In the S phase, DNA replication results in the formation of identical pairs of DNA molecules, sister chromatids, that are firmly attached to the centromeric region. The centrosome is duplicated during the S phase.
The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. At the center of each animal cell, the centrosomes of animal cells are associated with a pair of rod-like objects, the centrioles, which are at right angles to each other. Centrioles help organize cell division. Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi.
In the G 2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase.
There may be additional cell growth during G 2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. Learning Objectives Describe the events that occur during Interphase.
During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate. The ring contracts, forming a cleavage furrow, which divides the cell in two. In plant cells, Golgi vesicles coalesce at the former metaphase plate, forming a phragmoplast. A cell plate formed by the fusion of the vesicles of the phragmoplast grows from the center toward the cell walls, and the membranes of the vesicles fuse to form a plasma membrane that divides the cell in two.
In plant cells, a new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking into vesicles and dispersing throughout the dividing cell. During telophase, these Golgi vesicles are transported on microtubules to form a phragmoplast a vesicular structure at the metaphase plate.
There, the vesicles fuse and coalesce from the center toward the cell walls; this structure is called a cell plate. As more vesicles fuse, the cell plate enlarges until it merges with the cell walls at the periphery of the cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall.
The Golgi membranes become parts of the plasma membrane on either side of the new cell wall Figure 4. Figure 5 shows approximately how long a cell spends in each stage of the cell cycle:. Figure 5. The cell cycle consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei.
The cytoplasm is usually divided as well, resulting in two daughter cells. This video reviews all the steps of mitosis; seeing it all together is a great review at this stage. Answer the question s below to see how well you understand the topics covered in the previous section.
This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section. Privacy Policy. Skip to main content. Module 7: Cell Division. For single-celled organisms, this rate determines how quickly the organism can reproduce new, independent organisms.
For higher-order species the length of the cell cycle determines how long it takes to replace damaged cells. The duration of the cell cycle varies from organism to organism and from cell to cell. Certain fly embryos sport cell cycles that last only 8 minutes per cycle! Some mammals take much longer than that--up to a year in certain liver cells. Generally, however, for fast-dividing mammalian cells, the length of the cycle is approximately 24 hours.
Most of the differences in cell cycle duration between species and cells are found in the duration of specific cell cycle phases.
DNA replication, for example, generally proceeds faster the simpler the organisms. One reason for this trend is simply that prokaryotes have smaller genomes and not as much DNA to be replicated. Across species and organismal complexity, embryonic cells have an increased need for rapidity in the cell cycle because they need to multiply for the development of the embryo.
0コメント