有絲分裂

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有絲分裂,又稱為間接分裂,由W. Fleming (1882)年首次發現于動物及E. Strasburger(1880)年發現于植物。特點是有紡錘體染色體出現,子染色體被平均分配到子細胞,這種分裂方式普遍見于高等動植物(動物和高等植物)。是真核細胞分裂產生體細胞的過程。
植物細胞有絲分裂
動物細胞有絲分裂過程

  

目錄

特點

細胞進行有絲分裂具有周期性。即連續分裂的細胞,從一次分裂完成時開始,到下一次分裂完成時為止,為一個細胞周期。一個細胞周期包括兩個階段:分裂間期和分裂期,分裂期又分為分裂前期、分裂中期、分裂后期和分裂末期。細胞在分裂之前,必須進行一定的物質準備。細胞增殖包括物質準備和細胞分裂整個過程?! ?

過程

有絲分裂是一個連續的過程,為了描述方便起見,習慣上按先后順序劃分為間期、前期、中期、后期和末期四個時期,在前期和中期之間有時還劃分出一個前中期。

間期 有絲分裂間期分為G1、S、G2三個階段,其中G1期與G2期進行RNA(即核糖核酸)的復制與有關蛋白質的合成,S期進行DNA的復制。在有絲分裂間期,染色質沒有高度螺旋化形成染色體,而是以染色質的形式進行DNA(即脫氧核糖核酸單鏈復制。有絲分裂間期是有絲分裂全部過程重要準備過程,是一個重要的基礎工作。

前期
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自分裂期開始到核膜解體為止的時期。間期細胞進入有絲分裂前期時,核的體積增大,由染色質構成的細染色線逐漸縮短變粗,形成染色體。因為染色體在間期中已經復制,所以每條染色體由兩條染色單體組成。核仁在前期的后半漸漸消失。在前期末核膜破裂,于是染色體散于細胞質中。動物細胞有絲分裂前期時靠近核膜有兩個中心體。每個中心體由一對中心粒和圍繞它們的亮域,稱為中心質或中心球所組成。由中心體放射出星體絲,即放射狀微管。帶有星體絲的兩個中心體逐漸分開,移向相對的兩極(圖1)。這種分開過程推測是由于兩個中心體之間的星體絲微管相互作用,更快地增長,結果把兩個中心體(兩對中心粒)推向兩極,而于核膜破裂后終于形成兩極之間的紡錘體。

前中期 自核膜破裂起到染色體排列在赤道面上為止。核膜的斷片殘留于細胞質中,與內質網不易區別,在紡錘體的周圍有時可以看到它們。

前中期的主要過程是紡錘體的最終形成和染色體向赤道面的運動。紡錘體有兩種類型:一為有星紡錘體,即兩極各有一個以一對中心粒為核心的星體,見于絕大多數動物細胞和某些低等植物細胞。一為無星紡錘體。兩極無星體,見于高等植物細胞(圖2)。
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曾經認為有星紡錘體含有三種紡錘絲,即三種微管。一種是星體微管,由星體散射出的微管;二是極微管,是由兩極分別向相對一級方向伸展的微管,在赤道區來自兩極的極微管互相重疊?,F在認為極微管可能是由星體微管伸長形成的。三是著絲點微管,與著絲點聯結的微管,亦稱著絲點絲或牽引絲。著絲點是在染色體的著絲粒的兩側發育出的結構。有報告說著絲點有使微管蛋白聚合成微管的功能。無星紡錘體只有極微管與著絲點微管。

核膜破裂后染色體分散于細胞質中。每條染色體的兩條染色單體其著絲點分別通過著絲點與兩極相連。由于極微管和著絲微管之間的相互作用,染色體向赤道面運動。最后各種力達到平衡,染色體乃排列到赤道面上。

