within the nucleus. It is transcriptionally inactive during the interphase
stage of the cell cycle, when the genetic material is normally duplicated.
Heterochromatin is one of two subclassifications of chromatin on a mor-
phologic basis. Euchromatin (B)is actively transcribed chromatin and is
visible only with the use of electron microscopy. Cells with extensive
euchromatin are considered metabolically active.
The nucleolus (E)is the site of ribosomal RNA synthesis.^3 H-uridine
may be localized in the nucleolus by use of autoradiography and is often
used as a marker for RNA synthesis because uridine is preferentially incor-
porated into RNA. RNA is packaged with ribosomal proteins to form ribo-
somes. The nuclear envelope (C)shields the nucleus from the cytoplasm,
which allows the sequestration of the genetic material from mechanical
cytoplasmic forces. The separate nuclear compartment also allows for sep-
aration of the cellular processes of transcription and translation. The
nuclear envelope consists of two concentric unit membranes. The outer
membrane is continuous with the rough endoplasmic reticulum. The inner
nuclear membrane is associated with a lamina of fibrous proteins including
intermediate filament proteins, known as lamins, that regulate the assem-
bly and disassembly of the nuclear membrane during mitosis.
Nuclear pores (D)are interruptions in the nuclear envelope that func-
tion as aqueous channels for the passage of soluble molecules from the
nucleus to the cytoplasm (ribosomal subunits) and from the cytoplasm to
the nucleus (nuclear proteins synthesized in the cytoplasm and transported
to the nucleus). The nuclear envelope is highly selective, with selection
based on pore size, the presence of nuclear import signals, and receptor
recognition of RNAs. Translation of mRNA occurs in the cytoplasm.
67.The answer is d.(Alberts, pp 1005, 1335–1336. Kumar p 100. Ross and
Pawlina, pp 80–81, 91.)Increased transcription of the transcription factor,
E2F leads to loss of cell cycle control. E2F is regulated through phospho-
rylation and dephosphorylation of Retinoblastoma protein (Rb), a key
“negative” regulator of the cell cycle. Cells that enter G 1 have dephospho-
rylated Rb protein that is subsequently phosphorylated, allowing passage
of cells from G 1 to “S.” Dephosphorylated Rb is inhibitory because it
sequesters E2F. Upon Rb phosphorylation, E2F is released and induces the
expression of various genes associated with the initiation of the cell cycle.
The phosphorylation or absence of Rb facilitates E2F binding to DNA
(answer e).Bcl-2 is an antiapoptotic gene (answer c).Accumulation of
Cell Biology: Nucleus Answers 149