AP Biology Unit 6 Essentials

DNA stores genetic information, but it doesn't do anything on its own — it has to be expressed. Transcription copies a gene's DNA sequence into mRNA, and translation reads that mRNA at the ribosome to build a specific protein, one amino acid at a time. This DNA → RNA → Protein pipeline (the central dogma) is how the static information in a genome becomes the dynamic structures and enzymes that actually run a cell. Every topic in this unit — replication, transcription, translation, regulation, mutation — is really a question about some part of this pipeline.

Transcribing and translating a gene costs energy and resources, so cells don't express every gene all the time. In prokaryotes, operons like the lac operon switch whole sets of genes on or off based on environmental conditions (is lactose available?). In eukaryotes, transcription factors, enhancers, and epigenetic marks like DNA methylation control which genes in a cell's genome actually get used. This regulation is also why every cell in your body can have identical DNA and yet a neuron, a muscle cell, and a skin cell look and behave completely differently — differential gene expression creates cell specialization.

A mutation is simply a change to the DNA sequence — a substitution, insertion, or deletion — that can ripple forward through transcription and translation to change (or not change) a protein's function. Modern biotechnology exploits the very same chemistry: restriction enzymes cut DNA at known sequences, PCR uses the cell's own replication machinery to amplify a target sequence, and CRISPR borrows a bacterial immune system to cut DNA at a chosen location. Understanding how DNA is read and copied naturally is exactly what lets scientists read and edit it deliberately.