a single fertilized egg that splits in two and creates
two genetic replicas, while fraternal twins develop from
separate eggs and sperm.
tympanic membrane (ear drum) A stretched, thin,
semitransparent membrane attached to the ossicles of
the middle ear and located at the end of the auditory
canal. Sounds in the canal cause the tympanic mem-
brane to vibrate. It is attached to the malleus (hammer)
and its vibrations move the malleus, which in turn
moves the incus (anvil).
type 1,2,3 copper Different classes of copper-
BINDING SITEs in proteins, classified by their spectro-
scopic properties as Cu(II). In type 1, or BLUE COPPER
centers, the copper is coordinated to at least two imi-
dazole nitrogens from histidine and one sulfur from
cysteine. They are characterized by small copper
HYPERFINEcouplings and a strong visible absorption
in the Cu(II) state. In type 2, or non-blue copper
sites, the copper is mainly bound to imidazole nitro-
gens from histidine. Type 3 copper centers comprise
two spin-coupled copper ions bound to imidazole
nitrogens.
See alsoCOORDINATION.
type species Refers to a species that was selected as
the representative of a genus.
type specimen The actual specimen from which a
new species is named. If the original author does not
designate an individual, a single specimen from the
original series can be selected as the lectotype.
typhus An infectious disease transmitted by the
microorganism of genus Rickettisia (e.g., Rickettsia
prowazekii) by lice, mites, ticks, and fleas. Also called
typhus fever. Fatality is as high as 60 percent if not
treated.tyrosinase A copper protein containing an antiferro-
magnetically coupled dinuclear copper unit (TYPE3-like
site) that oxygenates the tyrosine group to catechol and
further oxidizes this to the quinone.
See alsoFERROMAGNETIC;NUCLEARITY.tyrosine kinase(PTKs) Protein enzymes that modu-
late a wide variety of cellular events, including differen-
tiation, growth, metabolism, and apoptosis. Protein
kinases add phosphate groups to proteins. Enzymes
that add phosphate groups to tyrosine residues are
called protein tyrosine kinases. These enzymes have
important roles in signal transduction and regulation of
cell growth, and their activity is regulated by a set of
molecules called protein tyrosine phosphatases that
remove the phosphate from the tyrosine residues. A
tyrosine kinase is an enzyme that specifically phospho-
rylates (attach phosphate groups to) tyrosine residues
in proteins and are critical in T- and B-cell activation.tyrosine kinase receptor Proteins found in the plas-
ma membrane of the cell that can phosphorylate
(attach phosphate groups to) on a tyrosine residue in a
protein. Insulin is an example of a hormone whose
receptor is a tyrosine kinase. Following binding of the
hormone, the receptor undergoes a conformational
change, phosphorylates itself, then phosphorylates a
variety of intracellular targets.332 tympanic membrane