Replication and division of nuclei and cells Understanding of chromosome behaviour in mitosis

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1 Cell and nuclear division Replication and division of nuclei and cells Understanding of chromosome behaviour in mitosis Learning objective: a. Explain the importance of mitosis in the production of genetically identical cells, growth, repair and asexual reproduction; Mitosis the division of a nucleus into two so that the two daughter cells have exactly the same number and type of chromosomes as the parent cell. Growth. The two daughter cells formed have the same number of chromosomes as the parent cell and are genetically identical (clones). This allows growth of multicellular organisms from unicellular zygotes. Growth may occur over the entire body, as in animals, or be confined to certain regions, as in meristems (growing points) of plants. Replacement of cells and repair of tissues is possible using mitosis followed by cell division. Cells are constantly dying and being replaced by identical cells. In the human body, for example, cell replacement is particularly rapid in the skin and in the lining of the gut. Some animals are able to regenerate whole parts of the body; for example, starfish can regenerate new arms. Asexual reproduction. Mitosis is the basis of asexual reproduction, the production of new individuals of a species by a single parent organism. This can take many forms. For a unicellular organism such as Amoeba, cell division inevitably results in reproduction. For multicellular organisms, new individuals may be produced which bud off from the parent in various ways. Budding is particularly common in plants; it is most commonly a form of vegetative propagation in which a bud on part of the stem simply grows a new plant. The new plant eventually becomes detached from the parent and lives independently. The bud may be part of the stem of an overwintering structure such as a bulb or tuber. The ability to generate whole organisms from single cells or small groups of cells is important in biotechnology and genetic engineering. Identical cells. Of B- and T lymphocytes during the immune response is dependent on mitosis. b. Describe, with the aid of diagrams, the behaviour of chromosomes during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell membrane, centrioles and spindle (names of the main stages are expected);

2 The cell cycle The cell cycle is regularly sequence of events that takes place between one cell division and the next. It has three phases, namely interphase, nuclear division and cell division. During interphase, the cell grows to its normal size after cell division and carries out its normal functions, synthesising many substances, especially proteins, in the process. At some point during interphase, a single may be received that the cell should divide again. If this happens, the DNA in the nucleus replicates so that each chromosome consists of two identical chromatids. This phase of the cell cycle is described as the S phase S stands for synthesis (of DNA). Interphase can therefore be divided into two growth phases, known as the G 1 phase and the G 2 phase, separated by the S phase. During G 2 the new DNA is checked and any errors are usually repaired. Nuclear division follows interphase. This may be referred to as the M phase (M for mitosis). Growth stops temporarily during mitosis. After the M phase, when the nucleus has divided into two, the whole cell divides. The length of the cell cycle is very variable, depending on environmental conditions and cell type. On average, root tip cells of onion divide once every 20 hours; epithelial cells in the human intestine every 10 hours.

3 In animal cells, cell division involves constriction of the cytoplasm between the two new nuclei, a process called cytokinesis. In plant cells, it involves the formation of the new cell wall between the two new nuclei.

4 c. Explain how uncontrolled cell division can result in cancer and identify factors that can increase the chances of cancerous growth; Control of cell division Each cell contains genes that help to control when it divides. It is important that cells divide by osmosis only when they are required to do so. This usually involves signals from neighbouring cell, to which the cell responds by either dividing or not dividing. If this control goes wrong, then cells mat not divide when they should (so growth does not take place, or wounds do not heal) or they mat divide when they should not (so that a tumour mat form). Cancer is a disease that can result when genes that normally control cell division mutate. The cell may divide over and over again, forming an irregular mass of cells. A small group of tumour cell is called a primary growth. There are two types of tumours: 1. Benign tumours, which do not spread from their site of origin, but can compares and displace surrounding tissues for example warts, ovarian cysts and some brain tumours. 2. Malignant (cancerous) tumours, which are far more dangerous since they spread throughout the body, invade other tissues and eventually destroy them. Malignant tumours interfere with the normal functioning of the area where they have started to grow. They may block the intestines, lungs or blood vessels. Cells can break off and spread through the blood and lymphatic system to other parts of the body to form secondary growths. It is thought that several different control genes must mutate before a cell becomes cancerous. This can happen just by chance. The risk is increased by any factor that can cause mutation, including: Ionising radiation Ultraviolet radiation Various chemicals, including several contained in tar from tobacco smoke Some viruses, for example the human papilloma virus (HPV), which can cause cervical cancer. d. Explain the meanings of the terms haploid and diploid and the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction; Haploid one that possesses one complete set of chromosomes; the abbreviation for haploid is n

5 Diploid one that possesses two complete sets of chromosomes; the abbreviation for diploid is 2n. Meiosis the type of division that results in a halving of chromosomes number and a reshuffling of alleles; it occurs in the formation of gametes Most of the cells in your body are diploid cells. This means that they contain two complete sets of chromosomes. Each cell has 2 sets of 23 chromosomes, making 46 chromosomes altogether. In the original cell from which you began, one of these sets came from your father and one from your mother, as this cell divided by mitosis, each daughter cell obtained a complete copy of each set. The sperm and egg that fused at fertilisation to produce that original cell each contained only one set of chromosomes. They were haploid cells. They each contained 23 chromosomes. When they fused together, this produced a diploid zygote with two sets of chromosomes. Sperm and egg cells are produced from diploid cells by a special type of nuclear division called meiosis. In meiosis, the chromosomes are shared out so that each daughter cell gets only half of the original number of chromosomes. Meiosis produced haploid cells from diploid cells. In humans, meiosis only happens in testes and ovaries. Meiosis is sometimes known as reduction division, because it reduces the number of chromosomes in a cell by half.