The molecular biology unit. How DNA replicates itself, how the information in a gene becomes a protein through transcription and translation, how cells switch genes on and off, and what happens when mutations and biotechnology enter the picture.
Eight topics from the College Board CED, in order.
Topic 6.1
DNA and RNA Structure
The double helix, antiparallel strands, base pairing, and the structural differences between DNA and RNA.
Topic 6.2
DNA Replication
Semiconservative replication, helicase, DNA polymerase, leading and lagging strands, and proofreading.
Topic 6.3
Transcription and RNA Processing
RNA polymerase builds mRNA from a DNA template; introns are spliced out and a 5' cap and poly-A tail are added.
Topic 6.4
Translation
Ribosomes read mRNA codons and tRNA delivers amino acids to build a polypeptide. The genetic code and the players involved.
Topic 6.5
Regulation of Gene Expression
The lac and trp operons in prokaryotes; transcription factors, enhancers, and epigenetic regulation in eukaryotes.
Topic 6.6
Gene Expression and Cell Specialization
All cells share the same DNA, but differential gene expression is what makes a neuron different from a liver cell.
Topic 6.7
Mutations
Point mutations (substitution, insertion, deletion), frameshift effects, and how mutations can change phenotype — or not.
Topic 6.8
Biotechnology
Restriction enzymes, gel electrophoresis, PCR, and CRISPR — the tools scientists use to read and edit DNA.
About Unit 6
Unit 6 is the molecular biology unit — the mechanics behind the central dogma of biology: DNA → RNA → protein. You'll learn how DNA replication copies the genome before every cell division, how transcription turns a gene into mRNA, and how translation reads that mRNA to build a protein. From there you'll see how cells control WHICH genes get expressed — through operons in bacteria and transcription factors and epigenetic marks in eukaryotes — and how that differential expression is what makes a skin cell different from a neuron despite sharing identical DNA.
This unit is 12–16% of the AP Bio exam and takes about 18–20 class periods — one of the most heavily weighted and content-dense units in the course. The most-tested topics are the steps of transcription and translation (in order), how the lac operon turns on and off, how a single point mutation can change a protein (or not), and reading data from gel electrophoresis or PCR experiments.
The College Board ties Unit 6 to three of the four Big Ideas:
Big Idea 2
Energetics — transcription and translation require energy input
Big Idea 3
Information — DNA's information is copied, transmitted, and expressed as protein
Big Idea 4
Systems — gene regulation lets cells respond to signals and specialize