RT Book, Section
A1 Mescher, Anthony L.
SR Print(0)
ID 1184199289
T1 The Nucleus
T2 Junqueira's Basic Histology Text and Atlas, 16e
YR 2021
FD 2021
PB McGraw Hill
PP New York, NY
SN 9781260462982
LK accessmedicine.mhmedical.com/content.aspx?aid=1184199289
RD 2024/04/19
AB The  Nucleus  SUMMARY  OF  KEY  POINTSNuclear  EnvelopeCytoplasm  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.ChromatinChromatin  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.NucleolusThe  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  CycleThe  cell  cycle  is  the  sequence  of  events  that  controls  cell  growth  and  division.The  G1phase,  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  G2phase  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  G1restriction  point;  only  when  each  phase’s  activities  are  completed  are  the  cyclins  changed  to  trigger  those  of  the  next  phase.MitosisStages  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  RenewalStem  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  stem  cells  before  slowing  or  stopping  division  to  differentiate.MeiosisMeiosis  is  the  process  by  which  two  successive  cell  divisions  produce  cells  called  gametes  containing  half  the  number  of  chromosomes  found  in  somatic  cells.Prophase  of  the  first  meiotic  division  is  a  unique,  extended  period  in  which  homologous  chromosomes  pair  and  undergo  genetic  recombination  during  the  process  called  synapsis.Synaptic  pairs  separate  toward  two  daughter  cells  at  the  first  ...