gene expression

Meaning: Information flows DNA → RNA → protein. Transcription makes RNA. In eukaryotes the mRNA is processed (5′ cap, poly-A tail, splicing) before translation at ribosomes. Cells control expression at many steps: chromatin accessibility (histone acetylation generally loosens chromatin and promotes transcription; DNA methylation generally silences), transcription factors, enhancers and silencers, mRNA stability, translation, and protein degradation. In prokaryotes, operons such as lac and trp put several genes under one promoter, with repressors blocking RNA polymerase and activators helping it.

On the MCAT: Passages describe a reporter gene, a knockout, or a mutation and ask whether expression goes up or down. Expect Northern blots, qPCR or RNA-seq (RNA level) and Western blots (protein level). A typical chain: an inducer binds a repressor, the repressor leaves the operator, and transcription increases. A promoter mutation usually changes how much product is made, while a coding mutation changes the protein's sequence.

Common confusion: Having a gene is not the same as expressing it. Nearly all cells of an organism carry the same genome, and cell types differ in which genes are expressed. Also, mRNA level does not always predict protein level, because translation rate and degradation of mRNA and protein also matter.