Cell Biology: Nucleus
Answers
61.The answer is d.(Junqueira, pp 52, 56–57. Ross and Pawlina, pp 71,
74–75.)The TCOF1 gene encodes treacle. Expression of treacle is critical
during early embryonic development in structures that form bones and
other facial structures. Treacle is active in the nucleolus, the structure
labeled in the transmission electron micrograph. The nucleolus is the site
of ribosomal protein transcription and treacle regulates ribosomal DNA
transcription and therefore ribosomal RNA (rRNA) synthesis. The nucleo-
lus is a highly organized, heterogeneous structure within the nucleus, with
distinct regions visible by electron microscopy: (1) fibrillar centers, which
represent the nucleolar organizer regions where DNA is not being actively
transcribed; (2) dense fibrillar components (pars fibrosa) where RNA mole-
cules are being transcribed; and (3) a granular component (pars granulosa)
where ribosomal subunits undergo maturation. The nucleolar organizer
contains clusters of rRNA genes (DNA). The size and number of nucleoli
differ with the metabolic activity of cells.
Ribosomal synthesis occurs in the nucleolus, but the complete assembly
and maturation of ribosomes requires transport to the cytoplasm (answer a).
Ribosomal proteins as well as all proteins that function in the nucleus are
synthesized in the cytosol and transported into the nucleus (answer b).
Nuclear proteins are also translated on ribosomes in the cytoplasm (answer
c)and targeted to the nucleus (traveling through the nuclear pores) by spe-
cific nuclear localization signals (NLSs). Cytosolic proteins are synthesized
on isolated ribosomes compared with most protein synthesis that occurs on
polyribosomes. Nuclear proteins are transcribed on euchromatin (answer
c).Lysosomes carry out degradation of organelles (answer e).
62.The answer is b.(Alberts, pp 1039–1041, 1046–1051.)The mitotic
cell in the micrograph is undergoing cytokinesis, or the cleavage of the
cytoplasm to form two cells. It occurs after completion of nuclear division
(i.e., nuclear condensation and separation of the chromosomes that are
mediated by microtubules). Cytokinesis requires the action of the contrac-
tile ring composed of actin and myosin. The force for cytokinesis is gener-
ated by the action of actin and myosin, which may be inhibited by
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