生物专业英语第三版蒋悟生编__课文翻译(12)

As mitosis proceeds, the spindle microtubules play a crucial role in ensuring that both paired and separated chromatids 染色单体 move in the right directions at the proper times. Each half of the spindle forms as microtubules extend from each pole of a dividing cell to the region of the metaphase plate. During prophase, other microtubules, the centromeric 着丝粒fibers, extend outward from the spindle poles to structures on the chromosomes called kinetochores着丝粒. During anaphase the fibers begin to shorten, and the chromatids begin to move apart.

在有丝分裂过程中，是纺锤体微管确保了染色单体在适当时间以正确方向进行分离。纺锤体微管由两极向赤道板延伸。在分裂前期，其它微管，着丝粒纤维延伸到染色体的动粒。在分裂后期，纤维开始变短，染色单体分离。

The spindle forms differently in plant and animal cells. In animals it is associated with centriole 中心粒, while in plant and fungal 真菌 cells spindle formation is associated with reions called microtubule organizing centers.

植物和动物细胞形成的纺锤体不同。动物细胞与中心粒相连，而在植物和真菌细胞中，纺锤体与微管组织中心的离子相连。

Cytokinesis: Partitioning the Cytoplasm

胞质分裂：细胞质分离

The division of the cell cytoplasm at the end of mitosis is called cytokinesis 细胞浆移动. In animal cells it takes place as a ring of actin filaments contracts使缩短 合同契约 around the cell equator赤道, pinching收聚 the cell in two. In plant cells, which are bounded 有限制的 by a cell wall, cytokinesis involves the building of a new cell plate across the dividing cell at its equator. Cell wall material is then deposited 存放堆积 in the region of the cell plate.

在动物细胞中，环形肌动蛋白丝延赤道板收缩而使细胞一分为二。在植物细胞中，在赤道板形成新的细胞板。

Meiosis: The Basis of Sexual Reproduction

Meiosis 减数 is a special form of cell division that takes place in the reproductive 生殖 organs that produce sex cells. Like mitosis, it takes place after DNA replication has occurred and involves two sequential 连续的 nuclear divisions (meiosis I and meiosis Ⅱ). These divisions result in four daughter cells, each with half the number of chromosomes of the parent cell. The phenomenon of crossing over during meiosis results in exchanges of genetic information between chromosomes. Hence因此, the homologous 同源的 chromosomes distributed 分布 to different progeny 后裔 cells are not identical同样的.

减数分裂是性细胞分裂的特殊形式。如有丝分裂，它也是发生在DNA复制后并有连续的两个核分裂。产生4个子细胞，分别含有亲本一半的染色体数。

As in mitosis two chromatids exist for each chromosome at the beginning of prophase 1. During this phase the homologous 同源 chromosomes undergo 经历sy-napsis 联会 , or pairing, which is brought about by a bridging structure of proteins and RNA called the synaptonemal 联会丝的 complex. The homologous pairs stay together when they align 排列 on the metaphase 中 plate. Unlike the anaphase of mitosis, however, during anaphase I the two chromatids of each chromosome stay joined at the centromere 着丝点 and move together to one of the two poles of the cell. It is this event that results in the halving 减半的 of the chromosome number in the four daughter cells that result from meiosis.

正如在有丝分裂中一样，两个同源染色单体通过蛋白质和RNA桥配对形成联会复合体。与有丝分裂不同的是，每组染色体的两个染色单体连接在着丝点上并一起移向细胞两极的一级。由此而导致4个子细胞染色体数减半。

During telophase末期 I nuclear envelopes enclose the chromosomes in nuclei, and in most species cytokinesis 细胞浆流动 (the first nuclear division) follows. The second nuclear division begins with metaphase Ⅱ, in which the chromosomes in each daughter cell again align 排列on a

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