116
Cell Biology:
Cytoplasm
Answers
37.The answer is a.(Alberts, pp 923–929.)Anti-vimentin is specific for mes-
enchymal cells such as fibroblasts, macrophages, endothelial cells, and
smooth muscle of the vasculature. In the salivary glands fibrous stromal tissue
is derived from mesenchyme. The acini and ducts (answers c, d, and e)are
derived from epithelium. The parasympathetic ganglia will stain with pan-
neuronal markers such as peripherin (answer b).The type of intermediate fil-
ament protein is relatively specific for cells derived from the three embryonic
germ layers. Antibodies to intermediate filament proteins have been used by
pathologists to determine the origin of tumors. Intermediate filament proteins
have a structural role but also are involved in the anchorage of the proteins
that form ion channels. Cytokeratins (also known as keratins) are specific for
epithelial cells. Neurofilament proteins (NFL, NFM, and NFH) are found in
neurons. In Alzheimer’s disease, extensive plaques of neurofilament proteins
occur. Desmin is found in striated and most smooth muscle, except vascular
smooth muscle. Glial fibrillary acidic protein, GFAP, is specific for astrocytes,
not microglia or oligodendrocytes.
38.The answer is c.(Alberts, pp 343–346. Ross and Pawlina, p 46.)The
large subunit of the ribosome catalyzes peptide bond formation by activa-
tion of peptidyl transferase. The small ribosomal subunit contains the
peptidyl-tRNA-binding (P) site that binds the tRNA molecule (answer a)
attached to the carboxyl end of the growing end of the polypeptide chain.
The small subunit also contains the aminoacyl-tRNA-binding (A) site that
holds the incoming tRNA and amino acid. The initiation factors are loaded
on the small ribosomal subunit that must locate the AUG (start) codon to
initiate protein synthesis. This occurs before binding of the large subunit.
In addition, the initiator tRNA containing methionine provides the amino
acid necessary to start protein synthesis. The initiator tRNA is also located
on the small subunit. It resides at the P site (the normal peptidyl site) even
though it is an aminoacyl-tRNA. This occurs before binding to the mRNA
(answer b).Therefore, the initiation phase of protein synthesis is regulated