Cel Division

Cell division

Division fashion of eukaryotic cell: 无丝分裂 (amitosis) 有丝分裂 (mitosis) 减数分裂 (meiosis)

amitosis characters ： 1.Spindle and chromosome are not formed. 2.Genetic material was separated to two cells unevenly.

What is (and is not) mitosis? Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase, metaphase, anaphase, and telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle.

有 丝 分 裂 由核分裂（ karyokinesis) 与 胞质分裂（ cytokinesis) 两个连续 的过程所组成。 在有丝分裂过程中出现了纺 锤丝、纺锤体，核仁、核膜、染 色质均发生相应改变，从而保证 将复制的染色体准确地分配到两 个子细胞中。

Both the alignment and separation processes are the consequence of the chromosomes interacting with filamentous proteinaceous structures, known as microtubules. The microtubules become organized into a biconical array known as a spindle, which forms early in mitosis, and then disassembles as mitosis nears completion. Mitotic spindles are visible in living cells with the polarizing light microscope. Some of the spindle microtubules become attached to the chromosomes at sites known as kinetochores. The kinetochores cannot be seen with the light microscope, but they reside near the place on the chromosome known as its centromere, which can be observed with the light microscope. There are two kinetochores on each replicated chromosome (one on each chromatid), and when the replicated chromosome splits apart at its centromere at the onset of anaphase, each daughter chromosome possesses one centromere and one kinetochore. The linkages between kinetochores and microtubules are thought to be central in controlling both the positioning of the replicated chromosome at the central portion of the spindle during the alignment phase, and in moving the daughter chromosomes apart after they split at their centromeres. The separation of daughter cells from each other is a process known as cytokinesis, and is separate from mitosis. In cytokinesis, animal and plant cells differ considerably from each other. These differences are the consequence of having or not having a cell wall. Cytokinesis in fungi reveals some similarities with plant cells, and exhibits other features unique to the group.

一、 structure of mitosis apparatus 纺锤体 （ spindle) 中心体

(central body)

中心粒

(centriole)

星体

(aster)

染色体

(chromosome)

Basic structure of spindle

kinetochore fibers continuous fibers interzonal fibers aster fibers

星体微管 中心粒 极微管 （连续 丝）

管

着丝点 染色体微管 中心体

纺 锤 体 微 区间微 管 染色体微管

Structure of spindle and aster

Structure of centrosome

Structure of centromere

纺锤体微管

姐妹 着丝点 （动原 粒） 主要成分蛋白质 外层

主缢痕（初级缢痕） 着丝粒 含高度重复 DNA

中层

内层

着丝点 kinetochore （动原粒）：是两条染色 单体外表面在初缢痕处的特殊附加结构，为染色体 的运动中心，也是微管组织中心之一。

Function of mitosis apparatus Mitosis apparatus is a temporal apparatus which is formed by centrosome. It ensures genetic material to be separated to two daughter cells evenly when the energy is provided by the ATP.

Stage

of

mitosis

前期 （ prophase) 前中期 (prometaphase) 中期 (metaphase) 后期 (anaphase) 末期 (telophase) 胞质分裂 (cytokinisis)

Interphase & mitosis

Interphase The cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules.

Prophase

During prophase, the replicated chromosomes undergo extensive condensation (i.e., coiling). The chromosomes are greatly thickened and shortened but are still contained within the nuclear envelope. Late in prophase, within about 6 min of nuclear envelope breakdown, the mitotic spindle begins to grow, and two triangular 'clear zones' become visible, with one on each side of the nucleus. In three dimensions, the clear zones are actually conical and the nucleus is spherical. With continued spindle expansion, the nucleus rapidly becomes compressed and appears eliptical in the optical section provided by differential interference contrast optics. Prophase ends with the sudden dispersion of the nuclear envelope (nuclear envelope breakdown), and the chromosomal mass is no longer occupying a discrete, spherically-shaped zone in the cell. Prophase in stamen hair cells can last for as long as several hours. It can also be observed by downloading the movie.

Prometaphase Once the nuclear envelope has broken down, the spindle microtubules and the chromosomes are no longer separated by a (double) membrane boundary. The microtubules begin to interact with the chromosomes, and the chromosomes undergo what is known as congressional movement, where they ultimately end up with their centromeres all situated in middle of the spindle, at a site known as the metaphase plate. Each kinetochore of the replicated chromosome is pointed toward one side of the spindle; later, in anaphase, each kinetochore moves to one of the two spindle pole regions as the daughter chromosome. The congression of chromosomes and the alignment of centromeres on the metaphase plate represent essential prerequisites for the orderly separation of the replicated genome into two equal parts. The mechanisms underlying congression are under intense scrutiny.

Metaphase The replicated chromosomes converge toward the center of the spindle, and once they get there, significant movements cease. On either side of each centromere are sites for microtubule attachment to the chromosome; electron microscopists called these plate-like structures kinetochores. The kinetochores are not visible with the light microscope. At several points during metaphase, the chromatid arms may unwind from each other. This unwinding is especially apparent late in metaphase, just 1 or 2 minutes before the chromatids will split apart at their centromeres, with each replicated chromosome giving rise to two daughter chromosomes. The interval between nuclear envelope breakdown and anaphase onset includes prometaphaseand metaphase, and requires approximately 33 minutes.

Anaphase Anaphase commences with the initial splitting of sister chromatids at their centromeres. These daughter chromosomes then begin to separate from each other, each moving away from the metaphase plate and toward one of the two spindle pole regions. The rate of chromosome separation in stamen hair cells is approximately 1.4 micrometers/min. The mechanisms that control chromosome separation clearly involve the interactions between Microtubules andcomponents in or near the kinetochore. Anaphase chromosomal movement is a topic of intense interest in the Cell Biology research community.

