64.The answer is a.(Alberts, pp 997–1000.)When a cell in any phase of
the cell cycle is fused with a mitotic cell, the mitotic cell dominates. The
reason for this is the dominance of M-Cdk complex (formerly called MPF).
The G 1 -phase cell is quickly pulled or driven into mitosis as the chromo-
somes condense. The M-Cdk complex will cause the phosphorylation of
the lamins. The lamins are a subclass of intermediate filaments including
three nuclear proteins: lamins A, B, and C. Phosphorylation of intermedi-
ate filaments leads to disassembly, as occurs with the lamins. The disas-
sembly of lamins results in the dissolution of the nuclear envelope in
prometaphase of the cell cycle. The S-phase activator cannot be expressed
in the M-phase cell, and it also cannot reduplicate its DNA because of the
re-replication block (answers b, c, and d). A re-replication block cannot
occur in the G 1 cell because it has not gone through S (answer e).The
lamins differ from other intermediate filament proteins in the presence of a
nuclear import signal. The lamins form the core of the nuclear lamina,
interact with nuclear envelope proteins, and play a role in the maintenance
of the shape of the nucleus. Dephosphorylation of the lamins is associated
with the reassembly of the nuclear envelope in telophase.
65.The answer is d.(Alberts, pp 927–928.)At a mitosis-inhibiting dose,
colchicine functions by binding specifically and irreversibly to tubulin. The
colchicine-tubulin complex is added at the positive end of the kinetochore,
but it inhibits further addition of tubulin (answer c).The result is a bio-
chemical capping of the tubulin at the growth end, preventing further
tubulin addition. Cells are blocked in metaphase and cannot escape
because microtubule motors are unable to function in generating the forces
required for anaphase. At higher doses of colchicine, cytosolic micro-
tubules depolymerize. Actin and myosin are involved in cytokinesis [the
division of cytoplasm (answers a and b)], whereas tubulin and the micro-
tubules regulate separation of the daughter nuclei and their contents.
Taxol, like colchicine, inhibits mitosis, but it uses a different mechanism.
Taxol binds and stabilizes microtubules (answer e),causing a disruption
of microtubule dynamics and inhibition of mitosis. Taxol and colchicine
are similar in binding only to α,β-tubulin dimers and microtubules.
66.The answer is b.(Junqueira, pp 51–52, 56–57. Alberts, pp 222–229.)Het-
erochromatin (A)is visible with the light microscope as condensed basophilic
clumps and with the electron microscope as compact, electron-densematerial
148 Anatomy, Histology, and Cell Biology