01 Prokaryotes, eukaryotes and plasmids
The structural organization of genetic material differs between organisms: DNA lies bare in the cytoplasm of prokaryotes, while in eukaryotes it is organised inside a defined nucleus bounded by a nuclear membrane and condensed by specialised proteins.
Comparison table
- Nuclear membrane — Prokaryotes (e.g. bacteria): the genetic material is not surrounded by a nuclear membrane; it lies free in the cytoplasm and is attached to the plasma membrane at one or more points (the starting point of replication). Eukaryotes (yeast, plants, animals): the genetic material is surrounded by a double nuclear membrane that separates it from the cytoplasm, and it replicates from multiple points along the molecule.
- Shape of the DNA molecule — Prokaryotes: a circular molecule whose two ends are joined, and it is not complexed with protein (naked DNA). Eukaryotes: long linear molecules whose ends are not joined, strongly complexed with histone and non-histone proteins.
- Plasmids — Prokaryotes: small additional circular DNA molecules that replicate as the cell divides, are used as vectors in genetic engineering and carry antibiotic-resistance genes. Eukaryotes: plasmids do not occur at all, except in yeast — the single eukaryotic organism that contains plasmids.
The three-dimensional structure of the double helix
- Two antiparallel sugar–phosphate backbones run 5′→3′ and 3′→5′.
- Complementary base pairing by hydrogen bonds: A=T with two bonds, G≡C with three.
- The helix shows a major groove and a minor groove along its length.