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La replicazione cellulare
(parte II)
La fase M • La fase M è dominata da riarrangiamenti del
citoscheletro che servono a segregare i cromosomi (MITOSI) e a dividere la cellula (CITOCHINESI)
• Gli eventi caratteristici della fase M sono – Condensazione cromosomiale
• Precede la mitosi e la citochinesi – Formazione del fuso mitotico
• Composto da microtubuli e da proteine associate • Allinea i cromosomi in un piano lungo la bisettrice della
cellula – Formazione dell’anello contrattile
• Formato da actina e miosina • Si forma su di un piano perpendicolare al fuso mitotico, subito sotto la membrana plasmatica, e serve a dividere la cellula
Il centrosoma
• La divisione cellulare dipende dalla duplicazione del centrosoma
• Il centrosoma è una struttura composta da – una matrice, scarsamente definita – 2 centrioli
• Il centrosoma funge durante l’interfase da centro organizzatore dei microtubuli
626 PANEL 18–1 The principal stages of M phase in an animal cell
CELL DIVISION AND THE CELL CYCLE INTERPHASE
INTERPHASE
M PHASE
S
CELL CYCLE
PROPHASE 1 CYTOKINESIS 6
PROMETAPHASE 2 TELOPHASE 5
METAPHASE 3 ANAPHASE 4
The division of a cell into two daughters occurs in the M phase of the cell cycle. M phase consists of nuclear division, or mitosis, and cytoplasmic division, or cytokinesis. In this figure, M phase has been expanded for clarity. Mitosis is itself divided into five stages, and these, together with cytokinesis, are described in this panel.
microtubules
cytosol
plasma membrane
duplicated centrosome
nuclear envelope
decondensed chromosomes in nucleus
During interphase, the cell increases in size. The DNA of the chromosomes is replicated, and the centrosome is duplicated.
In the light micrographs of dividing animal cells shown in this panel, chromosomes are stained orange and microtubules are green. (Micrographs courtesy of Julie Canman and Ted Salmon; “Metaphase” from cover of J. Cell. Sci. 115(9), 2002, with permission from The Company of Biologists; “Telophase” from J.C. Canman et al., Nature 424:1074–1078, 2003, with permission from Macmillan Publishers Ltd.)
PROPHASE 1
time = 0 min
centrosome
forming mitotic spindle
condensing chromosome with two sister chromatids held together along their length
intact nuclear envelope
kinetochore
At prophase, the replicated chromosomes, each consisting of two closely associated sister chromatids, condense. Outside the nucleus, the mitotic spindle assembles between the two centrosomes, which have begun to move apart. For simplicity, only three chromosomes are drawn.
PROMETAPHASE 2
time = 79 min
Prometaphase starts abruptly with the breakdown of the nuclear envelope. Chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement.
spindle pole
kinetochore microtubule
chromosome in motion
fragments of nuclear envelpoe
G 1 G 2
626 PANEL 18–1 The principal stages of M phase in an animal cell
CELL DIVISION AND THE CELL CYCLE INTERPHASE
INTERPHASE
M PHASE
S
CELL CYCLE
PROPHASE 1 CYTOKINESIS 6
PROMETAPHASE 2 TELOPHASE 5
METAPHASE 3 ANAPHASE 4
The division of a cell into two daughters occurs in the M phase of the cell cycle. M phase consists of nuclear division, or mitosis, and cytoplasmic division, or cytokinesis. In this figure, M phase has been expanded for clarity. Mitosis is itself divided into five stages, and these, together with cytokinesis, are described in this panel.
microtubules
cytosol
plasma membrane
duplicated centrosome
nuclear envelope
decondensed chromosomes in nucleus
During interphase, the cell increases in size. The DNA of the chromosomes is replicated, and the centrosome is duplicated.
In the light micrographs of dividing animal cells shown in this panel, chromosomes are stained orange and microtubules are green. (Micrographs courtesy of Julie Canman and Ted Salmon; “Metaphase” from cover of J. Cell. Sci. 115(9), 2002, with permission from The Company of Biologists; “Telophase” from J.C. Canman et al., Nature 424:1074–1078, 2003, with permission from Macmillan Publishers Ltd.)
PROPHASE 1
time = 0 min
centrosome
forming mitotic spindle
condensing chromosome with two sister chromatids held together along their length
intact nuclear envelope
kinetochore
At prophase, the replicated chromosomes, each consisting of two closely associated sister chromatids, condense. Outside the nucleus, the mitotic spindle assembles between the two centrosomes, which have begun to move apart. For simplicity, only three chromosomes are drawn.
