大学生物专业英语lesson_three.pptVIP

Lesson three Cellular reproduction: mitosis and meiosis The cell nucleus is the main repository of genetic information. 细胞核是贮藏遗传信息的主要场所。 Within the nucleus are the chromosomes—tightly coiled strands of DNA and clusters of associated proteins. 细胞核里的是染色体－－紧密盘绕成螺旋的DNA链和相关的成簇蛋白质。 More coiling and supercoiling produces a dense chromosome structure. 进一步的螺旋和超螺旋形成致密的染色体组结构。 Each long strand of DNA combine with histones and nonhistone proteins to make up the substance chromatin. 每个长链DNA与组蛋白和非组蛋白一起构成染色质物质。 Non-sex chromosomes are called autosomes. 非性染色体称常染色体。 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. 细胞里含有两套亲本染色体的生物体称二倍体；细胞里含有单套染色体的生物体称单倍体。 The 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. 细胞周期遵循特定的程序：细胞生长，分裂准备，分裂成2个子细胞，每个子细胞再循环。 Such cycling in effect makes single-celled organisms immortal. 此循环可以有效的使得单细胞生物永生。 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. 前三个时期包括G1,正常新陈代谢时期；S期,　生物分子的合成继续，同时，DNA复制，组蛋白合成； G2 期，新陈代谢和进一步生长的短暂时期。 Together the G1, S, and G2 phases are called interphase. G1, S, 和G2一起称为分裂间期。 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. 细胞周期的第四个时期是M期，有丝分裂期，复制的染色体浓缩，移动并细胞分裂。 Biologists divide the mitotic cycle into four phases. 生物学家将有丝分裂划分为4个阶段。 At the beginning of prophase the chromosomes each consist of two highly condensed chromatids attached to each other at a centromere. 分裂前期，每个染色体是由高度浓缩的通过着丝粒连接在一起的两个染色单体组成。(分裂前期，高度浓缩的两个染色单体通过着丝粒连接在一起形成染色体) Next, during anaphase, the two sister chromatids of each chromosome split, an