关于对澳洲教材“细胞的增殖”部分学生李文甫译本

 14.3 CELL DIVISION

Each species has a specific number of chromosomes, and different chromosomes carry different sorts of genetic information. The dividing cells must ensure that the new cells have both the same number and the same types of chromosomes as the original.

Usually the cells of the adult organism contain pairs of chromosomes. There are two of each type of chromosome which carry the same kinds of genetic information. These cells are said to be in the diploid condition and are represented as 2n, where n is the number of types of chromosomes. If the cell contains only one of each type of chromosome, it is said to be in the haploid condition and is represented as n.

Cell division is initiated by the division of the nucleus. Two types of nuclear division occur: mitosis and meiosis.

Mitosis takes place during the growth of an organism and is the basis of the reproductive processes of binary and multiple fission in unicellular organisms, gamete formation in plants and budding and vegetative propagation in multicellular organisms. It yields daughter cells that have the same number and types of chromosomes as the parent cell.

Meiosis generally takes place in the formation of gametes, although in plants it occurs in the formation of spores. During sexual reproduction, a gamete from each of two different parents fuses to form the zygote, the start of a new individual. In order to maintain species continuity the zygote must have the same number and types of chromosomes as the adults. Thus gametes must have half the number but the same types of chromosomes as an adult cell. During meiosis, therefore, haploid cells are formed from a diploid cell.

14.3细胞分裂

每个物种都有特定的染色体数目，和不同的染色体携带不同种类的遗传信息。细胞的分裂必须确保新的细胞具有相同数量和相同类型的染色体就像模板一样。

通常的成人有机体细胞含有成对的染色体。有两组染色体携带相同种类的遗传信息。这些细胞被称为是在二倍体生物中，并表示为2n，其中n是的染色体组数。如果每个细胞包含一组染色体，它被称为是在单倍体状中，并表示为n。

细胞分裂是由细胞核的分裂而发起的。有两种类型的核分裂发生： 有丝分裂和减数分裂。

有丝分裂发生在生长的生物体，是单细胞生物二分裂和复分裂基础的步骤、植物中的配子形成和多细胞生物体中的萌芽和植物生长繁殖的生殖过程的基础。它产生的子细胞具有与亲代染色体相同数量和类型的染色体。

减数分裂通常发生在配子形成中，在植物中在孢子的形成也如此。在有性生殖过程中从父母每个配子结合形成受精卵，开始一个新的个体。为了保持物种的连续性合子必须有相同数量和类型的染色体，像成年人一样。因此配子必须作为一个成人的细胞具有相同类型的染色体总数的一半。在减数分裂，因此，单倍体的生物是从二倍体生物演变而来。

 14.3.1  THE CELL CYCLE

The  cells of multicellular organisms grow and multiply through a cycle of four phases. The first three stages of this cycle involve growth and high metabolic activity within the cell and the fourth phase involves division. The length of the cell cycle is different in different cells in the body or at different stages of overall growth of the body. Thus the cycle of some cells may only take a matter of hours, whereas for others (like human brain cells) the cycle may take many years to complete.

  Three of the phases can be grouped together as interphase and the fourth phase (involving nuclear division, or mitosis, and separation of the daughter cells, or cytokinesis) is called cell division.

 14.3.1 细胞周期

多细胞生物的细胞生长和繁殖周期要通过四个阶段。这个周期的前三个阶段涉及生长和细胞内的高程度的新陈代谢活动，第四阶段涉及到分裂。在不同的细胞，在身体的细胞或身体的生长的细胞的不同时期，细胞周期的长度是不同的。因此一些细胞的周期可能只需要短短几小时内，而另一些 （如人脑细胞） 周期可能需要很多年才能完成。

三个阶段可以组合在一起成为间期，第四阶段 （涉及核的分裂，或子细胞的有丝分裂或胞质分裂） 被称为细胞分裂。

  The first phase of interphase is known as the G1 phase, where G represents growth.  It is a time of very rapid growth which involves a high degree of metabolic activity resulting in the synthesis of a RNA and a large number of proteins. This allows the increase in size and number of cell organelles such as mitochondria, chloroplasts (in plants), the endoplasmic reticulum and the Golgi apparatus.

间期的第一阶段被称为 G1 期，其中 G 代表成长。它是增长的非常迅速，其中涉及很高程度新陈代谢，导致大量的蛋白质和 RNA 合成。这将允许细胞细胞器如线粒体、叶绿体 （在植物里）、内质网、高尔基体的大小和数量的增加。

    The second. S, phase involves the synthesis of nucleotides in the nucleolus and replication of DNA. During interphase the chromosomes are not visible as distinct bodies but are strung out in long chromatin threads which are swollen at intervals. As the cell grows and matures, the genetic material (DNA) of each chromatin thread replicates (is copied), using the existing DNA molecules as the patterns and nucleotides in the nucleus for material. The original and replicated strands of DNA remain connected at a point called the centromere. Protein molecules, called histones, are produced and cover each DNA strand. Each chromosome has become two chromatids.

