AP Biology · Lesson 6 of 8 · Unit 6

Transcription factors, small RNAs and cell specialization

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Multicellular organisms build hundreds of distinct cell types from a single genome through precise transcriptional and post-transcriptional control. On the AP Biology exam (Topic 6.6), you will analyze diagrams of promoter and enhancer regions, evaluate experimental data from deletion studies or RNA interference (RNAi), and explain how differential gene expression determines cell phenotype.

By the end you’ll be able to

Explain how RNA polymerase and transcription factors binding to promoter or enhancer sequences initiate transcription.
Explain how negative regulatory molecules inhibit gene expression by binding to DNA and blocking transcription.
Explain how certain small RNA molecules regulate gene expression.
Explain how differential gene expression produces differences in cell products, functions and phenotypes.

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What the exam asks

Section I (Multiple Choice): Stimulus sets often show a gene map with numbered upstream/downstream regulatory sequences alongside bar graphs of mRNA or protein levels when specific sequences are deleted or mutated.
Section II (Free Response): Frequently appears in Q1/Q2 (Experimental Results) or Q4 (Conceptual Analysis), asking you to predict the effect of a mutated transcription factor, explain how two cells with identical DNA produce different proteins, or support a claim about a regulatory molecule using data.

Core ideas

Promoters, enhancers, and transcription factors

In eukaryotes, RNA polymerase cannot initiate transcription on its own; it requires regulatory proteins called transcription factors.

Promoter: A DNA sequence located just upstream of the transcription start site where general transcription factors and RNA polymerase assemble.
Enhancers (distal control elements): DNA sequences located far upstream, downstream, or even within an intron of a gene. Activators (specific positive transcription factors) bind to enhancers.
DNA bending: A DNA-bending protein brings the bound activators into contact with mediator proteins and general transcription factors at the promoter, forming the transcription initiation complex and helping RNA polymerase begin transcription.
Negative regulatory molecules (repressors): Inhibit gene expression by binding directly to regulatory DNA sequences (silencers or operators), blocking activator binding, or physically preventing RNA polymerase from attaching to or moving along the DNA.

Regulatory Element / Molecule	Type	Mechanism of Action
Promoter	DNA sequence	Proximal binding site for RNA polymerase and general transcription factors.
Enhancer	DNA sequence	Distal binding site (upstream or downstream) for specific activator proteins.
Activator	Protein (TF)	Binds enhancer; interacts with mediator proteins to recruit/stabilize RNA polymerase.
Repressor	Protein (TF)	Negative regulator; binds DNA to block activator binding or halt RNA polymerase.
Small RNAs (miRNA / siRNA)	Non-coding RNA	Base-pair with complementary mRNA to trigger mRNA degradation or block translation.