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 metaphase plate. The centromeres着丝粒 finally divide, and each sister chromatid moves to one of the poles of the spindle. The next phase is telophase , followed again by cytokinesis. The result of the entire process is four haploid cells in which parental chromosomes are randomly distributed.

第二次核分裂开始于分裂中期，子细胞中染色体重新排列在赤道板上。着丝粒最终分离，每个姊妹染色单体分向两极。接着胞质分裂。产生4个单倍体，父母染色体随机分配。 Asexual Versus Sexual Reproduction

Mitosis and meiosis, respectively分别, make simple cell division and sexual reproduction possible. Each means of passing on hereditary 遗传 information has advantages. In asexual 无性的 reproduction the parent organism生物体 gives rise to offspring 后代that are genetic clones of the parent. The advantages of this type of reproduction are that it preserves the parent's successful genetic complement 遗传互补, requires little or no specialization of reproductive organs, and is more rapid than sexual reproduction. A major disadvantage of the asexual mode is that a single catastrophic灾难的 event or disease may destroy

an entire population of genetically identical organisms. A prime benefit of sexual reproduction is that it provides genetic variability and a ready mechanism机制 for the elimination of deleterious mutations. It also allows \gene forms to arise and spread through populations.

有丝分裂和减数分裂在传递遗传信息过程中各有优势。体细胞的繁殖就是父母本的克隆，其优势是保留了父母本的成功遗传信息，不需要特殊器官，比性复制快的多。但一个简单灾难性事件或疾病都可能摧毁一个细胞群体。性复制的优势是它提供了遗传可变性和现存排除有害突变的机制。也可以产生新的基因并在种群中蔓延。

课后作业：第一篇阅读材料 答案：1C,2B,3D,4B,5A,6D

Glossary

mitosis 有丝分裂

Process that results in equal and identical distribution of replicated chromosomes into two newly formed nuclei. meiosis 减数分裂

The process by which a nucleus divides into four daughter nuclei, each containing half the number of chromosomes of the parent nucleus.

histone 组蛋白

One of a group of simple proteins that have a high content of the basic amino acids arginine or lysine and are found associated with nucleic acids in the chromatin of eukaryotic cells. nucleosome 核小体

The basic unit of chromatin structure in eukaryotic cells. A nucleosome consists of eight histone molecules of four different types, together with about 140 base pairs of DNA coiled around it. chromatin 染色质

The material of which the chromosomes are composed. DNA and protein are the main constituents of chromatin, most of the protein being histone.

karyotype 核型，染色体组型 ,

The appearance of the chromosome complement of an organism or cell.

homologous pair 同源染色体对

Chromosomes that pair during meiosis. Each homologue is a duplicate of one of the chromosomes contributed at syngamy by the mother or father. Homologous chromosomes contain the same linear sequence of genes and as a consequence each gene is present in duplicate. diploid二倍体

Any nucleus, cell, or organism that possesses twice the haploid number of chromosomes. haploid 单倍体

Any nucleus, cell, or organism that possesses a single set of unpaired chromosomes,

interphase (cycle) 分裂间期（周期）

In a cell preparing to duplicate, the interphase stage can divided into a period of initial growth (G1), a period of DNA synthesis (S), and a second growth period (G2). chalone 抑素

Substances found in mammalian tissue homogenates that, when applied to intact tissue cells, inhibit mitosis, particularly in the presence of adrenaline and corticosteroids. prophase （分裂）前期

First stage of mitosis, during which individual chromosomes become visible.

chromatid 染色单体

A replicated chromosome physically attached to an identical chromatid at the centromere. centromere 着丝粒