    第二个。S，此阶段核仁中核苷酸的合成及 DNA 的复制。在间期时染色体并不像完整的个体，是不可见的，但在长时间染色质间高度螺旋化形成。随着细胞成长，成熟，染色质的上的的遗传物质 （DNA） 复制，使用现有的 DNA 分子作为模板和核苷酸在核仁的原材料。原来的和复制股 DNA 保持连接点称为着丝点。蛋白质分子，称为组蛋白并产生了和涵盖每个 DNA 链。每个染色体成为两条染色单体。

 The third, G2, stage involves cellular synthesis in preparation for cell division-a process  that is energy-intensive and requires many proteins. In animal cells the centrioles replicate. Strands of protein that will form the spindle fibres start to assemble.

 The cell has now accumulated the chemical resources and energy necessary to undertake nuclear division. This involves the separation of chromatids and the segregation of the chromosomes at opposite sides of the cell. At the completion of mitosis the entire cell divides so that each new daughter cell has a complete set of chromosomes as well as cytoplasm and organelles. Each of these cells then enters interphase and the cycle repeats itself.

第三个，G2，此阶段涉及为细胞分裂做细胞内合成的准备，这是耗能并需要大量蛋白质的。在动物细胞中中心粒会复制。丝蛋白会形成纺锤丝并开始聚集。

细胞现在积累了化学资源和必须的能量来进行核分裂。这涉及到的染色单体的分离和染色体向各自反方向分离到细胞两级。在有丝分裂完成时整个细胞的分裂，每个新的子细胞有一套完整的染色体，以及作为细胞本身。每一个这些细胞进入间期和循环往复。

Mitosis and cytokinesis

There are four stages in nuclear division or mitosis: prophase, metaphase, anaphase and telophase. These phases are followed by division of the cytoplasm to form two cells (by cytokinesis).

有丝分裂和细胞质分裂

在核分裂或细胞有丝分裂有四个阶段： 前期、中期、后期和末期。其次是细胞质分裂形成两个细胞 （由细胞质分裂）。

Prophase

Prophase mobilises mitotic division. The DNA threads condense to form distinct, thickened chromosomes. Each of these chromosomes consists of two strands, chromatids, which lie parallel to each other alone their length and are joined at a point called the centromere. Each chromatid contains one side of the original DNA and a complementary. replicated side. While  the chromosomes are condensing, a series of protein threads are formed in the cytoplasm to form a spindle which spans the length of the cell. The two ends of the spindle are called the poles and the middle region the equator. In animal cells the centrioles separate and migrate to opposite poles of the spindle. The nucleolus of the nucleus breaks down and the nuclear membrane begins to separate.

前期

前期使有丝分裂动员。DNA 链浓缩变得更加鲜明，高度螺旋化的染色体。每个这些染色体包含两条链，染色单体，平行于彼此之间他们的长度和联接点称为着丝点。每个染色单体包含原始 DNA 和互补的另一条链，复制一条链。当染色体高度螺旋化，一系列在细胞质形成的蛋白质丝，形成一个贯穿该细胞的纺锤体。纺锤丝的两端被称为中部赤道和两极。在动物细胞中中心粒分开，并移动到相反的两极放出星射线。原子核的核仁消失，核膜开始分开。

Metaphase

With the disappearance of the nuclear membrane，the chromosomes migrate to the equator of the spindle, each chromosome attaching to an individual spindle fibre at its centromere. The arrangement of the chromosomes on the spindle fibres is apparently random and may change from division to division.

   中期

核膜消失后，染色体移动到纺锤体的赤道板位置上，每条染色体连接到纺锤丝的赤道板位置上，每个染色体附属于在每一条纺锤丝上的着丝点上。染色体在纺锤丝上的排列显然是随机的和会随着分裂的改变而改变。

Anaphase

During anaphase the chromatids separate to opposite poles of the spindle. The exact mechanism is not known but congregation of mitochondria around the spindle suggests that large amounts of energy are required. It has been suggested that the spindle fibres, with the input of energy, contract and separate at the equator and as they pull towards opposite poles  the centromere, and thus the chromatids part company.

后期

在后期时，染色单体被纺锤丝牵引分开到相反的两极。真正的原理尚不清楚，但线粒体集合在纺锤体表明大量的能量所需。它结果表明纺锤丝，与投入的能量，在赤道板处的收缩和分离，和他们拉向相反两极的染色体着丝粒，因此染色单体分离。

Telophase

On reaching the polar ends of the spindle, the chromatids, which will become the chromosomes of the daughter cells, congregate tightly together. The spindle breaks down and a new nuclear membrane is formed around each bundle of chromosomes. The nucleolus reappears and the chromosomes uncoil to revert to chromatin threads.

