reject it, because p is less than α). siRNA has appeared in a released College Board long FRQ (2026 Long FRQ Q2) framing it as eukaryotic cells using siRNA to bind complementary target mRNAs, with the AGO protein involved. The skill they're testing: recognize that this is post-transcriptional regulation and connect lower protein levels to mRNA degradation, not blocked transcription.
mRNA is the messenger that carries the gene's instructions to be translated into protein. siRNA is a short regulatory RNA that destroys a target mRNA. They're opposites in job: one delivers the message, the other deletes it. Mixing them up wrecks any question about where in the process expression is being controlled.
siRNA (small interfering RNA) silences a gene by binding to a complementary mRNA and triggering its degradation.
Because the mRNA already exists, siRNA is post-transcriptional regulation, not transcriptional control.
The process is called RNA interference (RNAi), and the protein AGO helps cut the targeted mRNA.
siRNA fits AP Bio Topic 6.5 and supports learning objective [AP Bio 6.5.A] on interactions that regulate gene expression.
If a question shows protein levels dropping after siRNA treatment, the cause is mRNA breakdown, so less protein gets translated.
siRNA lowers expression without changing the DNA sequence, which puts it alongside epigenetic changes as a reversible off-switch.
siRNA (small interfering RNA) is a short RNA molecule that binds a complementary target mRNA and causes it to be degraded, so less protein is made from that gene. In AP Bio it's an example of post-transcriptional gene regulation in Topic 6.5.
No. siRNA acts after transcription has already happened. It targets the finished mRNA for destruction, which is different from transcription factors or regulatory sequences that control whether the mRNA is made at all.
mRNA carries a gene's coded instructions to the ribosome to be translated into protein. siRNA is a short regulatory RNA that destroys a matching mRNA. One delivers the message, the other silences it.
Yes. It appears in multiple choice questions about RNA interference and gene silencing, and it was used in a released 2026 long FRQ framing siRNA binding complementary mRNAs with the AGO protein. Expect to connect siRNA to lowered protein levels and to gene regulation.
When siRNA binds and triggers degradation of a gene's mRNA, there's less mRNA available for translation, so the cell makes less of that protein. That's why treated cells in a data set often show protein at a small fraction of control levels.
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