Primer
An already existing short RNA chain bound to template DNA to which DNA nucleotides are
added during DNA synthesis.
Primate
A member of the order of mammals that includes anthropoids and prosimians.
Primitive
Not specialized; at an early stage of evolution or development.
Primitive streak
A dense, opaque band of ectoderm in the bilaminar blastoderm associated with the
morphogenetic movements and proliferation of the mesoderm and notochord; indicates the
first trace of the vertebrate embryo.
Primordium
A cell or organ in its earliest stage of differentiation.
Principle of allocation
The concept that each organism has an energy budget, or a limited amount of total energy
available for all of its maintenance and reproductive needs.
Prion
An infectious form of protein that may increase in number by converting related proteins to
more prions.
Probe
Any identifiable substance that is used to detect, isolate, or identify another substance, as a
labeled strand of DNA that hybridizes with its complementary RNA or a monoclonal
antibody that combines with a specific protein.
Proboscis
A long flexible snout or trunk, as of an elephant.
Procambium
A primary meristem of roots and shoots that forms the vascular tissue.
Proglottids
One of the segments of a tapeworm, containing both male and female reproductive organs
Prokaryotes
Prokaryotes are a group of organisms that lack a cell nucleus, or any other membrane-
bound organelles.
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