prokaryotic
Summary
Prokaryotic describes cells, bacteria and archaea, that lack a membrane-bound nucleus and other membrane-bound organelles. The most testable point is that transcription and translation occur together in the cytoplasm, often on polycistronic mRNA read by 70S ribosomes.
Detail
Meaning: Prokaryotic cells have no nuclear envelope or membrane-bound organelles. The DNA lies in the nucleoid, usually as one circular chromosome, sometimes with small plasmids. Without a nucleus, ribosomes can begin translating an mRNA while RNA polymerase is still transcribing it. Prokaryotic mRNA is often polycistronic, meaning one transcript encodes several proteins; it comes from an operon, a group of genes under one promoter, such as the lac operon. It typically has no 5′ cap or poly-A tail, and splicing is generally absent. Ribosomes are 70S (30S + 50S subunits) and find the start codon using the Shine-Dalgarno sequence.
On the MCAT: Passages compare prokaryotic and eukaryotic processes, such as operon regulation, replication from a single origin, ribosome size, or where translation happens. A typical question asks you to predict how a mutation in a repressor, operator or promoter changes expression, or which step differs in a eukaryotic cell. Binary fission versus mitosis also appears.
Common confusion: Students equate prokaryotic with bacteria only. Archaea are also prokaryotic, and the defining feature is the lack of a membrane-bound nucleus. Another trap: 30S + 50S = 70S, not 80S, because S measures sedimentation rate, not mass.
Sources
- Lehninger Principles of Biochemistry
- Molecular Biology of the Cell
- Campbell Biology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.