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Main page | Structure | Departments | Department of Histology and Embryology | Materials for students | Modul 1 | NUCLEUS. MITOSIS.

NUCLEU S.  MITOSIS.
           Nucleus is composed of :   1) Nuclear envelope  (2 membranes  + nuclear pores), 
                    2) Nucleoplasm ( contains  enzymes:  DNApolymerase and RNApolymerases),    
                    3) CHROMOSOMES (or CHROMATIN),         4) Nucleolus.
 
         CHROMATIN is UNcoiled  chromosomes of interphase nucleus .It is composed of :  
                                    2/3 is DNA,       1/3 are  RNAs and  histone  proteins.
    2 types of chromatin (CH): 1) HETEROCHROMATIN.  It is: a) condensed form of CH,   
                                                                      b) DARK stained,   c) transcriptionally INactive.        
                       2) EUCHROMATIN.   It is: a) decondensed form of CH,     b) PALE stained,
                                                   c) transcriptionally ACTIVE -  along euchromatin fibres
  TRANSCRIPTION happens. Transcription is synthesis o RNA molecules along DNA chain.
  Funct. of chromosomes (and chromatin) is STORAGE of genetic information in the form of GENES.
    Main events happening in nucleus:1) TRANSCRIPTION, including synthesis of  rRNA and mRNA.
                                                           2) DNA DUPLICATION  (during S-phase),
                3) MUTATIONS. It initiate APOPTOSIS   (programmed  self-destruction of cell).
                                Many somatic cells have DIPLOID chromosomal  set  (2n chromosomes). 
                  But  MEGAKARYOCYTES  are POLYploid cells. It have 64n chromosomes.
      ( POLYploidy is result of many times DNA duplication without following mitotic division).
   NUCLEOLUS is composed of 2 parts : 1) pars FIBROSA in the center –it are fibres of rRNA.
                           2) pars GRANULOSA  in periphery – it are forming RIBOSOMAL  SUBUNITS.
      Funct.:   nucleolus is place for production of  RIBOSOMAL  SUBUNITS.
                 Cell cycle includes 2 periods:     1) INTERPHASE,    2)  MITOSIS proper.
                                                        INTERPHASE has 4 phases:
         1) G1-phase: -dauther cells  GROWTH, --transcription of RNAs.
         2) S-phase (synthetic phase): -–DUPLICATION of DNA (and CHROMOSOMES).
         3) G2-phase: -- synthesis of proteins TUBILINS with aim of following mitotic SPINDLE
           formation.         4) Some cells locate in G0-phase.  It  NEVER divide by mitosis.
                                            ( It are : CARDIAC MUSCLE cells   and   NEURONS).
                                                           M I T O S I S   includes 4 phases:
1) PROphase : – CONDENSATION of chromosomes,     --desintegration of nuclear envelope and 
                         nucleolus,       --  centriols form fibres of MITOTIC SPINDLE.
2) METAphase: -- duplicated chromosomes locate on EQUATORIAL PLATE  
                                                                                         ( or metaphase  PLATE)
3) ANAphase: -- sister CHROMATIDS move to opposite cell poles.
4) TELOphase:   --  both  nuclear envelopes  are reconstituted and cell is BInucleated,
                            -- chromosomes uncoil,            -- desintegration of mitotic spindle fibres,
                            -- cytokinesis,  resulting into formation of 2 daughter cells.
 
                                         REGENERATION is realized  by 2 mechanisms:
 Mitotic (and meiotic) divisions lead to increasing  of  cells AMOUNT —it is PROLIFERATION.
 It is main mechanism of CELLULAR (or tissue) REGENERATION   (epithelial cells are actively proliferating).  During restoration cells also are able to increase in SIZES -- it is HYPERTROPHY.  It is main mechanism of  INTRAcellular REGENERATION (cardiac muscle cells in G0- phase are  HYPERTROPHIED, because are NOT able to DIVIDE).
 
                                                        There are 2 types of cellular DEATH:
  1) NECROSIS – is accidental cell death under influence of extreme environmental factors (flame,     
                                hot vapor,  strong acids, X-rays,  TOXINS of virus and bacteria).
   2) APOPTOSIS -  is programmed  self-destruction of cell. Apoptosis is triggered by MUTATIONS in DNA.  Proapoptotic gene P53 become active and cell cleaves itself by enzymes CASPASES.
Составитель – доцент В.В. Бондаренко.
      
 
 
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