sib Short for sibling, a brother or sister.
sibling species Species that are closely related and
are so similar morphologically that it is difficult to dis-
tinguish between them.
sickle cell A genetically inherited disease where
defective hemoglobin, the oxygen-carrying pigment in
red blood cells, causes sickling (distortion) as well as
becoming hard and sticky, resulting in loss of red blood
cells and causing anemia. There are three types in the
United States: hemoglobin SS or sickle-cell anemia,
hemoglobin SC disease, and hemoglobin sickle beta-
thalassemia. While anyone can get sickle-cell anemia,
8.5 percent of the African-American population carry
the trait but do not have the disease. One out of 400
African Americans has sickle cell, and sickle cell affects
eight out of every 100,000 people.
siderophore Generic term for Fe(III)-complexing
compounds released into the cell medium by bacteria
for the purpose of scavenging iron.
See alsoSCAVENGER.
sieve-tube member Living elongated cells arranged
in chains that formsieve tubes in phloem. Transports
sucrose, amino acids, other food materials, and hor-
mones throughout the plant.
signal peptide A sequence of amino acids that deter-
mines the location of a protein in a eukaryotic cell.
signal transductions pathway Signal transduction
refers to the movement of signals from outside the cell
tothe inside and is a mechanism connecting the stimu-
lus to a cellular response.
sign stimulus A stimulus or releaser such as an envi-
ronmental cue that elicits a fixed action response. If the
sign is not present, then the instinct will not occur.
sink habitat A habitat where deaths are greater than
births and where the population would cease to exist
without immigration from more productive habitats.
See alsoSOURCE HABITAT.sinus Sinusmeans “cavity,” and many structures of
the human body are thus called sinuses. However, the
term generally refers to the paranasal sinus. The sinuses
(paranasal sinuses) are air cavities within the facial
bones. They are lined by mucous membranes similar to
those in other parts of the airways. The paranasal
sinuses consist of the ethmoid sinus, frontal sinus, max-
illary sinus, and sphenoid sinus.
An inflammation of the sinuses is called sinusitis.siroheme AHEMElike PROSTHETIC GROUPfound in a
class of ENZYMEs that catalyze the six-electron reduc-
tion of sulfite and nitrite to sulfide and ammonia,
respectively.
See alsoNITRITE REDUCTASE;SULFITE REDUCTASE.sister chromatids Two identical copies of a single
chromosome that are connected by a centromere. After a
dividing cell has duplicated its chromosomes, it does not
separate the copies until it is positive that duplication was
a complete success and that the duplicated copies have
been lined up correctly. While the chromosomes are
duplicated in S phase, the two copies or sister chromatids
arekept together until later, when they are pulled apart
at the metaphase-to-anaphase transition during mitosis.
The sister chromatids are pulled to opposite halves of the
cell by microtubules coming from spindle poles located at
opposite sides of the cell. Sister chromatids segregate
away from each other because their kinetochores attach
to microtubules emanating from opposite poles.sister species Species that have evolved from a com-
mon ancestral species and shared by no other species.
The mushroom Chalciporus piperatoides is a sister
species of Chalciporus piperatus.site-directed mutagenesis SeeMUTAGENESIS.site-directed mutagenesis 307