PROMETAPHASE 2
time = 79 min
Prometaphase starts abruptly with the breakdown of the nuclear envelope. Chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement.
spindle pole
kinetochore microtubule
chromosome in motion
fragments of nuclear envelpoe
G 1 G 2
627
METAPHASE 3
time = 250 min
At metaphase, the chromosomes are aligned at the equator of the spindle, midway between the spindle poles. The paired kinetochore microtubules on each chromosome attach to opposite poles of the spindle.
ANAPHASE 4
time = 279 min
At anaphase, the sister chromatids synchronously separate, and each is pulled slowly toward the spindle pole it is attached to. The kinetochore microtubules get shorter, and the spindle poles also move apart, both contributing to chromosome segregation.
TELOPHASE 5
time = 315 min
During telophase, the two sets of chromosomes arrive at the poles of the spindle. A new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis. The division of the cytoplasm begins with the assembly of the contractile ring.
CYTOKINESIS 6
time = 362 min
During cytokinesis of an animal cell, the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches in the cell to create two daughters, each with one nucleus.
spindle pole
kinetochore microtubule
spindle pole
kinetochores of all chromosomes aligned in a plane midway between two spindle poles
shortening kinetochore microtubule
spindle pole moving outward
set of chromosomesat spindle pole
contractile ring starting to form
spindle pole interpolar microtubules
nuclear envelope reassembling around individual chromosomes
chromosomes
completed nuclear envelope surrounds decondensing chromosomes
contractile ring creating cleavage furrow
re-formation of interphase array of microtubules nucleated by the centrosome
astral microtubule
627
METAPHASE 3
time = 250 min
At metaphase, the chromosomes are aligned at the equator of the spindle, midway between the spindle poles. The paired kinetochore microtubules on each chromosome attach to opposite poles of the spindle.
ANAPHASE 4
time = 279 min
At anaphase, the sister chromatids synchronously separate, and each is pulled slowly toward the spindle pole it is attached to. The kinetochore microtubules get shorter, and the spindle poles also move apart, both contributing to chromosome segregation.
TELOPHASE 5
time = 315 min
During telophase, the two sets of chromosomes arrive at the poles of the spindle. A new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis. The division of the cytoplasm begins with the assembly of the contractile ring.
CYTOKINESIS 6
time = 362 min
During cytokinesis of an animal cell, the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches in the cell to create two daughters, each with one nucleus.
spindle pole
kinetochore microtubule
spindle pole
kinetochores of all chromosomes aligned in a plane midway between two spindle poles
shortening kinetochore microtubule
spindle pole moving outward
set of chromosomesat spindle pole
contractile ring starting to form
spindle pole interpolar microtubules
nuclear envelope reassembling around individual chromosomes
chromosomes
completed nuclear envelope surrounds decondensing chromosomes
contractile ring creating cleavage furrow
re-formation of interphase array of microtubules nucleated by the centrosome
astral microtubule
Il FUSO MITOTICO E la divisione cellulare
La Meiosi
• La meiosi (“diminuzione” in greco) è un particolare processo di divisione cellulare che produce gameti, le cellule germinali protagoniste della riproduzione sessuale, aploidi (con corredo cromosomico singolo)
• La riproduzione sessuale è vantaggiosa perché genera una discendenza di individui geneticamente unici (combinazioni inedite ogni volta)
• Negli animali due sono i gameti: – La cellula uovo – Lo spermatozoo
• Due gameti aploidi si fonderanno generando una cellula diploide, lo zigote, portatore di una nuova combinazione di cromosomi
• La meiosi comincia con la replicazione del DNA che porta al raddoppio del numero di cromosomi. La riduzione finale del loro numero avviene perché a questa replicazione seguono DUE divisioni cellulari successive
• Ogni gamete alla fine avrà una sola serie di cromosomi, composta da un mix di cromosomi materni e paterni, con un assortimento casuale
• Vi sono principalmente tre differenze sostanziali con la mitosi:
• I cromosomi omologhi (paterni e materni) già replicati si appaiano prima di disporsi sul fuso. Ciò permette la corretta
segregazione. • L’appaiamento degli omologhi è
accompagnato dalla ricombinazione (processo in cui due sequenze nucleotidiche identiche si scambiano tratti di DNA). L’effetto è lo scambio fisico di segmenti omologhi tra cromosomi paterni e materni (crossing over). Nella prima divisione meiotica (meiosi I) si separano quindi gli omologhi ricombinati.
• Segue un secondo turno di divisione (meiosi II) in assenza di replicazione del DNA, in cui i cromatidi fratelli segregheranno, come in una normale mitosi, alle 4 cellule figlie aploidi.
