用英语描述细胞有丝分裂和减数分裂的过程、意义!

用英语描述细胞有丝分裂和减数分裂的过程、意义!
用英语描述细胞有丝分裂和减数分裂的过程、意义!

用英语描述细胞有丝分裂和减数分裂的过程、意义!
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.

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