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B io Factsheet www.curriculum-press.co.uk Number 237 Mitosis vs Meiosis Students often get overwhelmed by the details when they first begin to study mitosis and meiosis. This Factsheet: • Gives a simple overview of the processes • Summarises and compares mitosis and meiosis • Reviews some of the more straightforward exam questions which pop up on these topics Note: For a much more detailed account of mitosis and meiosis see Factsheets: 50: Sources of genetic variation 76: Eukaryotic cell cycle 168: Gamete formation in animals Mitosis vs. Meiosis The processes that your body must perform every day to keep you alive are carried out by millions of different cells, many with individual, specialised functions. However, these cells age and eventually die – so in order for you to survive, these cells must be continually replaced. To ensure this happens, the cells in your body are able to divide and produce copies of themselves. There are two different ways a cell can divide, either by mitosis or meiosis – and this Factsheet outlines how, when and why each process happens. Why are there two different ways a cell can divide? When a cell is preparing to divide, it first copies everything inside it – including your DNA. Your DNA is divided into 46 pieces called chromosomes, and is a mixture of information from your parents: you have 23 chromosomes from your father, and 23 from your mother. Every cell in your body has 46 chromosomes apart from your red blood cells (that don't contain DNA) and your gametes. These cells (sperm in males or eggs in females) only have 23 chromosomes. This means that during fertilisation, when a sperm cell fuses with an egg cell, the fertilised egg has 46 chromosomes in total. Every other cell type that forms from the fertilised egg as it develops into an embryo therefore has 46 chromosomes. If sperm and egg cells had 46 chromosomes each, then the cell formed when they fused would have 92 chromosomes in total, and would die. . So, you have two groups of cells in your body: those with 23 chromosomes (sperm or egg cells) or those with 46 chromosomes (most other cell types). Cells with 23 chromosomes are referred to as haploid, whereas cells with 46 chromosomes are referred to as diploid. Because these cells have different numbers of chromosomes, the way in which they divide is also different. This is why there are two different ways a cell can divide: mitosis maintains the number of chromsomes in a cell, meiosis halves the number. Typical Exam Question (a) Complete the table to indicate the processes in which mitosis occurs in humans. Tick the box to indicate that mitosis does occur, put a cross if it does not. (4). (b) Epithelial cell 48; Sperm 24; Brain cell 48; Repair of tissue Cancer Cell growth Replacement of skin cells Process 9 if mitosis does occur X if mitosis does not occur 9 9 X 9 9 if mitosis does occur X if mitosis does not occur Repair of tissue Cancer Cell growth Replacement of skin cells Process (a) Markscheme (b) The diploid number of chimpanzees is 48. How many chromosomes would the following cells contain? (3) Epithelial cell Sperm Brain cell 1 237. Mitosis Vs Meiosis Bio Factsheet www.curriculum-press.co.uk Fig 1 shows why sperm and egg cells have 23 chromosomes, how they fuse to form a cell with 46 chromosomes and how this cell can either divide by mitosis to produce more diploid cells, or divide by meiosis to form haploid cells. Fig 1. Mitosis maintains ploidy but Meiosis results in haploid cells Haploid gametes Sperm 23 chromosomes (5 shown) Egg 23 chromosomes (5 shown) Diploid Fertilised egg 46 chromosomes (10 shown) Mitosis Meiosis 46 chromosomes DIPLOID 23 chromosomes HAPLOID Diploid Diploid Diploid Diploid Haploid Haploid Diploid Haploid Haploid Meiotic cell division ONLY produces sperm cells and egg cells Mitotic cell division produces all types of cells EXCEPT sperm cells and egg cells The next two sections describe the key events that occur during the processes of mitosis and meiosis. These events are illustrated in Fig 2 and 3. 2 237. Mitosis Vs Meiosis Bio Factsheet www.curriculum-press.co.uk Mitosis 1. The 46 chromosomes in your cells are found in the nucleus in pairs. In each pair, one chromosome is from your father, and one from your mother. The chromosomes in each pair are called homologous chromosomes. Because you have 46 individual chromosomes in total, you have 23 pairs of homologous chromosomes. 2. When the cell is ready to divide, each chromosome is copied. The two copies of each chromosome are called sister chromatids, and are held together in the middle by a structure called the centromere. 3. The centromere fixes sister chromatids to fibres within the cell. These fibres separate the sister chromatids by pulling them to opposite sides of the cell. This process results in identical copies of the original homologous chromosomes being located at opposite sides of the cell (Fig. 2). 4. The cell then physically divides to form two new diploid cells, each with the same number of chromosomes as the original cell. Fig 2 Mitosis One pair of homologous chromosomes = from father = from mother Sister chromatids Each chromosome copies itself to form sister chromatids Centromere forms Centromere Extract from Chief Examiner’s Report It was surprising to find a significant number of candidates that confused the terms chromatid, centrosome and centromere. Weaker candidates were confused about which membrane was breaking down – some thought it was the cell membrane… The sister chromatids attach to fibres by the centromere and are pulled to opposite ends of the cell The fibres break down and the cell begins to divide Two identical DIPLOID cells are formed (46 chromosomes) N.B. Only one pair of homologous chromosomes are shown per cell for simplicity – the same process occurs to all 23 pairs found in the nucleus of a cell. 3 237. Mitosis Vs Meiosis Bio Factsheet www.curriculum-press.co.uk Meiosis 1. As in mitosis, when the cell is ready to divide it first copies its chromosomes. However, there is a key additional step in meiosis at this point that does NOT happen in mitosis. After the chromosomes have been copied, the sister chromatids of each homologous chromosome are found next to each other in the nucleus (Fig. 3). At this point, sister chromatids from DIFFERENT chromosomes in a homologous pair can cross over each other, forming a structure called a chiasma. 2. The reason chiasma occur in meiosis and not mitosis is because the formation of chiasma results in mixing of genetic information. When cells divide by mitosis they want to produce identical copies of themselves – so they do not want to mix chromosomes. However, meiosis produces sex cells which pass on your genetic information to the next generation. The mixing of genetic information in these cells introduces variation into a species, and is why you are not identical to either of your parents, but a mixture of characteristics from them both. 3. After the formation of the chiasma, the chromatids are pulled to opposite ends of the cell and two diploid cells are produced, as in mitosis. However, instead of ending here, during meiosis the two chromosomes of a homologous pair in the newly formed cells are reattached to the fibres, and again pulled to opposite ends of the cell (Fig. 3). 4. Because this time the chromosomes have not been copied, when the cell then divides each daughter cell only has one chromosome from each pair, and therefore the cell now has only 23 chromosomes. It is haploid – it only has half the number of chromosomes as the original cell. Fig 3. Meiosis Two diploid cells are produced by mitosis after mixing of genetic information by chiasma formation Chromatids reattach to fibres in the cell and are pulled to opposite ends without copying themselves Chiasma Four HAPLOID sex cells are produced (23 chromosomes) N.B. Only one pair of homologous chromosomes are shown per cell for simplicity – the same process occurs to all 23 pairs found in the nucleus of a cell. Summary: Mitosis vs. Meiosis MITOSIS MEIOSIS Number of chromosomes in daughter cells 46 23 Genetic definition Diploid Haploid Type of cell produced Cells required for growth, maintenance and repair Sex cells (sperm or egg cells) No. of times cell division occurs Once Twice No. of daughter cells produced Two Four Formation of chiasma? No Yes Acknowledgements: This Factsheet was researched and written by Bryony Graham 4