In the G 2 phase, or second gap, the cell replenishes its energy stores and synthesizes the proteins necessary for chromosome manipulation. Centrioles are not present in the centrosomes of many eukaryotic species, such as plants and most fungi. The centrosome consists of a pair of rod-like centrioles 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 duplicated during the S phase. At this stage, each chromosome is made of two sister chromatids and is a duplicated chromosome. In the S phase (synthesis phase), DNA replication results in the formation of two identical copies of each chromosome-sister chromatids-that are firmly attached at the centromere region. 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 enough 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, or first gap, because little change is visible. The three stages of interphase are called G 1, S, and G 2. For a cell to move from interphase to the mitotic phase, many internal and external conditions must be met. 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. Watch this video about the cell cycle: Figure 6.3 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 6.3). Cells 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. The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Explain how the three internal control checkpoints occur at the end of G 1, at the G 2–M transition, and during metaphase.Discuss the behavior of chromosomes during mitosis and how the cytoplasmic content divides during cytokinesis.Describe the three stages of interphase.By the end of this section, you will be able to:
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