Biology · Unit 6: Gene Expression and Regulation · 14 min read · Updated 2026-05-10
AP Biology Transcription and RNA Processing — AP Biology
AP Biology · Unit 6: Gene Expression and Regulation · 14 min read
1. Core Steps of Transcription★★☆☆☆⏱ 4 min
Transcription is the first step of gene expression, where RNA polymerase synthesizes a complementary RNA strand from a DNA template. In addition to messenger RNA (mRNA) that encodes proteins, transcription also produces functional non-coding RNAs like rRNA, tRNA, and siRNA. Transcription occurs in three core steps shared by all organisms, with key differences between prokaryotes and eukaryotes.
In initiation, RNA polymerase binds to a promoter sequence upstream of the target gene, assisted by sigma factors (prokaryotes) or general transcription factors and a TATA box (eukaryotes). Only one DNA strand, the *template strand*, is transcribed; the other strand, the *coding strand*, matches the mRNA sequence except thymine replaces uracil. During elongation, RNA polymerase reads the template strand 3'→5', building the mRNA strand 5'→3' by adding complementary ribonucleotides to the free 3' hydroxyl. Unlike DNA polymerase, RNA polymerase does not require a primer to start synthesis. In termination, transcription ends when RNA polymerase reaches a termination sequence: rho-dependent or rho-independent in prokaryotes, and a polyadenylation signal sequence in eukaryotes.
2. Eukaryotic Pre-mRNA Processing★★★☆☆⏱ 4 min
RNA processing refers to a set of post-transcriptional modifications that only occur in eukaryotic cells, converting the primary pre-mRNA transcript into mature, translation-ready mRNA that can be exported from the nucleus to the cytoplasm for translation.
There are three key modifications to all eukaryotic pre-mRNA: (1) A modified guanine 5' cap added to the 5' end, which protects mRNA from degradation and helps ribosomes bind for translation initiation; (2) A poly-A tail (50-250 adenine nucleotides) added to the 3' end, which also protects against degradation and aids nuclear export; (3) RNA splicing, which removes non-coding intervening sequences called introns and joins coding expressed sequences called exons. Splicing is carried out by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs) that recognize splice sites at intron ends.
3. Alternative Splicing★★★☆☆⏱ 3 min
Alternative splicing is a regulated post-transcriptional process unique to eukaryotes, where different combinations of exons from the same pre-mRNA are assembled into different mature mRNA molecules. This process explains the 'gene number paradox': humans have only ~20,000 protein-coding genes, but produce hundreds of thousands of distinct proteins. Alternative splicing is also a form of gene regulation, allowing different cell types to produce different protein isoforms with distinct functions from the same gene.
4. AP-Style Concept Check★★★★☆⏱ 3 min
Common Pitfalls
Why: Students confuse prokaryotic coupled transcription-translation with eukaryotic compartmentalization, forgetting processing is only for eukaryotic nuclear pre-mRNA.
Why: Many students memorize that mRNA is complementary to DNA, but forget that only the template strand is transcribed, and the coding strand matches mRNA except T→U.
Why: Students remember introns are non-coding, but forget they are completely removed during splicing before the mRNA is mature.
Why: Many historical sources called introns junk, but AP Biology now tests that introns can have regulatory functions and can be processed into non-coding RNAs.
Why: Students confuse transcription with DNA replication, which copies the entire genome.
Why: Students confuse alternative splicing with DNA recombination.