中期 從染色體排列到赤道面上,到它們的染色單體開始分向兩極之前,這段時間稱為中期。有時把前中期也包括在中期之內。中期染色體在赤道面形成所謂赤道板。從一端觀察可見這些染色體在赤道面呈放射狀排列,這時它們不是靜止不動的,而是處于不斷擺動的狀態。中期染色體濃縮變粗,顯示出該物種所特有的數目和形態。因此有絲分裂中期適于做染色體的形態、結構和數目的研究,適于核型分析。

后期 每條染色體的兩條姊妹染色單體分開并移向兩極的時期。分開的染色體稱為子染色體。子染色體到達兩極時后期結束。染色單體的分開常從著絲點處開始,然后兩個染色單體的臂逐漸分開。當它們完全分開后就向相對的兩極移動。這種移動的速度依細胞種類而異,大體上在0.2~5微米/分間。平均速度為 1微米/分。同一細胞內的各條染色體都差不多以同樣速度同步地移向兩極。子染色體向兩極的移動是靠紡錘體的活動實現的。

末期 從子染色體到達兩極開始至形成兩個子細胞為止稱為末期。此期的主要過程是子核的形成和細胞體的分裂。子核的形成大體上是經歷一個與前期相反的過程。到達兩極的子染色體首先解螺旋而輪廓消失,全部子染色體構成一個大染色質塊,在其周圍集合核膜成分,融合而形成子核的核膜,隨著子細胞核的重新組成,核內出現核仁。核仁的形成與特定染色體上的核仁組織區的活動有關。
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細胞體的分裂稱胞質分裂。動物和某些低等植物細胞的胞質分裂是以縊束或起溝的方式完成的??O束的動力一般推測是由于赤道區的細胞質周邊的微絲收縮的結果。微絲的緊縮使細胞在此區域產生縊束,縊束逐漸加深使細胞體最后一分為二。

高等植物細胞的胞質分裂是靠細胞板的形成。在末期,紡錘絲首先在靠近兩極處解體消失,但中間區的紡錘絲保留下來,并且微管增加數量,向周圍擴展,形成桶狀結構,稱為成膜體。與形成成膜體的同時,來自內質網和高爾基器的一些小泡和顆粒成分被運輸到赤道區,它們經過改組融合而參加細胞板的形成。細胞板逐漸擴展到原來的細胞壁乃把細胞質一分為二(圖3)。細胞板由兩層薄膜組成,兩層薄膜之間積累果膠質,發育成胞間層,兩側的薄膜積累纖維素,各自發育成子細胞的初生壁。

【細胞有絲分裂記憶口訣】有絲分裂并不難

間前中后末相連

前期:膜仁消失現兩體

中期:形定數晰赤道齊

后期:點裂數加均兩極

末期:兩消兩現重開始(動物)

兩消兩現新壁建(植物)  

動植物的不同

動物細胞有絲分裂的過程,與植物細胞的基本相同.不同的特點是:

1.動物細胞有中心體,在細胞分裂的間期,中心體的兩個中心粒各自產生了一個的中心粒,因而細胞中有兩組中心粒.在細胞分裂的過程中,兩組中心粒分別移向細胞的兩極.在這兩組中心粒的周圍,發出無數條放射線,兩組中心粒之間的星射線形成了紡錘絲.

2.動物細胞分裂末期,細胞的中部并不形成細胞板,而是細胞膜從細胞的中部向內凹陷,最后把細胞縊裂成兩部分,每部分都含有一個細胞核.這樣,一個細胞就分裂成了兩個子細胞  

動植物的相同

動物細胞有絲分裂的過程與植物細胞的分裂過程存在兩個個十分重要的相同點:

無論是動物細胞分裂過程還是植物細胞分裂過程都會有染色體的出現和紡錘體的形成。(植物:無星射線紡錘體;動物:星射線紡錘體)?! ?

意義

有絲分裂的重要意義:是將親代細胞的染色體經過復制(實質為DNA的復制)以后,精確地平均分配到兩個子細胞中去。由于染色體上有遺傳物質DNA,因而在生物的親代和子代之間保持了遺傳性狀的穩定性??梢?,細胞的有絲分裂對于生物的遺傳有重要意義?! ?

