减数分裂的意义 英文
The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are interphase, prophase, prometaphase, metaphase, anaphase and telophase.
The mitotic phase is a relatively short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for cell division. Interphase is therefore not part of mitosis.
Prophase: The two round objects above the nucleus are the centrosomes. The chromatin has condensed.Prometaphase: The nuclear membrane has degraded, and microtubules have invaded the nuclear space. These microtubules can attach to kinetochores or they can interact with opposing microtubules.
Metaphase: The chromosomes have aligned at the metaphase plate.
Early anaphase: The kinetochore microtubules shorten.
Telophase: The decondensing chromosomes are surrounded by nuclear membranes. Cytokinesis has already begun; the pinched area is known as the cleavage furrow.
Mitosis is important for the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell. Transcription is generally believed to cease during mitosis, but epigenetic mechanisms such as bookmarking function during this stage of the cell cycle to ensure that the "memory" of which genes were active prior to entry into mitosis are transmitted to the daughter cells.
Meiosis is a special type of cell division necessary for sexual reproduction.In animals, meiosis produces gametes like sperm and egg cells, while in other organisms like fungi it generates spores. In many organisms, including humans, meiosis begins with one cell containing two copies of each .
Interphase: Before a dividing cell enters meiosis, it undergoes a period of growth called interphase.
* G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.
* S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.
* G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.
* In the latter part of interphase, the cell still has nucleoli present.
* The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.
* In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.
Prophase I:
* Chromosomes condense and attach to the nuclear envelope.
* Synapsis occurs (a pair of homologous chromosomes lines up closely together) and a tetrad is formed. Each tetrad is composed of four chromatids.
* Crossing over may occur.
* Chromosomes thicken and detach from the nuclear envelope.
* Similar to mitosis, the centrioles migrate away from one another and both the nuclear envelope and nucleoli break down.
* Likewise, the chromosomes begin their migration to the metaphase plate.
Metaphase I:
* Tetrads align at the metaphase plate.
* Note that the centromeres of homologous chromosomes are oriented toward the opposite cell poles.
Anaphase I:
* Chromosomes move to the opposite cell poles. Similar to mitosis, the microtubules and the kinetochore fibers interact to cause the movement.
* Unlike in mitosis, the homologous chromosomes move to opposite poles yet the sister chromatids remain together.
Telophase I:
* The spindles continue to move the homologous chromosomes to the poles.
* Once movement is complete, each pole has a haploid number of chromosomes.
* In most cases, cytokinesis occurs at the same time as telophase I.
* At the end of telophase I and cytokinesis, two daughter cells are produced, each with one half the number of chromosomes of the original parent cell.
* Depending on the kind of cell, various processes occur in preparation for meiosis II. There is however a constant: The genetic material does not replicate again.
Prophase II:
* The nuclear membrane and nuclei break up while the spindle network appears.
* Chromosomes do not replicate any further in this phase of meiosis.
* The chromosomes begin migrating to the metaphase II plate (at the cell's equator).
Metaphase II:
* The chromosomes line up at the metaphase II plate at the cell's center.
* The kinetochores of the sister chromatids point toward opposite poles.
Anaphase II:
* The sister chromatids separate and move toward the opposite cell poles.
Telophase II:
* Distinct nuclei form at the opposite poles and cytokinesis occurs.
* At the end of meiosis II, there are four daughter cells each with one half the number of chromosomes of the original parent cell.
Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count.Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the offspring. This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring. Therefore a gene for meiosis will be favoured by natural selection over an allele for mitotic reproduction, because any selection pressure which acts against any clone will act against all clones, whilst inevitably favoring some offspring which are the result of sexual reproduction.
特点：染色体复制一次，而细胞连续分裂两次，结果子细胞中的染色体数目比原来减少一半。
意义：对于有性生殖的生物来说，减数分裂对于维持每种生物前后代体细胞中染色体数目的恒定性，对于生物的遗传性和变异性，都有十分重要的意义。
减数分裂是由相继的两次分裂组成的，分别称为减数分裂Ⅰ和减数分裂Ⅱ。 Meiotic division is composed of two successive, and are called meiosis meiosis Ⅰ and Ⅱ. 在这两次分裂之间一般有一很短的间期，不进行DNA合成，从而也不发生染色体复制。 In both cases there is a split between the generally very short interval, did not carry out DNA synthesis, and thus does not occur and chromosome replication. 由于细胞核分裂两次，而染色体只复制一次，所以经过减数分裂染色体数目减半。 As the nucleus divided twice, and chromosomes replicate only once, so halve the number of chromosomes after meiosis.
