Encyclopedia of Biology

primordial germ cells Fetal cells that develop in the
early fetus and that will develop into the gametes, the
male sperm or female eggs.

principle of allocation Each organism has a limited,
finite energy budget that is used for living out its life
processes of growth, reproduction, obtaining nutrients,
predator flight, and adjusting to environmental
changes.

prion The smallest infectious particle known
(though its existence is challenged by some scientists),
consisting of hydrophobic protein without any nucle-
ic acid. There are a number of prion-caused diseases.
Scrapie, found in sheep and goats, causes those ani-
mals to lose coordination, and eventually they
become incapacitated and develop an intense itch
that leads them to scrape off their wool or hair
(hence the name “scrapie”). Other diseases are trans-
missible mink encephalopathy, chronic wasting dis-
ease of mule deer and elk, feline spongiform
encephalopathy, and bovine spongiform encephalopa-
thy (mad cow disease). Kuru has been seen among the
Fore highlanders tribesmen of Papua New Guinea,
the result of cannibalism (but has almost disap-
peared). Creutzfeldt–Jakob disease affects one person
in amillion. The other two human prion diseases are
Gerstmann–Straussler–Scheinker disease and fatal
familial insomnia.

probability The statistical measure of an event’s like-
lihood.

procambium Aprimary meristem tissue that differ-
entiates into the vascular bundle and vascular tissues
(xylem and phloem).

prodrug Any compound that undergoes BIOTRANS-
FORMATION before exhibiting its pharmacological
effects. Prodrugs can thus be viewed as DRUGs contain-
ing specialized nontoxic protective groups used in a
transient manner to alter or to eliminate undesirable
properties in the parent molecule.
See alsoDOUBLE PRODRUG.

prognosis Prediction of the future course of a disease.

prokaryote Aunicellular organism characterized by

the absence of a membrane-bound nucleus.

See alsoEUKARYOTES.

promoter The DNA region, usually upstream from

the coding SEQUENCEof a GENEor OPERON, that binds

and directs RNA polymerase to the correct transcrip-

tional start site and thus permits TRANSCRIPTIONat a

specific initiation site. (In catalysis, a promoter is used

differently: a cocatalyst usually present in much smaller

amounts than the catalyst.)

prophage A phage (bacteriophage, a virus that

infects bacteria) chromosome that has been inserted

on a specific part of the DNA chromosome of a

bacterium.

prophage 275

Occurrence

Average diameter

Nuclear material

DNA

Nucleolus

Cell division

Cytoplasmic

streaming

Vacuoles

Plastids

Ribosomes

Endoplasmic

reticulum

Golgi apparatus

Eukaryotic

animals, plants, and

fungi

20μm

bounded by nuclear

membrane

linear and divided into

a number of chromo-

somes

+

usually by mitosis or

meiosis

+

+

+

larger (80S)

+

+

Prokaryotic

bacteria

1μm

not separated

from cytoplasm

by membrane

circular and

forming only one

linkage group

• amitotic


• 
  


• 
  


• 
  

  smaller (70S)

  
  


• 
  


• 
  

Comparison of Prokaryotic and Eukaryotic Cells

+ indicates presence; - indicates absence