英文版介紹(Mitosis)

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle - the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell.
Mitosis divides the chromosomes

Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species. For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus. Prokaryotic cells, which lack a nucleus, divide by a process called binary fission.

The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase and telophase. During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two identical daughter cells.

Because cytokinesis usually occurs in conjunction with mitosis, "mitosis" is often used interchangeably with "mitotic phase". However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. This occurs most notably among the fungi and slime moulds, but is found in various different groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development.Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to cancer.  

Phases of cell cycle and mitosis

Interphase

The mitotic phase is a relatively short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for cell division. Interphase is therefore not part of mitosis. Interphase is divided into three phases, G1 (first gap), S (synthesis), and G2 (second gap). During all three phases, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only during the S phase. Thus, a cell grows (G1), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G2), and divides (M).

Preprophase

In plant cells only, prophase is preceded by a pre-prophase stage. In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules and actin filaments (called preprophase band) underneath the plasma membrane around the equatorial plane of the future mitotic spindle. This band marks the position where the cell will eventually divide. The cells of higher plants (such as the flowering plants) lack centrioles: with microtubules forming a spindle on the surface of the nucleus and then being organized into a spindle by the chromosomes themselves, after the nuclear membrane breaks down. The preprophase band disappears during nuclear envelope disassembly and spindle formation in prometaphase.

Prophase

Prophase

Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin. At the onset of prophase, chromatin condenses together into a highly ordered structure called a chromosome. Since the genetic material has already been duplicated earlier in S phase, the replicated chromosomes have two sister chromatids, bound together at the centromere by the cohesion complex. Chromosomes are visible at high magnification through a light microscope.

Close to the nucleus are structures called centrosomes, which are made of a pair of centriole. The centrosome is the coordinating center for the cell's microtubules. A cell inherits a single centrosome at cell division, which replicates before a new mitosis begins, giving a pair of centrosomes. The two centrosomes nucleate microtubules (which may be thought of as cellular ropes or poles) to form the spindle by polymerizing soluble tubulin. Molecular motor proteins then push the centrosomes along these microtubules to opposite side of the cell. Although centrosomes help organize microtubule assembly, they are not essential for the formation of the spindle, since they are absent from plants, and centrosomes are not always used in meiosis.

Metaphase
Metaphase


As microtubules find and attach to kinetochores in prometaphase, the centromeres of the chromosomes convene along the metaphase plate or equatorial plane, an imaginary line that is equidistant from the two centrosome poles. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug-of-war between people of equal strength. In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline. Metaphase comes from the Greek μετα meaning "after."

Because proper chromosome separation requires that every kinetochore be attached to a bundle of microtubules (spindle fibres), it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned. The signal creates the mitotic spindle checkpoint.

Anaphase
Anaphase


When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase (from the Greek ανα meaning “up,” “against,” “back,” or “re-”).

Two events then occur; First, the proteins that bind sister chromatids together are cleaved, allowing them to separate. These sister chromatids, which have now become distinct sister chromosomes, are pulled apart by shortening kinetochore microtubules and move toward the respective centrosomes to which they are attached. Next, the nonkinetochore microtubules elongate, pushing the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. The force that causes the centrosomes to move towards the ends of the cell is still unknown, although there is a theory that suggests that the rapid assembly and breakdown of microtubules may cause this movement.

These two stages are sometimes called early and late anaphase. Early anaphase is usually defined as the separation of the sister chromatids, while late anaphase is the elongation of the microtubules and the microtubules being pulled farther apart. At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations.

Telophase
Telophase


Telophase (from the Greek τελο? meaning "end") is a reversal of prophase and prometaphase events. It "cleans up" the after effects of mitosis. At telophase, the nonkinetochore microtubules continue to lengthen, elongating the cell even more. Corresponding sister chromosomes attach at opposite ends of the cell. A new nuclear envelope, using fragments of the parent cell's nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Mitosis is complete, but cell division is not yet complete.