10.3
Fissione binaria
La fissione è la divisione di una singola entità in due o più parti, che si rigenerano in entità separate che assomigliano all'originale. Gli organism…
- [Istruttore] I procarioti si propagano attraverso la
divisione cellulare, anche se utilizzando un processo
diverso dagli eucarioti, di cui uno che è asessuale,
formando due cloni geneticamente identici di se stesso.
Questo tipo di riproduzione è chiamato fissione binaria,
che significa divisione a metà.
Per dividersi, la cellula prima sostituisce il suo DNA,
copiando il cromosoma dalla sua origine di replicazione,
un'area vicino al contatto cromosomico
con la membrana plasmatica delle cellule.
Gli enzimi di replica copiano il cromosoma in modo
bidirezionale, muovendosi in direzioni opposte dall'origine
per creare due cromosomi a doppio filamento.
Quando avviene la replica, entrambi i cromosomi
iniziano ad allontanarsi l'uno dall'altro,
ai lati opposti della cellula.
La cellula stessa si allunga,
stimolando la separazione cromosomica o nucleotidica.
Una volta che i cromosomi si sono spostati sui lati opposti,
la separazione citoplasmatica, la citochinesi, può iniziare.
Ora, un anello di proteine autoassemblanti
si forma al punto medio della cellula,
direzionando la formazione di un anello del setto,
una parete cellulare in via di sviluppo,
dalle periferie del punto medio al centro.
Quando il setto ha finito di svilupparsi,
pizzica la cellula,
dividendosi per produrre due cellule figlie identiche.
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Q1: How does binary fission differ from eukaryotic cell division?
Binary fission is an asexual prokaryotic process producing two genetically identical daughter cells, unlike eukaryotic cell division mitosis and cytokinesis. Prokaryotes replicate their single chromosome bidirectionally from an origin of replication, then separate chromosomes to opposite cell poles. A septum ring forms at the cell midpoint, pinching the cell in half to yield clones without the complex nuclear envelope dynamics of eukaryotes.
Q2: What is the role of the origin of replication in binary fission?
The origin of replication is the chromosomal region where DNA replication begins in prokaryotes. Located near the cell plasma membrane, it serves as the starting point for bidirectional replication enzymes that copy the chromosome in opposite directions. This process creates two identical double-stranded chromosomes that subsequently move to opposite cell poles before cytokinesis occurs.
Q3: How fast do prokaryotes reproduce through binary fission?
Binary fission speed varies by species but is generally rapid. Escherichia coli, for example, typically divides every 20 minutes, doubling the cell population in that timeframe. This period is called the doubling time. However, bacterial growth slows significantly when cells encounter growth-limiting factors such as nutrient depletion or space constraints, entering a stationary growth phase.
Q4: What happens during the cytokinesis stage of binary fission?
Cytokinesis begins after chromosomes separate to opposite cell poles. A ring of self-assembling proteins forms at the cell midpoint, directing septum formation—a developing cell wall extending from the periphery toward the middle. When the septum completes, it pinches off the cell, splitting it into two identical daughter cells that can each grow to the original parent cell size.
Q5: Do eukaryotic organelles reproduce using binary fission?
Yes, some eukaryotic organelles reproduce via prokaryotic binary fission. Mitochondria, for example, divide using this mechanism, requiring division of mitochondrial proteins and DNA. This reflects the evolutionary origin of mitochondria from prokaryotic ancestors and demonstrates that binary fission is not limited to free-living bacteria and archaea.
Q6: Why are the daughter cells produced by binary fission genetically identical?
Binary fission is an asexual reproduction process in which a parent prokaryote copies its single chromosome bidirectionally and distributes one copy to each daughter cell. Since no genetic recombination or meiosis occurs, each daughter cell receives an exact replica of the parent's genomic DNA, making them genetic clones with identical genetic material.
Q7: What factors limit bacterial growth during binary fission?
Although binary fission can occur rapidly, bacterial growth is constrained by nutrient availability and physical space. When cells exhaust nutrients or encounter spatial restrictions, they enter a stationary growth phase where binary fission occurs at much lower rates. These growth-limiting factors prevent unlimited exponential population expansion and reflect resource constraints in bacterial environments.