This unit is ≈14% of the Biology, across 8 lessons. Full syllabus
AP Biology · Lesson 3 of 8 · Unit 6
Transcription and RNA processing
3 quick checks ≈2% of the test7 min read · about 1 h 30 min with practicePreview
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Transcription and RNA processing bridge the hereditary blueprint of DNA and the synthesis of functional proteins. On the AP Biology exam (Topic 6.3), you will frequently analyze diagrams of transcription bubbles, determine mRNA sequences from antiparallel DNA strands, and predict how mutations in splice sites or polyadenylation signals affect mature eukaryotic transcripts in both multiple-choice sets and Free-Response Question 4 (Conceptual Analysis) or Question 5 (Analyze a Model).
By the end you’ll be able to
Describe the functions of mRNA, tRNA and rRNA in the flow of genetic information.
Explain how RNA polymerase uses a single template strand of DNA, read 3′ to 5′, to synthesize RNA in the 5′ to 3′ direction.
Determine the mRNA sequence transcribed from a given DNA template strand.
Describe eukaryotic mRNA processing: addition of a poly-A tail for stability, addition of a GTP cap for ribosomal recognition, and excision of introns.
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Explain how alternative splicing generates different mature mRNA molecules from one gene.
What the exam asks
Sequence and directionality: Given a double-stranded DNA segment with labeled 5′ and 3′ ends, identify the template strand or determine the exact 5′→3′ mRNA transcript.
Model analysis: Label or interpret visual representations of transcription, spliceosomes, or intron/exon arrangements.
Disruption predictions: Predict the consequence on mRNA stability, nuclear export, or polypeptide length if a mutation alters a promoter, a 5′ cap enzyme, a poly-A signal, or an intron-exon boundary.
Prokaryotic vs. eukaryotic comparison: Explain why prokaryotic translation can begin before transcription finishes, whereas eukaryotic pre-mRNA must be processed and exported from the nucleus first.
Core ideas
Functional classes of RNA
Genetic information flows from DNA to RNA to protein. Three main types of RNA are transcribed from DNA to execute this process:
RNA Type
Primary Function in Gene Expression
Messenger RNA (mRNA)
Carries genetic information (codons) copied from a DNA gene to the ribosome to specify the amino acid sequence of a polypeptide.
Transfer RNA (tRNA)
Recruits and delivers specific amino acids to the ribosome; pairs its three-base anticodon with a complementary mRNA codon during translation.
Ribosomal RNA (rRNA)
Combines with proteins to form the structural and catalytic core of ribosomes; catalyzes peptide bond formation between amino acids.
Mechanism of transcription
RNA polymerase synthesizes RNA using a single strand of the DNA double helix as a template. That selected strand is variably called the template strand, noncoding strand, minus strand, or antisense strand.
Initiation: RNA polymerase binds to a region of DNA called the promoter (assisted by transcription factors in eukaryotes) and pries the two DNA strands apart.
Elongation: RNA polymerase moves along the DNA template strand in the 3′→5′ direction. It synthesizes a single-stranded RNA molecule in the 5 direction, covalently adding ribonucleotides to the free group of the growing RNA chain.
Base-pairing rules: Adenine (A) on the DNA template pairs with uracil (U) in RNA (instead of thymine), thymine (T) pairs with adenine (A), cytosine (C) pairs with guanine (G), and guanine (G) pairs with cytosine (C).
Termination: Transcription proceeds until RNA polymerase transcribes a terminator sequence (in prokaryotes) or passes a polyadenylation signal sequence (in eukaryotes), releasing the RNA transcript.