The Mitotic Phase Figure 4: Mitosis in onion root cells. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. There may be additional cell growth during G 2. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. In the G 2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation.
(b) Centrioles give rise to the mitotic spindle (grey threadlike structures). Figure 3 (a) Structure of the centrioles making up the centrosome. Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi. 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. The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. The centrosome is also duplicated during the S phase. The chromosomes remain attached together at a region called the centromere. Figure 2 DNA replication during S phase copies each linear chromosome. In the S phase, DNA replication can proceed through the mechanisms that result in the formation of identical pairs of DNA molecules- sister chromatids-that are firmly attached to the centromeric region ( Figure 2). Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating sufficient energy reserves to complete the task of replicating each chromosome in the nucleus. However, during the G 1 stage, the cell is quite active at the biochemical level. The first stage of interphase is called the G 1 phase (first gap) because, from a microscopic aspect, little change is visible. For a cell to move from interphase to the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G 1, S, and G 2.
Interphaseĭuring interphase, the cell undergoes normal processes while also preparing for cell division. Usually the cell will divide after mitosis in a process called cytokinesis in which the cytoplasm is divided and two daughter cells are formed. Mitosis is nuclear division during which duplicated chromosomes are segregated and distributed into daughter nuclei. During interphase, G1 involves cell growth and protein synthesis, the S phase involves DNA replication and the replication of the centrosome, and G2 involves further growth and protein synthesis.
PROPHASE PICTURE SERIES
Figure 1: A cell moves through a series of phases in an orderly manner. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides. During interphase, the cell grows and DNA is replicated. The cell cycle has two major phases: interphase and the mitotic phase ( Figure 1). The timing of events in the cell cycle is controlled by mechanisms that are both internal and external to the cell.Ĭells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produce two genetically identical cells. When fast-dividing mammalian cells are grown in culture (outside the body under optimal growing conditions), the length of the cycle is approximately 24 hours. There is also variation in the time that a cell spends in each phase of the cell cycle. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development to an average of two to five days for epithelial cells, or to an entire human lifetime spent without dividing in specialized cells such as cortical neurons or cardiac muscle cells. The length of the cell cycle is highly variable even within the cells of an individual organism. The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells via mitosis. Meiosis will be discussed in a later chapter. Mitosis is used to produce new body cells for growth and healing, while meiosis is used to produce sex cells (eggs and sperm). Eukaryotes have two major types of cell division: mitosis and meiosis.