AP Biology Unit 6 Notes: Controlling Gene Activity and the Consequences of DNA Change

Gene regulation

Mechanisms cells use to control when, where, and how much a gene is expressed (RNA and often protein produced).

Operon

In prokaryotes, a cluster of genes controlled by one promoter and transcribed together into a single mRNA, allowing coordinated regulation of a pathway.

Promoter

DNA sequence where RNA polymerase binds to begin transcription.

Operator

Regulatory DNA region in an operon that can bind regulatory proteins (e.g., repressors) to control transcription.

Repressor

Regulatory protein that binds DNA (often the operator) and blocks transcription (negative control).

Activator

Regulatory protein that increases transcription by helping RNA polymerase bind and initiate transcription (positive control).

Inducible operon

Operon usually OFF that can be turned ON when an inducer is present (often when the substrate appears).

Inducer

Small molecule that turns on an inducible operon by binding to and inactivating a repressor (prevents the repressor from binding DNA).

lac operon

Inducible operon for lactose metabolism; lactose (allolactose) inactivates the repressor so transcription can occur when lactose is present.

Repressible operon

Operon usually ON that can be turned OFF when a corepressor is present (often when the end product is abundant).

Corepressor

Small molecule (often the pathway end product) that binds to a repressor and activates it, enabling the repressor to bind the operator and shut down transcription.

trp operon

Repressible operon for tryptophan synthesis; when tryptophan is abundant it acts as a corepressor to shut off transcription (negative feedback).

Epigenetic regulation

Heritable changes in gene expression that do not alter DNA base sequence; often involves DNA methylation and histone modifications affecting DNA accessibility.

Chromatin remodeling

Changes to chromatin packing that alter DNA accessibility, helping determine whether genes are available for transcription.

Histone acetylation

Addition of acetyl groups to histones; typically loosens DNA-histone interactions and is associated with increased transcription.

DNA methylation

Addition of methyl groups to DNA (often near promoters); commonly correlates with reduced transcription.

Transcription factor

Protein that binds specific DNA sequences to activate or repress transcription; different cell types have different sets of transcription factors.

Enhancer

Regulatory DNA sequence (can be far from a gene) that increases transcription when bound by activator proteins; DNA looping can bring it near the promoter.

Alternative splicing

Post-transcriptional process in eukaryotes where different combinations of exons are joined, producing different mRNAs (and protein variants) from the same gene.

RNA interference (RNAi)

Gene-silencing mechanism where small RNAs (miRNA/siRNA) bind complementary mRNAs, leading to mRNA degradation or blocked translation (acts post-transcriptionally).

Cell differentiation (cell specialization)

Process by which genetically similar cells become different in structure and function, mainly by expressing different subsets of genes and at different levels.

Induction (cell-to-cell signaling)

When one group of cells influences another’s fate through signaling molecules that alter transcription factor activity and gene expression patterns.

Point mutation

Single-nucleotide substitution; in coding regions it can be silent (same amino acid), missense (different amino acid), or nonsense (premature stop codon).

Frameshift mutation

Insertion or deletion of nucleotides not in multiples of three, shifting the reading frame and usually altering many downstream amino acids (often severe).

Nucleotide excision repair

DNA repair pathway that removes damaged DNA segments (e.g., UV-induced distortions like thymine dimers) and replaces them using the undamaged strand as a template.