1111 SDA
during such operations as throwing, winding, weav-
screw. (auger; worm). A simple machine em-
ing, knitting, etc. ploying the principle of the inclined plane, invented
by the Greek scientist Archimedes. It consists of a
central shaft around which winds a spiral of ribs
SCP. Abbreviation for single-cell protein.
(called “flights”) that are integral with the shaft. The
See protein; single-cell.
distance between the flights is the pitch, or angle of
inclination of the screw. This distance may be uni-
screen. A woven, fabriclike structure made of
form throughout the length of the screw or it may
intersecting strands of wire or plastic, usually
vary from one point to another, depending on use
mounted in a steel frame. They are available in a
requirements. The shorter the distance between
wide range of sizes, weaves, and meshes from as
flights, the more pressure the screw will deliver.
coarse as 25 to as fine as 400. The mesh is the
Screws have a number of important applications in
number of apertures per square inch; it is the square
industrial operations. (1) Extrusion of rubber, plas-
of the number of strands of metal or plastic per linear
tics, and food products: the screw rotates in a cham-
inch. The strands can be made of any suitable metal
ber or barrel, the product being introduced through a
(copper, nickel), alloy (steel, bronze), or synthetic
port where the flights are farthest apart; it is forced
(nylon, PVC). Some types of screen are mechanical-
through a die at the opposite end of the barrel, which
ly vibrated or gyrated to facilitate solids separation;
molds it to the form desired. (2) Mixing of solids:
fine mesh screens require application of force, such
continuous mixing is possible with screws having a
as a stream of water, to effect separation. Screens are
wide variety of pitches and contours that impart a
used for filtration, clarification of suspensions, se-
back-and-forth motion to the product being mixed
paration and classification of solids, and removal of
without moving it to the discharge end until it is
contaminants from semisolid materials. Lab sizes
pushed along by added material. Two screws may
are available. A special application is the wire of a
operate in parallel, their pitches opposing each other
fourdrinier papermaking machine; it may be 38–60
in such a way as to effect maximum mixing. Some
inches. wide and 55–85 mesh; it moves continuous-
screws are cored for circulation of cooling water. (3)
ly over return rolls, the sheet being formed upon it by
Conveying of solids: for this purpose screws with
filtration of wood pulp slurry.
uniform and fairly wide pitch are used; as the screw
See filter media.
turns the solids are passed along from one flight to
the next. These are used for conveying wet and dry
screening. To screen a library (see “Library”) is
solids, wood chips, and similar particulates. (4) An
to select and isolate individual clones out of the
engineering application of the screw is the mecha-
mixture of clones. For example, if you needed a
nism known as a worm gear, in which the flights of
cDNA clone of the pituitary glycoprotein hormone
the screw engage corresponding indentations or
alpha subunit, you would need to make (or buy) a
notches in a shaft or wheel, causing it to turn.
pituitary cDNA library, then screen that library in
order to detect and isolate those few bacteria carry-
scrubber. See absorption tower; scrubbing.
ing alpha subunit cDNA.
(1) Screening by hybridization involves spreading
scrubbing. Process for removing one or more
the mixture of bacteria out on a dozen or so agar
components from a mixture of gases and vapors by
plates to grow several ten thousand isolated colo-
passing it upward and usually countercurrent to and
nies. Membranes are laid onto each plate, and some
in intimate contact with a stream of descending
of the bacteria from each colony stick, producing
liquid, the latter being chosen so as to dissolve the
replicas of each colony in their original growth posi-
desired components and not others. The gas or vapor
tion. The membranes are lifted and the adherent
may be broken into fine bubbles upon entering a
bacteria are lysed, then hybridized to a radioactive
tower filled with liquid, but more frequently the
piece of alpha DNA (the source of which is a story in
tower is filled with coke, broken stone, or other
itself—see “Probe”). When X-ray film is laid on the
packing, over which the liquid flows while exposing
filter, only colonies carrying alpha sequences will
a relatively large surface to the rising gas or vapor.
“light up”. Their position on the membranes show
See absorption (1).
where they grew on the original plates, so you now
can go back to the original plate (where the remnants
scruple. Unit of weight used in pharmacy equiva-
of the colonies are still alive), pick the colony off the
lent to 20 grains or 1/3 dram.
plate and grow it up. You now have an unlimited
source of alpha cDNA.
SDA. Abbreviation for specially denatured al-
(2) Screening by antibody is an option if the bacteria
cohol.
and plasmid are designed to express proteins from
the cDNA inserts (see “Expression clones”). The
SDDC. Abbreviation for sodium dimethyldithio-
principle is similar to hybridization, in that you lift
carbamate.
replica filters from bacterial plates, but then you use
the antibody (perhaps generated after olde tyme
protein purification rituals) to show which colony
SDA. Abbreviation for the Soap and Detergent
expresses the desired protein. Association.