Biologia per il liceo/Divisione cellulare: differenze tra le versioni
Contenuto cancellato Contenuto aggiunto
m Bot: correggo errori comuni |
m →Mitosi |
||
Riga 8:
== Mitosi ==
[[Immagine:Major events in mitosis.svg|400px|right||thumb|Principali fasi della mitosi]]
Il processo di '''mitosi''', consiste in una divisione cellulare in cui il nucleo cellulare si divide in modo tale che ciascuna cellula figlia abbia esattamente lo stesso patrimonio cromosomico della cellula madre.Durante il processo di divisione mitodica, i cromosomi della cellula madre vengono duplicati e distributi in due cellule figlie identiche tra di loro.▼
▲Il processo di '''mitosi''', consiste in una divisione cellulare in cui il nucleo cellulare si divide in modo tale che ciascuna cellula figlia abbia esattamente lo stesso patrimonio cromosomico della cellula madre.Durante il processo di divisione
Ecco di seguito illustrate brevemente le fasi della divisione mitodica:▼
#I cromosomi appaiono come filamenti attorcigliati. All'esterno del nucleo compaiono delle strutture dette '''centrioli'''.
Line 18 ⟶ 19:
#Si formano nuove membrare nucleari e il citoplasma inizierà a dividersi.
#I cromosomi non sono più visibili e dividendosi il citoplasma, forma le due cellule figlie.
[[Image:Major events in mitosis.svg|right|thumb|350px|Mitosis divides genetic information during cell division.]]
In biology, Mitosis is the process of chromosome segregation and nuclear division that follows replication of the genetic material in eukaryotic cells. This process assures that each daughter nucleus receives a complete copy of the organism's genetic material. In most eukaryotes, mitosis is accompanied with cell division or cytokinesis, but there are many exceptions, for instance among fungi. There is another process called meiosis, in which the daughter nuclei receive half the chromosomes of the parent, which is involved in gamete formation and other similar processes, which makes the parent cell still active.
Mitosis is divided into several stages, with the remainder of the cell's growth cycle considered interphase. Properly speaking, a typical cell cycle involves a series of stages: G1, the first growth phase; S, where the genetic material is duplicated; G2, the second growth phase; and M, where the nucleus divides through mitosis. Mitosis is divided into prophase, prometaphase, metaphase, anaphase and telophase.
The whole procedure is very similar among most eukaryotes, with only minor variations. As prokaryotes lack a nucleus and only have a single chromosome with no centromere, they cannot be properly said to undergo mitosis.
== Profase ==
[[Image:Prophase.jpg|right|frame|'''Prophase:''' The two round objects above the nucleus are the centrosomes. Note the condensed chromatin.]]
Il materiale genetico (DNA), che normalmente esiste in forma di cromatina, condensa in una struttura altamente ordinata chiamata cromosoma. Dal momento che il materiale genetico è stato duplicato , esistono due copie identiche di ogni cromosoma nella cellula . I cromosomi identici (detti anche ''fratelli'') sono uniti in una zona detta '''centromero'''.
L'altro componente che entra in gioco nella mitosi è il centriolo , che funziona come una specie di ancora. Durante la profase , i due centrioli -che replicano in modo indipendente dalla mitosi-
The other important piece of hardware in mitosis is the centriole, which serves as a sort of anchor. During prophase, the two centrioles - which replicate independently of mitosis - begin recruiting microtubules (which may be thought of as cellular ropes or poles) and forming a mitotic spindle between them. By increasing the length of the spindle (growing the microtubules), the centrioles push apart to opposite ends of the cell nucleus. It should be noted that many eukaryotes, for instance plants, lack centrioles although the basic process is still similar.
==Prometaphase ==
Some biology texts do not include this phase, considering it a part of prophase. In this phase, the nuclear membrane dissolves in some eukaryotes, reforming later once mitosis is complete. This is called open mitosis, found in most multicellular forms. Many protists undergo closed mitosis, in which the nuclear membrane persists throughout.
Now kinetochores begin to form at the centromeres. This is a complex structure that may be thought of as an 'eyelet' for the microtubule 'rope' - it is the attaching point by which chromosomes may be secured. The kinetochore is an enormously complex structure that is not yet fully understood. Two kinetochores form on each chromosome - one for each chromatid.
When the spindle grows to sufficient length, the microtubules begin searching for kinetochores to attach to.
==Metaphase ==
[[Image:Metaphase.jpg|frame|right|'''Metaphase:''' The chromosomes have aligned at the metaphase plate.]]
As microtubules find and attach to kinetochores, they begin to line up in the middle of the cell. Proper segragation requires that every kinetochore be attached to a microtubule before separation begins. It is thought that unattached kinetochores control this process by generating a signal - the mitotic spindle checkpoint - that tells the cell to wait before proceeding to anaphase. There are many theories as to how this is accomplished, some of them involving the generation of tension when both microtubules are attached to the kinetochore.
When chromosomes are bivalently attached - when both kinetochores are attached to microtubules emanating from each centriole - they line up in the middle of the spindle, forming what is called the metaphase plate. This does not occur in every organism - in some cases chromosomes move back and forth between the centrioles randomly, only roughly lining up along the midline.
==Anaphase==
[[Image:Anaphase.jpg|frame|right|'''Early anaphase:''' Kinetochore microtubules shorten]]
Anaphase is the stage of meiosis or mitosis when chromosomes separate and move to opposite poles of the cell (opposite ends of the nuclear spindle). Centromeres are broken and chromatids rip apart.
When every kinetochore is attached to a microtubule and the chromosomes have lined up along the middle of the spindle, the cell proceeds to anaphase. This is divided into two phases. First, the proteins that bind the sister chromatids together are cloven, allowing them to separate. They are pulled apart by the microtubules, towards the respective centrioles to which they are attached. Next, the spindle axis elongates, driving the centrioles (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. These two stages are sometimes called 'early' and 'late' anaphase.
At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations.
==Telophase ==
Telophase is the opposite of prophase. A nuclear membrane reforms around each of the daughter cells; nucleoli reappear. The spindles and asters (in animals) disappear. The chromatids start to elongate and become less condensed, changing their form to the long and thin chromatin.
==Cytokinesis==
Cytokinesis refers to the physical division of one eukaryotic cell. Cytokinesis generally follows the replication of the cell's chromosomes, usually mitotically, but sometimes meiotically. Except for some special cases, the amount of cytoplasm in each daughter cell is the same. In animal cells, the cell membrane forms a cleavage furrow and pinches apart like a balloon. In plant cells, a cell plate forms, which becomes the new cell wall separating the daughters. Various patterns occur in other groups.
== Meiosi ==
| |||