How is the information in a gene copied into messenger RNA?
Topic 6.3 Transcription and RNA Processing: explain how RNA polymerase transcribes a gene into mRNA and how the primary transcript is processed in eukaryotes.
A focused answer to AP Biology Topic 6.3, covering RNA polymerase, the template strand, the differences between transcription and replication, and eukaryotic RNA processing (cap, tail, splicing), with a worked transcription example.
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What this topic is asking
The College Board (Topic 6.3) wants you to explain how RNA polymerase transcribes a gene into mRNA, using the template strand, and how the eukaryotic primary transcript is processed (capping, tailing and splicing) before translation.
Transcription
Transcription differs from replication: it copies only one gene (not the whole genome), uses RNA polymerase (which needs no primer), makes a single-stranded RNA product, and uses uracil in place of thymine. The product is also temporary: mRNA is made when a gene's product is needed and then degraded, which is one way the cell controls how much protein is made.
Transcription also stops at the right place: RNA polymerase reads to a terminator sequence, then releases the finished transcript and detaches from the DNA. Because many RNA polymerases can transcribe the same gene at once, a single gene can produce many mRNA copies quickly when a lot of its protein is needed.
RNA processing in eukaryotes
Alternative splicing
Alternative splicing helps explain how organisms with relatively few genes can make a much larger number of distinct proteins, and it is also a point of regulation: which version is made can be controlled in different cell types or conditions. This makes splicing one more layer at which gene expression is regulated, alongside the controls covered in the regulation topic.
Try this
Q1. State which DNA strand RNA polymerase uses to build mRNA. [1 point]
- Cue. The template (antisense) strand.
Q2. Explain how one gene can produce several different proteins. [2 points]
- Cue. Alternative splicing joins different combinations of exons, making several mRNAs from one pre-mRNA, each translated into a different protein.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (long FRQ excerpt). (a) Describe the role of RNA polymerase in transcription. (b) Eukaryotic pre-mRNA is processed before leaving the nucleus. Describe two processing steps and explain how alternative splicing allows one gene to encode more than one protein.Show worked answer →
A 4-point describe-and-explain FRQ on transcription and processing.
(a) Describe (1 point): RNA polymerase binds the promoter and synthesizes an mRNA strand complementary to the template DNA strand, adding RNA nucleotides to (with uracil pairing to adenine).
(b) Describe and explain (3 points): (1 point) a cap and a poly-A tail are added, protecting the mRNA and aiding export and translation; (1 point) introns are removed and exons joined by splicing; (1 point) alternative splicing joins different combinations of exons, so one gene can produce several different mRNAs and therefore several proteins.
Markers reward the role of RNA polymerase, naming two processing steps, and explaining alternative splicing as the source of protein diversity.
AP 2018 (style)1 marksSection I (multiple choice). During transcription, RNA polymerase builds an mRNA molecule that is complementary to: (A) the coding strand. (B) the template strand of DNA. (C) the finished protein. (D) a tRNA molecule.Show worked answer →
The correct answer is (B).
RNA polymerase reads the template (antisense) strand and builds a complementary mRNA. The mRNA therefore has the same sequence as the coding strand (except U replaces T). It is not complementary to the protein (C) or to tRNA (D).
Related dot points
- Topic 6.4 Translation: explain how the ribosome translates mRNA codons into a polypeptide, including the roles of tRNA and the genetic code.
A focused answer to AP Biology Topic 6.4, covering codons, the genetic code, the roles of mRNA, tRNA and ribosomes, the stages of translation, and using a codon table, with a worked translation problem.
- Topic 6.1 DNA and RNA Structure: describe the structure of DNA and RNA and explain how it suits their role in storing and transmitting genetic information.
A focused answer to AP Biology Topic 6.1, covering the double helix, antiparallel strands, complementary base pairing, the sugar-phosphate backbone, and the differences between DNA and RNA, with a worked base-pairing calculation.
- Topic 6.2 Replication: explain how DNA is replicated semiconservatively, including the roles of the key enzymes and the leading and lagging strands.
A focused answer to AP Biology Topic 6.2, covering semiconservative replication, helicase, DNA polymerase, the leading and lagging strands, Okazaki fragments and ligase, with a worked replication problem.
- Topic 6.5 Regulation of Gene Expression: explain how gene expression is regulated in prokaryotes and eukaryotes, including operons and regulatory sequences.
A focused answer to AP Biology Topic 6.5, covering the lac and trp operons, promoters, regulatory sequences, transcription factors and epigenetic control, and how regulation lets cells respond to the environment, with a worked operon example.
- Topic 6.7 Mutations: explain the types of mutations and how they affect gene products, phenotype and the variation available to a population.
A focused answer to AP Biology Topic 6.7, covering point mutations (silent, missense, nonsense), frameshift mutations, chromosomal mutations, their effects on proteins and phenotype, and their role as the source of new variation, with a worked example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)