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.