Physical Map
A map of the locations of identifiable landmarks on DNA (e.g., restriction-enzyme cutting sites, genes), regardless of inheritance. Distance is measured in base pairs. For the human genome, the lowest-resolution physical map is the banding patterns on the 24 different chromosomes; the highest-resolution map is the complete nucleotide sequence of the chromosomes.

Pinker, Steven
A psychologist and professor with a special interest in language, linguistic behavior, and cognitive science. Pinker’s publication include the popular science books The Language Instinct and How the Mind Works.

Pilus (plural: pili)
A proteinaceous filamentlike projection from the surface of a bacterial cell.

Plate techonics
The theory that the surface of the earth is made of a number of plates, which have moved throughout geological time resulting in the present-day positions of the continents. Plate tectonics explains the locations of mountain building as well as earthquakes and volcanoes. The rigid plates consist of continental and oceanic crust together with the upper mantle, which “float” on the semi-molten layer of the mantle beneath them, and move relative to each other across the earth. Six major plates (Eurasian, American, African, Pacific, Indian, and Antarctic) are recognized, together with a number of smaller ones. The plate margins coincide with zones of seismic and volcanic activity.

Many organisms that are sessile (attached to surface) as adults disperse by means of a planktonic larval stage.

Plasmid
Autonomously replicating extra-chromosomal circular DNA molecules, distinct from the normal bacterial genome and nonessential for cell survival under nonselective conditions. Some plasmids are capable of integrating into the host genome. A number of artificially constructed plasmids are used as cloning vectors. 2. An extrachromosomal circular DNA element that replicates independently of the host chromosome and may contain other genetic information. 3. A genetic element that exists (or can exist) independently of the main DNA in the cell. In bacteria, plasmids can exist as small loops of DNA and be passed between cells independently.