2.5 – Cell Division 2.5.1 - Outline the stages in the cell cycle, including interphase (G₁, S, G₂), mitosis and cytokinesis Interphase is when the DNA replicates. The cell will also replicate its centrosome, which is important for movement of chromosomes. This is split into three stages: G₁ is the first stage, and stands for Gap 1. During this time, the cytoplasm is still active, and the cell continues with its normal functions, such as protein synthesis, mitochondria replication or chloroplast replication. There is all the activity of a growing cell. S is the synthesis phase when the DNA is replicated. The mass of the DNA in the cell doubles. All the chromosomes are copied and form chromatids. These remain attached until they divide in mitosis. G₂ is the third stage, standing for Gap 2, when there is more growth of the cell, then preparation takes place for cell division. Mitosis then happens. This consists of four stages: prophase, metaphase, anaphase and telophase. The chromosomes are separated and distributed. Cytokinesis is the division of the cytoplasm to form two daughter cells. The cell cycle is then repeated.
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2.5.2 - State that tumours (cancers) are the result of uncontrolled cell division and that these can occur in any organic tissue Tumours, or cancers, are cell mass formed as a result of uncontrolled cell division. They can occur in any tissue. In a tumour, the normal repressed state of mitosis is disrupted by mutation to the protooncogene. As a result, the cells begin to divide uncontrollably. The proto-oncogene mutates into the oncogene, resulting in the loss of control of cell division. The cells form an irregular mass of cells; the tumour. Some cells may break away and form a secondary tumour elsewhere. Eventually they take over the surrounding, healthy cells, which leads to malfunction and death. It is caused by damage to DNA chromosomes. The accumulation of mistakes in DNA causes cancer, which is why it is more common in older people. Another cause is damage to the gene that codes for p53, the protein which stops the copying of damaged DNA. The damage to the DNA can result from ionising radiation (X-rays, gamma rays...), some chemicals (tar in tobacco smoke) as well as virus infections. Some factors are also inherited. The development of cancer requires at least two mutations; one of the proto-oncogene; two of the tumour suppressor. Cancer exerts its deleterious effect on the body by destroying the adjacent tissues (such as compressing nerves, eroding blood vessels), replacing normal functioning cells (such as replacing blood forming cells in the bone marrow or the heart muscles so that the heart fails).
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2.5.3 - State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts This is always the longest part of the cell cycle. During interphase, the nucleus undergoes many changes. The chromosomes disperse as chromatin and become actively involved in protein synthesis. Copies of the information in particular genes or groups of genes are taken from the chromosomes for use in the cytoplasm. Proteins are assembled in the ribosomes by combining amino acids in sequences dictated by the information from the gene. The synthesis of new organelles takes place in the cytoplasm during interphase. There is intense biochemical activity in the cytoplasm and the organelles, and there is an accumulation of stored energy before nuclear division occurs again. Also, each chromosome replicates into the two identical structures called chromatids. These remain attached until they divide in mitosis. In this time, the cell itself will continue to carry out its specialised function. The length of interphase varies between cell types. After cytokinesis, G1 occurs: when various proteins are synthesised to allow the cell to specialise. Then, in the S stage, the DNA is replicated. G2 is the preparation stage for mitosis. Mitochondria (and chloroplasts in plants) are replicated.
2.5.4 - Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase) During mitosis, the chromatids are separated and distributed to two daughter nuclei. Mitosis is a continuous process with no breaks in it, although we divide it into four stages. Prophase The chromosomes become visible as long, thin threads. They shorten and thicken through the process of supercoiling. In supercoiling, DNA is combined with histone proteins and
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non-histone proteins to form the readily stainable chromatin. The genes must be left in predictable positions and a distinctive overall chromosome shape. The supercoiling makes the structure so dense that it can be seen with a light microscope during mitosis. At the end of prophase, it is possible to see that the two chromatids are held together at the centromere. During this time, the nucleolus disappears and the nuclear membrane breaks down.
Metaphase The centrioles move to opposite ends of the cell. Microtubules in the cytoplasm start to form a spindle, radiating out from the centrioles. These attach to the centromeres and are arranged at the equator of the spindle. In plant cells, the same structure is formed, but without the presence of centrioles.
Anaphase The centromeres divide, the spindle fibres shorten, and the chromatids are pulled by their centromere to opposite poles. Once separated, they are referred to as chromosomes.
Telophase The nuclear membrane reforms around both groups of chromosomes at opposite ends of the cell. They begin to decondense and become chromatin again. The nucleolus reforms. Interphase then follows the division of the cytoplasm.
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2.5.5 - Explain how mitosis produces two genetically identical nuclei Cell division produces genetically identical daughter cells. They have a set of chromosomes identical to each other and to the parent cell from which they were formed. An exact copy of each chromosome is made by accurate replication during interphase. There is conservation of the chromosome number, so the chromosome numbers of the daughter cells are exactly the same as each other and the parent cell. The chromatids remain attached by their centromeres during metaphase, when each becomes attached to a spindle fibre at the equator of the spindle. Centromeres then divide during anaphase, and one copy of each chromosome moves to each pole of the spindle. The chromosomes then form two new nuclei when the cell splits into two, each with an exact copy of the original nucleus. Hence, the two nuclei from each new cell are genetically identical.
2.5.6 - State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis In the growth and development of an embryo, it is very important that all cells carry the same genetic information as the existing cells from which they are formed, and with which they share surrounding cells or tissues. Multicellular organisms increase their size through growth, which involves increasing the number of cells through mitosis. These cells can then differentiate and specialise their function. In the same way, when repair of damaged or worn out cells occurs, they are exact copies of the ones they replace. If this was not the case, different body part would begin working to conflicting blueprints, resulting in chaos. This form of cell division is also the basis of asexual reproduction, in which the offspring produced are identical to the parent. In other words, they are clones. This is very common in nature.
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