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SUMMARY OF KEY POINTS
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The Nucleus SUMMARY OF KEY POINTS Nuclear Envelope
Cytoplasm is separated from nucleoplasm by the nuclear envelope, a double set of membranes with a narrow perinuclear space; the outer membrane binds ribosomes and is continuous with the RER.
The nuclear envelope is penetrated by nuclear pore complexes, large assemblies of nucleoporins with eightfold symmetry through which proteins and protein–RNA complexes move in both directions.
The nuclear envelope is supported internally by a meshwork, the nuclear lamina, composed of intermediate filament subunits called lamins.
Chromatin Chromatin is the combination of DNA and its associated proteins.
Chromatin with DNA that is active in transcription stains lightly and is called euchromatin; inactive chromatin stains more darkly and is called heterochromatin.
The DNA molecule initially wraps around complexes of basic proteins called histones to form nucleosomes, producing a structure resembling beads on a string.
Additional levels of chromatin fiber condensation are less well understood and involve nonhistone proteins, including scaffolds of large protein complexes.
The extra X chromosome in cells of female mammals forms facultative heterochromatin and can be seen as the Barr body.
Nucleolus The nucleolus is a very basophilic or electron-dense area of chromatin localized where rRNA transcription and ribosomal subunits assembly occur.
By TEM, an active nucleolus is seen to have fibrous and granular parts where rRNA forms and ribosomal subunits are assembled, respectively.
The Cell Cycle The cell cycle is the sequence of events that controls cell growth and division.
The G1 phase, the longest part of the cycle, begins immediately after mitosis and includes all preparations for DNA replication.
The period of DNA (and histone) synthesis is the S phase.
In a short G2 phase the cell prepares for division during mitosis (M).
Cell cycling is controlled by the sequential appearance of key cytoplasmic proteins, the cyclins, which bind CDKs.
CDKs phosphorylate and activate the enzymes and transcription factors whose functions characterize each phase of the cell cycle.
Progress through the cell cycle stages is monitored at checkpoints, including the G1 restriction point; only when each phase’s activities are completed are the cyclins changed to trigger those of the next phase.
Mitosis Stages of mitotic cell divisions include prophase, when chromosomes condense, the nuclear envelope disassembles, and the microtubular spindle forms; metaphase, when chromosomes are aligned; anaphase, when they begin to separate toward the two centrosomes; and telophase, when nuclear envelope re-forms around the separated chromosomes.
Telophase ends with cytokinesis or cell cleavage into two daughter cells by a contractile ring of actin filaments and myosin.
Stem Cells & Tissue Renewal Stem cells occur in all tissues with rapid cell turnover; they divide slowly in an asymmetric manner, with one daughter cell remaining a stem cell and one becoming committed toward differentiation.
Cells committed to differentiate (transit amplifying or progenitor cells) typically divide more rapidly than ...