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increases the fidelity of gene expression by degrading aberrant mRNAs. RNA
surveillance systems also assess whether the transcripts possess the proper reg-
ulatory elements. Thus, mRNA surveillance is important in determining which
mRNA molecules become available for translation and, hence, which proteins
are produced within the cell. tRNAs also are monitored and destroyed if defec-
tive, as are defective rRNAs (
Doma and Parker 2007
).
2.10 Import and Export from the Nucleus
The presence of a nucleus distinguishes eukaryotes from prokaryotes. The
nucleus contains the nuclear genetic information and is the cell's control center
where DNA replication, transcription, and RNA processing take place. The final
stage of gene expression, translation, occurs in the cytoplasm.
The nucleus is surrounded by a nuclear envelope consisting of two nuclear
membranes constructed of phospholipid bilayers, an underlying lamina (a net-
work of lamin filaments that provide structural support), and nuclear pore
complexes. The nuclear lamina serves as a site to which the chromosomes
attach. When cells are not undergoing mitosis or meiosis, the chromosomes are
“unwound' and resemble a “plate of spaghetti.” The location of these chromo-
somes is not random; chromosomal location is important in determining when
and how well genes function (
Pennisi 2011
). The nucleolus, which consists of pro-
teins and RNAs where the cell's ribosomes are made before export to the cyto-
plasm, has a designated location, as do other nuclear structures (
Baker 2011
). It
seems that gene location varies within the nucleus depending upon the develop-
mental stage or tissue type. Active genes are usually near the edges of chromo-
some territories, whereas silenced genes are deep inside these territories. Gene
location within the nucleus plays a role in fully activating a gene and making tran-
scription and RNA processing efficient (
Pennisi 2011
).
The inner and outer membranes of the nuclear membrane are joined at the
nuclear pore complexes, the only channels through which small polar mol-
ecules and macromolecules are able to travel through the nuclear envelope.
The nuclear membranes prevent the free movement of molecules between the
nucleus and the cytoplasm, thus maintaining the nucleus as a distinct biochemi-
cal compartment. The outer nuclear membrane is similar in structure to the
membranes of the endoplasmic reticulum, to which it attaches.
The
nuclear pore complex
has a diameter of
≈
120nm. The pore functions as
a semipermeable filter that allows diffusion of ions, small molecules, and larger
molecules
<
60 kDa (
Grunwald et al. 2011
). Larger molecules that have a nuclear
localization signal enter the nucleus by transport factors, but the method by