Telophase Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.

Cytokinesis In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

Cytokinesis Character: 1.Sure the position of fission 2. Fiber ring forming 3. Contract pinching 4.Cell membrane fusing ， daughter cell separate 。

有丝分裂过程

核 仁

动 物 细 胞 的 有 丝 分 裂

中心粒

前

期

中 期

收缩环造成的分裂 沟

后

期

染色体形态动画

Meiosis Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete". Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished.

Function of meiosis Sexual reproduction occurs only in eukaryotes. During the formation of gametes, the number of chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during fertilization.

Process of meiosis Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis produces 4 haploid cells. Mitosis produces 2 diploid cells. The old name for meiosis was reduction/ division. Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). Most of the differences between the processes occur during Meiosis I.

Prophase I Prophase I has a unique event -- the pairing (by an as yet undiscovered mechanism) of homologous chromosome. Synapsis is the process of linking of the replicated homologous chromosomes. The resulting chromosome is termed a tetrad, being composed of two chromatids from each chromosome, forming a thick (4-strand) structure. Crossingover may occur at this point. During crossing-over chromatids break and may be reattached to a different homologous chromosome.

Prophase I Thus, instead of producing only two types of chromosome (all capital or all lower case), four different chromosomes are produced. This doubles the variability of gamete genotypes. The occurrence of a crossing-over is indicated by a special structure, a chiasma (plural chiasmata) since the recombined inner alleles will align more with others of the same type (e.g. a with a, B with B). Near the end of Prophase I, the homologous chromosomes begin to separate slightly, although they remain attached at chiasmata. Crossing-over between homologous chromosomes produces chromosomes with new associations of genes and alleles.

减数分裂过程 1 . 减数分裂 I

前期 I( 细线 期)

1. 同源染色体配对 1. 二价体四分体 ——联会 2. 非姐妹染色 单体之间出现 2. 二价体形成 交叉。

前期 I( 偶线 期)

前期 I( 粗线 期)

1. 联会复合体消 失 2. 同源染色体某 些部分分离

前期 I( 双线 期)

前期 I( 终变期

同源染色体：大小形态相同、结构相似、一条来自父亲一条来自母亲，上面载有等位基 姐妹染色单体 因的 一对染色体 ——同源染色体。 姐妹染色单体 3 和 4 的染色 1 和 2 的染色 质 同源染色体 I 的 同源染色体 II 质 中央组分 2 条姐妹染色单 的 2 条姐妹染色 体

单体

同源染色 体 I 的着丝 点

侧生组分

四分体

同源染色 体 II 的着 丝点

联会复合体

Prophase I

Events of Prophase I (save for synapsis and crossing over) are similar to those in Prophase of mitosis: chromatin condenses into chromosomes, the nucleolus dissolves, nuclear membrane is disassembled, and the spindle apparatus forms.

Metaphase I Metaphase I is when tetrads line-up along the equator of the spindle. Spindle fibers attach to the centromere region of each homologous chromosome pair. Other metaphase events as in mitosis.

Anaphase I Anaphase I is when the tetrads separate, and are drawn to opposite poles by the spindle fibers. The centromeres in Anaphase I remain intact.

Telophase I Telophase I is similar to Telophase of mitosis, except that only one set of (replicated) chromosomes is in each "cell". Depending on species, new nuclear envelopes may or may not form. Some animal cells may have division of the centrioles during this phase.

Prophase II During Prophase II, nuclear envelopes (if they formed during Telophase I) dissolve, and spindle fibers reform. All else is as in Prophase of mitosis. Indeed Meiosis II is very similar to mitosis.

Metaphase II Metaphase II is similar to mitosis, with spindles moving chromosomes into equatorial area and attaching to the opposite sides of the centromeres in the kinetochore region.

Anaphase II During Anaphase II, the centromeres split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell.

Telophase II Telophase II is identical to Telophase of mitosis. Cytokinesis separates the cells.

中期 I

前期 II

前期 I( 细线

前期 I( 偶线

前期 I( 粗线

前期 I( 双线

期)

期)

期)

期)

后期 I

中期 II

末期 I

后期 II

末期 II

前期 I( 终变期 )

Comparison of Mitosis and Meiosis |

Mitosis maintains ploidy level, while meiosis reduces it. Meiosis may be considered a reduction phase followed by a slightly altered mitosis. Meiosis occurs in a relative few cells of a multicellular organism, while mitosis is more common.

Meiosis

Gametogenesis Gametogenesis is the process of forming gametes (by definition haploid, n) from diploid cells of the germ line. Spermatogenesis is the process of forming sperm cells by meiosis (in animals, by mitosis in plants) in specialized organs known as gonads (in males these are termed testes). After division the cells undergo differentiation to become sperm cells. Oogenesis is the process of forming an ovum (egg) by meiosis (in animals, by mitosis in the gametophyte in plants) in specialized gonads known as ovaries. Whereas in spermatogenesis all 4 meiotic products develop into gametes, oogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop. This all the cytoplasm and organelles go into the egg. Human males produce 200,000,000 sperm per day, while the female produces one egg (usually) each menstrual cycle.

Spermatogenesis Sperm production begins at puberty at continues throughout life, with several hundred million sperm being produced each day. Once sperm form they move into the epididymis, where they mature and are stored.

Oogenesis The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells. Each egg begins oogenesis as a primary oocyte. At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.