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An in-depth explanation of the eukaryotic cell cycle, focusing on the process of mitosis. It covers the stages of mitosis, including prophase, metaphase, anaphase, and telophase, as well as the importance of mitosis in growth and repair. The document also includes questions for self-assessment and diagrams to aid understanding.
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The eukaryotic cell cycle This is illustrated by Fig 1. Cell division consists of two phases, mitosis and cytokinesis. Mitosis, is the division of the nucleus. Cytokinesis is the division of the cytoplasm which usually, but not always, occurs immediately after nuclear division. Only a small time period of the cell cycle consists of the cell division stages. The other phases, known as G1 , S and G2 take up the majority of the time. The total time of the cell cycle varies from around 30 minutes in growing yeast cells, 18 to 24 hours in animal sperm- producing cells, 10 to 30 hours in plant meristematic cells to several weeks in slowly regenerating tissues. For example in a cell with a 24 hour cycle the times of the phases would be around G1 10 hours, S 9 hours, G2 4 hours and mitosis with cytokinesis 1 hour. Fully differentiated cells generally remain arrested in the G1 stage and will not normally divide again. Some cells remain arrested in the G2 stage, for example, human cardiac muscle cells. The synthesis of DNA occurs in the S phase when the quantity of DNA in the cell is doubled. This is shown in Fig 2.
The G1 , S and G2 phases are termed interphase. Interphase is the stage of the cell cycle between cell divisions. It is not a resting stage, since in an actively dividing and growing cell new DNA and proteins are being synthesised and in a non-dividing mature cell, the G1 or G2 cell is performing all its metabolic cell functions and specific jobs. The importances of mitosis Cell division by mitosis is important during growth of eukaryotic organisms and is the way in which eukaryotes increase their cell numbers, either in a population of a single celled organism, such as Amoeba or yeast, or within the body of a multicellular organism. Growth may be allometric meaning that different parts of the organism grow at different rates. This can be due to mitosis occuring at different rates in different organs. Mitosis is also important during repair of damaged tissue. Mitosis produces two ‘daughter’ or offspring nuclei from the original parent nucleus. The chromosome number of each of the offspring nuclei is the same as the parent nucleus and the genome (nature and arrangement of the genes) is kept exactly the same. Thus mitosis maintains the same chromosome number and genotype throughout growth, life and repair. Mitosis ensures that every body cell throughout life, with the exception of gametes, has a genome identical to that of the original zygote. Mitosis of a diploid cell will produce two diploid cells and mitosis of a haploid cell will produce two haploid cells. For example, when the haploid leafy gametophyte stage of a moss or the haploid gametophyte prothallus of a fern produce haploid gametes they do so by mitosis. The process of mitosis For easy reference the process of mitosis is divided into four phases.These are prophase, metaphase, anaphase and telophase. Cytokinesis occurs at the end of telophase. Prophase: The events of prophase are shown in Fig 3.
Once the two sets of chromosomes have reached their respective poles they become enclosed in new nuclear membranes which are derived from the endoplasmic reticulum. Meanwhile the spindles disappear and the centriole of each nucleus replicates. The chromosomes become longer and thinner and return to their interphase form and the nucleoli reappear. Cytokinesis usually starts once the two ‘daughter’ nuclei are established. Cytokinesis: In animals a cleavage furrow develops and runs round the cell in line with the original equator of the spindle. Once ‘daughter’ nuclei have formed the cleavage furrow deepens and eventually cuts the cell in two. In plants a phragmoplast or cell plate is formed between the two ‘daughter’ nuclei. This cell plate is assembled from membraneous vesicles from the Golgi body and endoplasmic reticulum. The vesicles give rise to the middle lamella of calcium and magnesium pectates on which cellulose is deposited, forming new a cell wall. The two new genetically identical cells then pass into the G1 phase of the cell cycle. Practice questions
4(a) (b) (in animals) cytoplasm divides by constriction (between daughter nuclei); (in plants) a phragmoplast/cell plate/new cell wall is synthesised (between the daughter nuclei); (2) Exam questions 1 The image below is of a cell undergoing mitosis. (a) Suggest which phase of mitosis this cell is currently in and explain your reason. [3]
(b) Explain how the cell progresses from this stage until the end of telophase. [4] (c) Mitosis forms a part of a cyclical process of cell renewal, termed the cell cycle. It is made up of several stages (G 1 , S, G 2 and M). Fill in the graph below to show how the quantity of DNA in a cell changes as the cell progresses through the cell cycle. Use ‘n’ to represent a single set of chromosomes (haploid). [1]