末期

到达纺锤的两极末端，将成为对子细胞的染色体的染色单体，紧紧地聚集在一起。纺锤体分解，在每捆染色体周围形成新的核膜。核仁再现和染色体解旋恢复到染色质

cytokinesis

While the events of telophase are occurring, division of the cytoplasm between the two poles commences. In animal cells the cell the membrane constricts across the equator of the spindle to form two new cells. In plants a cell plate, derived from vesicles of the Golgi apparatus, grows across the middle of the cell to unite with the existing cell membrane. New cell walls are produced between the membrane of the plate prior to separation.

   细胞质分裂

虽然末期的事件正在发生，两极之间的细胞质分裂开始。在动物细胞细胞膜收缩横穿纺锤体的赤道板来形成两个新的细胞。在植物的细胞板，来源于高尔基产生囊泡，生长横穿细胞的中间部分与现有细胞膜融合在一起。新的细胞壁在分裂前细胞板和膜间产生。

Significant features in the cell cycle are:

· Just prior to division there is replication of DNA. Each chromosome now consists of two identical chromatids.

·Each chromosome attaches to a separate spindle fibre.

·Separation of the chromatids of each chromosome to opposite poles of the spindle ensures that each pole contains the same number and types of new chromosomes.

·The daughter cells will therefore have exactly the same number and type of chromosomes as the original cell. Thus diploid cells will divide to form diploid daughter cells, and haploid cells will form haploid daughter cells.

·At each division two daughter cells are produced.

·A period of time between divisions(interphase) is required for the cell to grow, store energy and replicate DNA and organelles.

在细胞周期中的显著特点是

·  只是在分列前有 DNA 复制。每个染色体现在由两个完全相同的染色单体组成。

·  每个染色体附着到单独的纺锤丝。

·  每条染色体上的染色单体由纺锤体拉开分离到细胞的两个几点可确保每个极点包含相同数量和类型的新染色体。

·  子细胞因此具有和原始细胞相同的数量和类型的染色体。因此二倍体细胞将划分为形成二倍体的子细胞，单倍体的细胞会形成单倍体子细胞。

·  在每次分裂中产生两个子细胞。

·  在两次分裂间的一段时间（间期）是必须让细胞生长的时间，储存能量和复制DNA和制造细胞器。

 Although cytokinesis normally follows mitosis, there are exceptions. The plasmodial phase in the life cycle of some slime moulds and the formation of skeletal muscle fibres in vertebrate animals result from mitosis without cytokinesis.

即使细胞质分裂通常跟随着有丝分裂，但也有例外。在生命周期的一些疟原虫等粘菌类和脊椎动物的骨骼肌纤维的形成由没有胞质分裂的有丝分裂来进行分裂。

Some cell types go through the cell cycle-growth, mitosis and cytokinesis-continuously throughout the life of the organism. Examples of this include single-called organism and certain plant and animal cells. Since red blood cells of vertebrates do not have a nucleus and live only for about 120 days, cells in the bone marrow constantly divide to maintain the correct number of red blood cells in the body. Other cell types (e.g. brain cells) do not undergo further division once the organism is mature.

某些类型的细胞去通过有丝分裂细胞周期增长和细胞质分裂不断整个贯穿整个生命的生物体。这方面的例子包括单叫有机体和某些植物和动物的细胞。因为脊椎动物血液红细胞没有细胞核，只住了大约 120 天，细胞在骨髓中的也不断分裂，从而保持正确的体内的红细胞数。其它细胞类型 （如脑细胞） 没有经过进一步分裂当生物体是成熟的。

  A third category of cells divide only at a sufficient rate to produce the number of cells that are required for growth and replacement. Human liver cells, for example, do not normally divide after the liver has reached a certain size. If some of the liver is surgically removed, the existing cells are triggered into division mode until the liver has regained its original size. Cell division then stops.

第三类细胞分裂只足够上的速度产生所需的增长和更换的细胞数目。例如人体的肝脏细胞，肝脏已达到一定的大小一般不继续分裂。如果一些肝脏被手术移除，现有细胞被触发进入分裂状态，直到肝脏已重新恢复其原始大小，然后停止细胞分裂。

The nature of the control system or systems that regulate the cell cycle are still unclear. Certain hormones (e.g. growth hormone) and other chemicals have been shown to stimulate cell division. Other chemicals or cell overcrowding can inhibit it. The control of cell division is of particular interest to researchers since cancers result from uncontrolled cell division in multicellular organisms.