【正文】 s, forming beadlike plexes known as nucleosomes. More coiling and supercoiling produces a dense chromosome structure. Each long strand of DNA bines with histones and nonhistone proteins to make up the substance chromatin.細(xì)胞核是貯藏遺傳信息的主要場所。DNA盤繞成螺旋線以及相關(guān)的成簇蛋白質(zhì)。DNA螺旋線纏繞成簇的組蛋白形成珠鏈狀的核小體。這些螺旋和超螺旋形成致密的染色體組結(jié)構(gòu)。每個長鏈DNA與組蛋白和非組蛋白一起構(gòu)成染色質(zhì)物質(zhì)。A pictorial display of an organism39。s chromosomes in the coiled, condensed state is known as a karyotype. Karyotype reveal that in most cells all but sex chromosomes are present as two copies, referred to as homologous pairs. Nonsex chromosomes are called autosomes. Organisms whose cells contain two sets of parental chromosomes are called diploid。 those with cells containing a single set of parental chromosomes are called haploid. 染色體致密的超螺旋狀態(tài)我們稱染色體組。除了性染色體外，大多數(shù)細(xì)胞的染色體組成對出現(xiàn)，稱同源染色體對。非性染色體稱常染色體。生物細(xì)胞含有兩套父母本染色體的稱二倍體；含有單套染色體的稱單倍體。The Cell CycleThe cell cycle is a regular sequence in which the cell grows, prepares for division, and divides to form two daughter cells, each of which then repeats the cycle. Such cycling in effect makes singlecelled organisms immortal. Many cells in multicellular organisms, including animal muscle and nerve cells, either slow the cycle or break out of it altogether.在細(xì)胞生長過程中，細(xì)胞循環(huán)遵循特定程序，分裂準(zhǔn)備，分裂成2個子細(xì)胞，子細(xì)胞再循環(huán)。此循環(huán)使得單細(xì)胞永生。多細(xì)胞生物中的許多細(xì)胞，包括動物肌肉和神經(jīng)細(xì)胞，要么降低循環(huán)速度，要么同時分裂。The normal cell cycle consists of four phases. The first three include G1, the period of normal metabolism。 S phase, during which normal synthesis of biological molecules continues, DNA is replicated, and histones are synthesized。 and G2, a brief period of metabolism and additional growth. Together the G1, S, and G2 phases are called interphase. The fourth phase of the cell cycle is M phase, the period of mitosis, during which the replicated chromosomes condense and move and the cell divides. It is believed that properties of the cell cytoplasm control the cell cycle, along with external stimulators and inhibitors such as chalones.正常細(xì)胞循環(huán)由4個時期組成。頭三期包括G1,正常新陳代謝；S期,正常新陳代謝同時，DNA復(fù)制，組蛋白合成； G2 期，短期的新陳代謝和少許生長。G1, S, 和G2稱分裂間期。最后是M期，有絲分裂期，復(fù)制的染色體組濃縮，移動并細(xì)胞分裂。據(jù)稱是染色質(zhì)控制了細(xì)胞循環(huán)，伴隨外部激活因子和抑制因子如抑素。 Mitosis: Partitioning the Hereditary MaterialBiologists divide the mitotic cycle into four phases. At the beginning of prophase the chromosomes each consist of two highly condensed chromatids attached to each other at a centromere. As prophase ends and metaphase begins, the condensed chromosomes bee associated with the spindle. Eventually the chromosomes bee arranged in a plane (called the metaphase plate) at a right angle to the spindle fibers. Next, during anaphase, the two sister chromatids of each chromosome split, and one from each pair is drawn toward each pole of the cell. During telophase nuclear envelopes begin to form around each set of chromosomes, and division of the cytoplasm takes place.生物學(xué)家將有絲分裂劃分為4個階段。分裂前期，高度濃縮的兩個染色單體通過著絲粒連接在一起。在分裂前期后期和分裂中期前期，濃縮的染色體與紡錘體相連，最后以正確的角度排列在赤道板上。在分裂后期，兩個姊妹單體分離，分別拽向細(xì)胞兩極。在分裂末期，在每套染色體周圍形成核膜，細(xì)胞質(zhì)發(fā)生分裂。As mitosis proceeds, the spindle microtubules play a crucial role in ensuring that both paired and separated chromatids 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 fibers, extend outward from the spindle poles to structures on the chromosomes called kinetochores. During anaphase the fibers begin to shorten, and the chromatids begin to move apart.在有絲分裂過程中，是紡錘體微管確保了染色單體在適當(dāng)時間以正確方向進行分離。紡錘體微管由兩極向赤道板延伸。在分裂前期，其它微管，著絲粒纖維延伸到染色體的動粒。在分裂后期，纖維開始變短，染色單體分離。The spindle forms differently in plant and animal cells. In animals it is associated with centriole, while in plant and fungal cells spindle formation is associated with reions called microtubule organizing centers. 植物和動物細(xì)胞形成的紡錘體不同。動物細(xì)胞與中心粒相連，而在植物和真菌細(xì)胞中，紡錘體與微管組織中心的離子相連。Cytokinesis: Partitioning the Cytoplasm胞質(zhì)分裂：細(xì)胞質(zhì)分離The division of the cell cytoplasm at the end of mitosis is called cytokinesis. In animal cells it takes place as a ring of actin filaments contracts around the cell equator, pinching the cell in two. In plant cells, which are bounded 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 in the region of the cell plate. 在動物細(xì)胞中，環(huán)形肌動蛋白絲延赤道板收縮而使細(xì)胞一分為二。在植物細(xì)胞中，在赤道板形成新的細(xì)胞板。Meiosis: The Basis of Sexual ReproductionMeiosis is a special form of cell division that takes place in the reproductive organs that produce sex cells. Like mitosis, it takes place after DNA replication has occurred and involves two sequential 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, the homologous chromosomes distributed to different progeny cells are not identical.減數(shù)分裂是性細(xì)胞分裂的特殊形式。如有絲分裂，它也是發(fā)生在DNA復(fù)制后并有連續(xù)的兩個核分裂。產(chǎn)生4個子細(xì)胞，分別含有親本一半的染色體數(shù)。As in mitosis two chromatids exist for each chromosome at the beginning of prophase 1. During this phase the homologous chromosomes undergo synapsis, or pairing, which is brought about by a bridging structure of proteins and RNA called the synaptonemal plex. The homologous pairs stay together when they align on the metaphase plate. Unlike the anaphase of mitosis, however, during anaphase I the two chromatids of each chromosome stay joined at the centromere and move together to one of the two poles of the cell. It is this event that results in the halving of the chromosome number in the four daughter cells that result from meiosis.正如在有絲分裂中一樣，兩個同源染色單體通過蛋白質(zhì)和RNA橋配對形成聯(lián)會復(fù)合體。與有絲分裂不同的是，每組染色體的兩個染色單體連接在著絲點上并一起移向細(xì)胞兩極的一級。由此而導(dǎo)致4個子細(xì)胞染色體數(shù)減半。During telophase I nuclear envelopes enclose the chromosomes in nuclei, and in most species cytokinesis (the first nuclear division)