9. Cell reproduction. Cell cycle. Mitosis. Flashcards
Cell cycle
-Controlled by set of genes called Cell Cycle Genes.
-Control is carried out by specific proteins (cyclins, cyclin-dependent kinases, growth factors, etc.)
-Duration - Varies by different cell types ( for normal human cells - about 24 hours).
A life cycle of animal cells is related to growth, development, and reproduction, according to the program of instuctions encoded in DNA and inherited from the parent cells.
A life cycle in bacteria consists of continuous alternation of growth and division
The cell life cycle consists of:
Mitosis
Interphase:
a) G1 (gap phase 1) - presynthetic (between end of mitosis and initiation of DNA synthesis)
-Lasts 8-10 hours
-Enzymes for synthesis of nucleotides are formed
-Formation of deoxyribonucleotides (monomers of DNA) begins
-Diploid (2n) nuclei
-Centrioles are doubled
-Cytoplasm begins to grow
b)S - synthetic
-Lasts 6-8 hours
-DNA and histones are synthesized
-DNA is duplicated
-Tetraploid (4n) nuclei
Cell Cycle. Mitosis
Mitotic phase (M) of the cell:
-division of the nucleus (karyokinesis)
-division of the cytoplasm (cytoplasm)
Duration - 30-60 minutes
mitosis is triggered by MPF (M-phase promoting factor)
5 Stages:
1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase
Cytokinesis completes the mitotic phase
Prophase
Early:
-Cell rounding
-Physiochemical changes in cytoplasm (Fragmentation of golgi and er, depolymerization of intermediate filaments, endo and Exocytosis processes stop)
-Each prophase chromosome contains pair of identical chromatids joined at centromeres and along arms by cohesin
-Centrosome divides and mitotic spindle begins to form (plants lack centrosomes and forms mitotic spindle w/out them)
Late:
-Nucleoli disappear
-Between late prophase and prometaphase, fragmentation of nuclear envelope begins
Prometaphase
-Nuclear envelope is fragmented
-Chromosomes attach with their kinetochores to the tubules of mitotic spindle
Types of microtubules:
-Aster - start from centrosomes and radiate in all directions
-Nonkinetochore - from centrosomes to equator, determines the poles of dividing spindle
-Kinetochore - from centrosomes to kinetochores (centromeres of chromosomes)
Metaphase
-Chromosomes have all arrived at equatorial part of mitotic spindle (metaphase plate)
-Chromosomes centromeres lie at metaphase plate
-Kinetochore of each chromatid attaches to a kinetochore microtubule
-Chromosomes are maximally spiralised and can be seen very well under light microscope
Anaphase
Early:
-Cohesin proteins are cleaved and two chromatids of each pair separate. Each chromatid becomes independant chromosome
Late:
-Two new daughter chromosomes begin moving toward opposite ends of cell as their kinetochore microtubules shorten
Telophase
Early:
-Chromosomes are at both poles and less condensed
-Nucleoli reappear
-Two daughter nuclei form in cell
-Nuclear envelopes arise from fragments of parent cells nuclear envelope and other portions of endomembrane system
-Begins formation of contractile ring of actin and myosin II in equatorial region of cell
-Nuclear pores are formed. Nuclear lamins enter nuclear thru here
Late:
-Nucleus gains interphase appearance.
Cytokinesis
-Formation of cleavage furrow, pinches cell in two
-Microtubules of mitotic spindle disappear
-Two daughter cells are formed
Atypical forms of Mitosis
Endomitosis:
After DNA replication, nucleus does not divide, resulting in polyploidy. Occurs due to damage to mitotic spindle as a result of chemical exposures, radiation, etc.
Amitosis (direct division):
-Rapid but incomplete cell division
-Nucleus elongated and contracts into 2 parts, may not be identical after cytokinesis
-Genetic material not equally divided into daughter cells
-Is default for protozoa
Amitosis in humans:
Reparative amitosis:
-regen of dmged cells
Degenerative amitosis:
-Often precedes apoptosis (programmed cell death)
-In differentating cells that have lost their ability to divide by mitosis
-In pathological conditions
