高中生物受精过程

fertilization is the epic story of a single sperm facing incredible odds to unite with an egg and form a new human life it is the story of all of us during sexual intercourse about 300 million sperm enter the vagina soon afterward millions of them will either flow out of the vagina or die in its acidic environment however， many survive because of the protective elements provided in the fluid surrounding them next the sperm must pass through the cervix and opening into the uterus usually it remains tightly closer， but here the cervix is open for a few days， while the woman ovulates the sperm swim through the cervical mucus which is thinned to a more watery consistency for easier passage once inside the cervix the sperm continue swimming toward the uterus though millions will die trying to make it through the mucus some sperm remain behind caught in the folds of the cervix， but they may later continue the journey as a backup to the first group inside the uterus muscular uterine contractions assist the sperm on their journey toward the egg however resident cells from the woman's immune system mistaking the sperm for foreign invaders destroy thousands more next half the sperm head for the empty fallopian tube while the other half swim toward the tube containing the unfertilized egg now only a few thousand remain inside the fallopian tube a tiny sillier pushed the egg toward the uterus to continue the sperm must surge against this motion to reach the egg some sperm get trapped in the sillier and die during this part of the journey chemicals in the reproductive tract caused the membranes covering the heads of the sperm to change as a result the sperm become hyperactive swimming harder and faster toward their destination at long last the sperm reached the egg only a few dozen of the original three hundred million sperm remain the egg is covered with a layer of cells called the corona radiator the sperm must push through this layer to reach the outer layer of the egg the zona pollucidor when sperm reached the zona pollucida they attached a specialized sperm receptors on the surface which triggers their acrosomes to release digestive enzymes enabling the sperm to borrow into the layer inside the zone of pollucida is a narrow fluid filled space just outside the excel membrane the first sperm to make contact will fertilize the egg after a perilous journey and against incredible odds a single sperm attaches to the egg cell membrane within a few minutes their outer membranes fuse and the egg pulls the sperm inside this event causes changes in the egg membrane that prevent other sperm from attaching to it next the egg releases chemicals that push other sperm away from the egg and create an impenetrable fertilization membrane as the reaction spreads outward the zone of pollucidor hardens trapping any sperm unlucky enough to be caught inside outside the egg sperm are no longer able to attach to the zona pollucidor， meanwhile inside the egg， the tightly packed male genetic material spreads out a new membrane forms around the genetic material creating the male pro nucleus inside the genetic material reforms into 23 chromosomes， the female genetic material awakened by the fusion of the sperm with the egg finishes dividing resulting in the female pronucleus， which also contains 23 chromosomes as the male and female pro nuclei form spider wear black threads called microtubules pull them toward each other the two sets of chromosomes join together completing the process of fertilization at this moment a unique genetic coder rises instantly determining gender hair， color， eye， color and hundreds of other characteristics this new single cell the zygote is the beginning of a new human being and now the sillier and the philopean tube gently sweep the zygote toward the uterus where he or she will implant in the rich uterine lining growing and maturing for the next nine months until ready for birth。

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动画展示动物细胞有丝分裂过程，在分裂间期完成了 dna 复制和相关蛋白质的合成，中心体也由一个复制成，两个细胞也有了适度的生长。 当一切准备工作完成后，细胞进入分裂期。分裂前期，染色体在细胞核内悄悄螺旋的时候，核仁区因没有松散的 dna 而无法再继续合成 rna， 最终解体， 紧接着核膜也消失了。染色体暴露在细胞质中，每条染色体的着丝率部位都连接着一些微管蛋白， 这些微管蛋白的另外一头是移动到细胞两极的中心体，形成两头尖中间骨的纺锤状结构，这就是纺锤体。 染色体螺旋化程度进一步增强，并受到纺锤体上微管蛋白的牵引，逐渐排列在一个平面上，这就是赤道板，也叫赤道面。赤道板与两个中心体的连线垂直，细胞进入到了分裂中期， 此时染色体形态稳定。排列规则是观察染色体结构和数目的最佳时期。 经过一些细胞周期蛋白的检测，如果每条啄丝粒部位都连接正确，就会发出信号，进入分裂后期， 此时分离酶会将粘连蛋白分解掉，导致啄丝粒分裂。碱胺染色单体分离，各自形成一条染色体，且这两条染色体所携带的遗传物质一模一样，从而实现遗传物质的平均分配。 当染色体到达细胞两极后，进入末期，染色体解螺旋形成丝状的染色体，同时核膜重新组装起来，形成两个新的细胞核。 注意，此时还是一个细胞。细胞质的分裂从后期就已经开始了，在赤道板的位置形成分裂沟，细胞膜逐渐凹陷，屹立在末期，结尾的时候屹立成两个细胞。