eukaryotic

Meaning: Eukaryotic refers to cells of animals, plants, fungi and protists. They have a nucleus and membrane-bound organelles such as mitochondria, the ER and the Golgi. The MCAT uses the term to contrast their molecular machinery with prokaryotes. Eukaryotic DNA is organized into linear chromosomes wound around histones. Transcription occurs in the nucleus, so the primary transcript is processed before export: a 5′ cap is added, a poly-A tail is added to the 3′ end, and introns are spliced out. Translation then occurs on cytoplasmic or ER-bound 80S ribosomes, made of a 60S and a 40S subunit.

On the MCAT: Passages compare eukaryotic and prokaryotic gene expression. Typical questions ask why a bacterial gene fails to work in yeast, where a processing defect would act, or which organisms an antibiotic aimed at 70S ribosomes would spare. Expect to predict the effect of blocking splicing, capping or nuclear export on protein production.

Common confusion: Svedberg units measure sedimentation rate, not mass, so they are not additive: 60S + 40S gives 80S, not 100S. Also, mitochondria contain 70S ribosomes like bacteria (endosymbiotic theory), so an eukaryotic cell contains more than one ribosome type. Coupled transcription and translation is a bacterial feature, because eukaryotes separate the two steps by the nuclear envelope.