The basics
What it covers: DNA replication; transcription and RNA processing; translation; regulation of gene expression in prokaryotes and eukaryotes; gene expression and cell specialization; mutations; and biotechnology.
Exam weight: 12–16% of the AP Biology exam.
The big question: How does the information stored in DNA get expressed as a protein, and how do cells control which genes get expressed when?
Big Ideas covered: Energetics (BI 2), Information Storage & Transmission (BI 3), Systems Interactions (BI 4).
Key topics at a glance
DNA Replication
Semiconservative: each new helix has one old strand + one new strand. Helicase unwinds; DNA polymerase synthesizes 5'→3'; needs a primer.
Leading vs. Lagging Strand
Leading: continuous synthesis. Lagging: discontinuous, built in Okazaki fragments, joined by DNA ligase.
Transcription
RNA polymerase builds mRNA from a DNA template (3'→5' template read, mRNA built 5'→3'). Occurs in the nucleus in eukaryotes.
RNA Processing
Pre-mRNA gets a 5' cap and poly-A tail; introns are spliced out, leaving only exons. Only then does mature mRNA leave the nucleus.
Translation
Ribosome reads mRNA codons (3 bases = 1 amino acid); tRNA's anticodon delivers the matching amino acid. Starts at AUG, ends at a stop codon.
The Lac Operon
OFF when lactose absent (repressor blocks transcription). ON when lactose present (lactose inactivates repressor, RNA polymerase transcribes the lac genes).
Eukaryotic Gene Regulation
Transcription factors and enhancers control which genes get transcribed; epigenetic marks (DNA methylation, histone modification) can silence genes without changing DNA sequence.
Differential Gene Expression
Every cell has the same DNA. Cell specialization happens because different cells express different subsets of that DNA.
Mutation Types
Silent: same amino acid, no effect. Missense: different amino acid. Nonsense: premature stop. Frameshift: insertion/deletion shifts every codon downstream.
Biotechnology Tools
Restriction enzymes cut DNA at specific sites. Gel electrophoresis separates fragments by size. PCR amplifies DNA. CRISPR edits specific sequences.
The key terms you must know
- Central dogma — DNA → RNA → Protein, the flow of genetic information in a cell.
- Semiconservative replication — each new DNA molecule has one parent strand and one new strand.
- Helicase / DNA polymerase — unwinds the double helix / synthesizes new DNA strands 5'→3'.
- Transcription — copying a gene's DNA into mRNA, carried out by RNA polymerase.
- RNA processing — adding a 5' cap and poly-A tail, and splicing out introns, before mRNA leaves the nucleus.
- Translation — building a polypeptide at the ribosome by reading mRNA codons.
- Codon / anticodon — the 3-base mRNA sequence specifying an amino acid / the complementary 3-base sequence on tRNA.
- Operon — a cluster of prokaryotic genes transcribed together under shared regulatory control (e.g., the lac operon).
- Transcription factor / enhancer — eukaryotic proteins and DNA sequences that control whether and how much a gene is transcribed.
- Epigenetics — heritable changes in gene expression that don't alter the DNA sequence (methylation, histone modification).
- Point mutation — a change to a single DNA nucleotide (substitution, insertion, or deletion).
- Frameshift mutation — an insertion/deletion not a multiple of 3, shifting every downstream codon.
- Restriction enzyme / gel electrophoresis / PCR / CRISPR — the core toolkit of modern biotechnology.
Key themes to remember
- The central dogma is the spine of the unit. Almost every question maps back to DNA → RNA → Protein and where in that pipeline something is happening.
- Direction matters. DNA polymerase only works 5'→3'. RNA polymerase reads the template 3'→5' and builds mRNA 5'→3'. Get the directions backwards and the whole answer falls apart.
- Regulation = efficiency. Cells don't transcribe everything all the time — operons and transcription factors let cells respond to their environment and not waste energy.
- Same DNA, different cells. Differential gene expression — not different genomes — is why a neuron differs from a liver cell.
- Not all mutations matter equally. Where the mutation happens, and whether it shifts the reading frame, determines how big the effect is.
- Biotech tools build on the same biology. PCR uses DNA polymerase's own replication mechanism; CRISPR borrows a bacterial immune defense system.
Common exam traps
- DNA polymerase can't start from scratch. It needs a primer (RNA primer in replication) to begin adding nucleotides — a common point lost on replication FRQs.
- Introns are spliced OUT; exons stay IN. Easy to mix up which one is "expressed" in the final mRNA.
- The lac operon is ON when lactose is present, not when it's absent. Lactose binds the repressor and removes it from the DNA, allowing transcription.
- A silent mutation is NOT the same as "no mutation." The DNA sequence did change — it's just that the resulting amino acid (and protein) didn't.
- Frameshift mutations are usually worse than single substitutions. Because they shift every codon downstream of the mutation, not just one.
- Epigenetic changes do NOT change the DNA sequence. Methylation and histone modification change whether a gene is accessible/transcribed — not the underlying code.
- Restriction enzymes cut at specific recognition sequences — not randomly. That's what makes them useful for predictable cloning and gene splicing.
- PCR amplifies DNA you already have — it doesn't create new sequences. Don't confuse it with gene editing tools like CRISPR.