减数分裂Ⅰ 前期Ⅰ比较复杂，减数分裂的许多特殊过程都发生在这一时期。 Meiotic prophase Ⅰ Ⅰ more complicated, many of the special process of meiosis have occurred in this period. 它又细分为：①细线期。 It is subdivided into: ① leptotene. 染色质已集缩成细长的线状结构，每条染色体通过附着板与核膜相连，此期核的体积增大，核仁也较大。 Chromatin has been set shrunk to thin linear structure, each chromosome and the nuclear membrane through the attachment plate connected to increase the size of this nucleus, nucleolus rate as well. ②合线期又称偶线期。 ② All-line phase is also called zygotene. 是同源染色体配对的时期。 Is a homologous chromosome pairing period. 这种配对称为联会。 This pairing is called the Federation. 联会一般是从靠近核膜的一端开始，有时在染色体全长的若干点上也同时进行。 Federation is usually near the nuclear membrane at one end from the beginning, and sometimes the full length of a number of points in the chromosome is also the same time. 配对是靠两条同源染色体间沿长轴形成的联会复合体实现的。 Relies on two homologous chromosomes pair along the long axis between the formation of synaptonemal complex achieved. 配对后的每对同源染色体称二价体。 After matching each pair of homologous chromosomes that bivalent. 由于联会，细胞中的染色体由2n条单价体成为n条二价体，虽然DNA含量未变，但数目看起来减少了一半。 As the Federation of cell chromosome by a 2n of Article n of univalents into bivalents, although the DNA content remained unchanged, but the number appears to reduce by half. ③粗线期。 ③ pachytene. 染色体明显缩短变粗。 Chromosome significantly shorter thicker. 联会的两条同源染色体结合紧密，只在局部位置上有时可分辨出是两条染色体。 Association of two homologous chromosomes are linked closely together, only in local positions can sometimes distinguish between a two chromosomes. 在粗线期每条染色体实际已由两条染色单体组成。 Each chromosome in pachytene actually have been composed of two chromatids. 粗线期核仁仍然很大，含有很多RNA。 Nucleolar Pachytene still large, containing a lot of RNA. ④双线期。 ④ diplotene. 联会的两条同源染色体开始分离，但在许多称作交叉的点上它们还连在一起。 Association of two homologous chromosomes began to separate, but in many of the points referred to as cross They also linked together. 此期可以看清，联会的两条染色体都分别由两条染色单体组成。 This period can see, the Federation of the two chromosomes are formed by two chromatids. 交叉发生在两条非姊妹染色单体之间。 Cross took place in between the two non-sister chromatid. 一般认为，交叉是发生了交换的结果。 Is generally believed that the cross is the result of the exchanges took place. 双线期的染色体进一步缩短，此时联会复合体已消失。 Diplotene chromosome to further reduce this time synaptonemal complex has disappeared. 人和动物的卵母细胞常长期停留在减数分裂的双线期。 Oocytes of humans and animals often have long remained at meiotic diplotene. ⑤终变期亦称浓缩期。 ⑤ diakinesis also known as condensed phase. 二价体显著收缩变粗，并向核的周边移动，在核内较均匀地分散开。 Bivalent experienced a significant contraction thicker peripheral to the nuclear movement, the more evenly in the nuclear dispersed. 核仁消失，但有的植物在终变期的早期核仁仍然很大。 Nucleolus disappeared, but some plants in the early diakinesis nucleolus remains high. 终变期末有些二价体的同源染色体只在末端连在一起。 Diakinesis bivalents end of some of the homologous chromosomes together only at the end.
中期Ⅰ核膜解体后二价体分散在细胞质中。 After the disintegration of the mid-Ⅰ membrane bivalents scattered in the cytoplasm. 二价体排列于赤道区，形成赤道板。 Bivalents arranged at the equatorial zone, the formation of equatorial plane.
后期Ⅰ每个二价体的两条同源染色体分开，移向两极。 Each post-Ⅰ bivalents of two homologous chromosomes separate move poles. n个二价体成为n条单价染色体，此时DNA含量减半。 n-bivalents become a unit of n chromosomes, DNA content in half the time.
末期Ⅰ染色体各自到达两极后逐渐解螺旋化，变成细线状。 Ⅰ to reach the end of each chromosome gradually after helicase of bipolar become thin-like. 核膜重建，核仁重新形成，同时进行细胞质分裂。 Membrane reconstruction, nucleolar re-formation, at the same time cytokinesis. 许多植物在减数分裂Ⅰ只发生核的分裂，而细胞质分裂在减数分裂Ⅱ的末期进行，使四个核同时分开。 Many plant species only occur in meiosis Ⅰ nuclear division, but cytokinesis Ⅱ at the end of meiosis to allow the separation of the four core at the same time.
间期在减数分裂Ⅰ和减数分裂Ⅱ之间的间期很短，不进行染色体复制。 Between the period of meiosis meiosis Ⅰ and Ⅱ a very short interval between, say no to chromosome replication. 这时每条染色体已是由两条染色单体构成了。 Then each chromosome is formed by the two chromatids. 在有些生物甚至没有这个间期，而由末期Ⅰ直接转为前期Ⅱ。 In some biological and even without this interval, but by the end of the direct to pre-Ⅰ Ⅱ.
减数分裂Ⅱ 这次分裂基本上与有丝分裂相同。 Meiosis Ⅱ The split is basically the same as mitosis. 前期Ⅱ时间较短。 Shorter pre-Ⅱ. 中期Ⅱ染色体排列于赤道面，形成赤道板。 Chromosomes arranged in metaphase Ⅱ equatorial plane, forming the equatorial plate. 后期Ⅱ时两条染色单体分开，移向两极。 Ⅱ later, when two chromatids separate move poles. 到达两极的子染色体为n数，并且每条子染色体只由一条染色单体构成。 Reach the poles, n the number of sub-chromosome, and each sub-chromosome composition of only one chromatid. 末期Ⅱ时两极的子染色体解螺旋化。 Ⅱ, when the end of the bipolar helicase of the sub-chromosome. 形成核膜，出现核仁，经过细胞质分裂，完成减数分裂过程。 The formation of nuclear membrane, there nucleoli, after cytoplasmic division, the completion of meiosis. 新产生的每个细胞都变成了单倍体。 Each cell in the emerging into a haploid.
郅疤痛必： Leading to germ cell division in the process of halving the chromosome number. 在所有进行有性生殖的生物的生活史中,细胞除进行数次有丝分裂外,还要进行一次减数分裂,使其染色体数目由二倍体(2n)变为单倍体(n). Identical in all ...
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