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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 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.

  • 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.

  • 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 ...

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