细胞有丝分裂的过程描述英语作文

细胞有丝分裂的过程描述英语作文

Mitosis is a continuous process, which is artificially divided into six periods for convenience of description: interphase, prophase, premetaphasem, metaphase, anaphase and telophase. The interval includes G1 phase, S phase and G2 phase, mainly for DNA replication and other preparatory work.

Through mitosis, each chromosome is accurately copied into two chromatids and equally divided into two daughter cells, so that the daughter cells contain the same genetic information as the mother cells. The process of cell mitosis can be divided into prophase, metaphase, anaphase and telophase. The morphology and behavior of chromosomes in different periods are different. The interval from the end of the last cell mitosis to the completion of the next cell mitosis is called a cell cycle. It includes mitosis interval and mitosis period (prophase, metaphase, anaphase and telophase). Interphase is the period when DNA synthesis and physiological metabolism of cells are vigorous, accounting for most of the cell cycle. According to the morphology of chromosomes and DNA synthesis in cells, the interval can be divided into G1 phase-at this time, there is no DNA replication, but RNA and protein synthesis. Stage S-At this time, DNA synthesis takes place in the cell, doubling the total amount of DNA. G2-At this time, the cell contains two sets of complete diploid chromosomes, and DNA synthesis is no longer carried out. M (mitosis)-At this time, the chromosomes really start to divide.

At the end of interphase, chromosomes exist in the nucleus as chromatin, and the cells function normally.

Prophase: Chromatin spirals together, gradually shortens and thickens, and highly spirals into chromosomes. Each chromosome consists of two parallel sister chromatids, which are connected by a common centromere. And sends out spindles from the two poles of the cell. (The spindles of higher plants emit directly from the two poles of the cell, while the spindles of higher animals and some lower plants are made from the centrosome.) Spindle-shaped spindles appear, chromosomes are scattered in the center of the spindles, the nuclei decompose, the nucleolus disappears, and the nuclear membrane gradually disintegrates.

Metaphase: The metaphase of cell division, in which the spindle is clearly visible. At this time, on both sides of the centromere of each chromosome, there are spindles attached to it, and the spindles pull the chromosomes to move, so that the centromere of each chromosome is arranged on a plane in the center of the cell. This plane is perpendicular to the axis of the spindle, similar to the position of the equator on the earth, so it is called the equatorial plate. In metaphase cells, the morphology of chromosomes is fixed, the number is clear, and it is easy to observe clearly.

Late stage: In the late stage of cell division, each centromere divides into two, and the two sister chromatids originally connected to the same centromere are separated into two daughter chromosomes. Spindle leads daughter chromosomes to move to two levels of cells respectively. At this time, all chromosomes in the nucleus are evenly distributed to the two levels of the cell, so that each level of the cell has a set of chromosomes.

Note: The number of chromosomes in a cell varies with the species. For example,

Drosophila melanogastert has 4 pairs of 8 chromosomes, human beings have 23 pairs of 46 chromosomes, onion cells have 8 pairs of 16 chromosomes and rice has 12 pairs of 24 chromosomes.

Terminal stage: After the chromosome reaches the poles, it unwinds to form chromatin filaments, one cell splits into two, the spindle disappears, and the nuclear membrane and nucleolus rebuild.