师：同学们，今天我们继续学习细胞增殖的内容。对于连续分裂的细胞，存在细胞周期。其中，细胞分裂间期时间很长，它在做怎样的物质准备呢？分裂期时间短暂，人为将其分为前、中、后、末四个时期，这期间又发生着怎样的变化？我们先来看。

　　“Adult human contains an estimated 100 trillion cells. And yet we start life as a single cell. To grow, develop and repair tissue damage, we rely on cell division in eukaryotic cells. This process is accomplished by a series of well - orchestrated steps called mitosis. Every day, our bodies must produce millions of skin cells to replace those lost through normal activity. From interphase to cytokinesis, the entire process of cell division can take on average 10 to 20 hours in a typical plant or animal cell, depending on the nature and use of the cell, the process can happen at different frequencies as well. In humans, our skin cells have a high turnover rate due to wear and tear and go through mitosis very frequently, whereas other cells, such as adult neurons and muscle cells, rarely divide prior to cell division, where the cell divides into two identical cells, the DNA needs to be replicated so that each daughter cell receives an exact copy.

　　The replication of DNA and the formation of sister chromatids is one part of the entire cell cycle to prepare for cell division, the cell goes through interphase, which can be divided into three distinct phases during G1 or GAP1 phase, all the organelles and cytoplasmic components, including the centrioles in animal cells, replicate. Then during s or synthesis phase, the DNA replicates. Finally, during G2 or GAP2 phase, all the enzymes needed to aid in the process of cell division are produced.

　　Most eukaryotic cells spend a great deal of time in interphase and a very short period of time actually dividing, a process called mitosis. The cell is now ready to go through mitosis, which consists of prophase metaphase, anaphase and telophase. During prophase, the chromosomes condense and become visible, appearing as two sister chromatids held together at the centromere. The cytoskeleton disassembles as the spindle begins to form. In animal cells, centrioles play an important role in the distribution of the chromosomes. In the dividing cell, the centrioles migrate to opposite poles, establishing a bridge of microtubules called the spindle apparatus. And the nuclear envelope breaks down towards the end of prophase. Chromosomes attach by proteins in their centromeres, called kinetochores, to microtubules from each pole, moving the chromosomes toward the equator of the cell. During metaphase, all chromosomes are aligned at the equator of the cell, called the metaphase plate.

　　Anaphase begins with the degradation of proteins that hold sister chromatids together, freeing individual chromosomes. The free chromosomes are then pulled by their kinetochores to opposite poles at telophase. A cleavage furrow forms in the center of the cell. This indentation is made from a constricting belt of actin filaments surrounding the inside of the cell's circumference. Chromosomen cluster at opposite poles and begin decondensing as the nuclear envelope reforms around them. The Spindel apparatus disassembles as the microtubules are broken down into tubulin monomers that can be used to form the cytoskeleton of the daughter cells. In animal cells, cytokinesis completes cell division by extending the cleavage furrow to completely separate the newly formed daughter.”

　　这是动物细胞的有丝分裂过程，那么高等植物细胞有丝分裂又是怎样的呢？我们先看末期，高等植物细胞要分裂形成两个子细胞，要考虑它有一个特殊结构——细胞壁。它是怎样形成的呢？在细胞中央部位出现一个特殊结构叫细胞板，它会逐渐扩张形成新的细胞壁。也就是说，高等植物细胞在有丝分裂末期和动物细胞不一样。不仅如此，前期也不一样。

　　刚才视频中提到，动物细胞有丝分裂密切相关的细胞器是中心体。细胞分裂间期，中心体完成复制，有丝分裂前期发出星射线形成纺锤体。但高等植物细胞没有中心体，它的纺锤体是从细胞两极发出纺锤丝形成的。请同学们按照表格比较动物细胞和高等植物细胞有丝分裂的相同与不同之处。

　　动物细胞由两组中心粒发出星射线，高等植物细胞从细胞两极发出纺锤丝，这是前期形成纺锤体方式的不同。末期时，动物细胞由中部细胞膜凹陷缢裂成两个子细胞，高等植物细胞在细胞中央赤道板位置出现细胞板，并逐渐扩张形成新细胞壁，即末期形成两个子细胞的方式不同，或者说细胞质的分裂方式不同。但着眼于染色体的变化，动植物细胞有丝分裂过程很相似，都历经染色质转变为染色体，再由染色体转变为染色质的过程，染色体都会历经散乱分布、排列在细胞中央、平均分配到两极的变化过程。