Cambridge International AS & A Level Biology Coursebook PDF

 CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK BEFORE YOU START During growth of multicellular organisms, the nucleus divides before the cell divides so that each new cell contains an identical nucleus. With a partner, discuss briefly why this is important. Then carry out the...

 CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK BEFORE YOU START During growth of multicellular organisms, the nucleus divides before the cell divides so that each new cell contains an identical nucleus. With a partner, discuss briefly why this is important. Then carry out the following exercise. Make a list of four structural features of the nucleus of eukaryotes. For each feature, outline its function (or an example of its function). WHY GROW OLD? Is it useful to prolong human life? The forerunners of modern chemists, the alchemists, thought so (Figure 5.1). They had two main aims: to discover how to transform ‘base’ metals (e.g. lead) into ‘noble’ metals (e.g. gold and silver) to discover the elixir of life, which would give eternal youth. By the early 20th century, scientists had relegated these aims to impossible dreams. Now, however, humans are once again challenging the idea that the process of ageing is inevitable. Why do organisms grow old and die? Interest in the process of ageing was rekindled with the discovery of telomeres in 1978. Telomeres are protective sequences of nucleotides found at the ends of chromosomes, which become shorter every Figure 5.1: A 19th-century oil painting showing an time a cell divides. A gradual degeneration of the alchemist at work. organism occurs, resulting in ageing. Some cells are able to replenish their telomeres Question for discussion using the enzyme telomerase. It is thought that If the ageing process could be slowed or cancer cells can do this and so remain immortal prevented, this would raise some important moral (will never die). It may therefore be possible to and ethical issues. Try to identify and discuss some prevent the ageing of normal cells by keeping the of these issues. enzyme telomerase active. In Chapter 6 you will learn how DNA can copy itself 5.1 Growth and accurately. In this chapter you will learn how whole cells can do the same. reproduction In Chapter 1 you saw that one of the most easily All living organisms grow and reproduce. Living recognised structures in eukaryotic cells is the nucleus. organisms are made of cells, so this means that cells The importance of the nucleus has been obvious ever must be able to grow and reproduce. Cells reproduce since it was realised that the nucleus always divides by dividing and passing on copies of their genes to before a cell divides. Each of the two daughter cells ‘daughter’ cells. The process must be very precisely therefore contains its own nucleus. This is important controlled so that no vital genetic information is lost. because the nucleus controls the cell’s activities. It does 124  5 The mitotic cell cycle this because it contains the genetic material, DNA, which Before their function was known, they were called acts as a set of instructions, or code, for life (Chapter 6). chromosomes because ‘chromo’ means coloured and ‘somes’ means bodies. All the cells in the bodies of multicellular organisms are genetically identical, apart from the reproductive The number of chromosomes is characteristic of cells known as gametes. This is because they all come the species. For example, in human cells there are from one cell, the zygote. This is the cell formed when 46 chromosomes; in fruit fly cells there are only 8 one gamete from your mother and one gamete from chromosomes. Figure 5.2 is a photograph of a set of your father fused. When the zygote starts the process chromosomes in the nucleus of a human cell. of growth, it divides into two cells with identical nuclei. This involves a type of nuclear division called mitosis. This process of nuclear division followed by cell division The structure of chromosomes continues to be repeated in a cycle called the mitotic Before studying nuclear division, you need to cell cycle to produce all the cells of your body, about understand a little about the structure of chromosomes. 30 trillion in an average human. Figure 5.3 is a simplified diagram of the structure of a You will study the process of mitosis and the mitotic cell chromosome just before cell division. cycle in this chapter. telomeres Genes for different characteristics – 5.2 Chromosomes in reality each chromosome is typically made up of several thousand genes. Just before a eukaryotic cell divides, a number of Centromere – holds the two threadlike structures called chromosomes gradually chromatids together. There are no become visible in the nucleus. They are easily seen, genes in this region. because they stain intensely with particular stains. Two identical chromatids make one chromosome. Each chromatid contains one DNA molecule. telomeres Figure 5.3: Simplified diagram of the structure of a chromosome. You can see that the chromosome at this stage is a double structure. It is made of two identical structures called chromatids, joined together. The two identical chromatids of one chromosome are known as sister chromatids. There are two chromatids because, during the period between nuclear divisions, known as interphase, each DNA molecule in a nucleus makes an identical copy of itself (Chapter 6, Section 6.3, DNA KEY WORD Figure 5.2: Photograph of a set of chromosomes in a chromatid: one of two identical parts of a human male, just before cell division. Each chromosome chromosome, held together by a centromere, is composed of two chromatids held together at the formed during interphase by the replication of centromere. Note the different sizes of the chromosomes the DNA strand and the different positions of the centromeres. 125  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK replication). Each chromatid contains one of these Histones help to package DNA into a smaller DNA copies. The sister chromatids are held together space. The packing ratio is a useful measure of the by a narrow region called the centromere, to form a degree of compactness achieved. If a 10 cm long single chromosome. The centromere could be anywhere piece of string was packed into a 5 cm long tube, along the length of the chromosome, but the position is the packing ratio would be 2 (2 cm of string per characteristic for a particular chromosome. cm of tube). The same idea can be applied to the problem of packing DNA into chromosomes. DNA is the molecule of inheritance and is made up of a series of genes. Each gene is one unit of inheritance. c Chromosomes vary in length. A chromosome The two DNA molecules, one in each of the sister 10 μm long was estimated to contain 8.7 cm of chromatids, are identical. This means the genes on the DNA. What is the packing ratio of DNA in chromatids are also identical. The fact that there are this chromosome? Show your working. two identical chromatids is the key to precise nuclear d There are 46 chromosomes in an adult human division. When cells divide, one chromatid goes into one cell. Their average length is about 6 μm. The daughter cell and one goes into the other daughter cell, total length of DNA in the 46 chromosomes making the daughter cells genetically identical. is about 1.8 m. What is the approximate overall packing ratio for DNA in human So much information is stored in DNA that it needs to be chromosomes? Show your working. a very long molecule. Although the molecule is only 2 nm wide, the total length of DNA in the 46 chromosomes e Explain briefly how histone proteins contribute of an adult human cell is about 1.8 metres. This has to to reducing the packing ratio for DNA. be packed into a nucleus which is only 6 μm in diameter. This is the equivalent of trying to get an 18 km length of string into a ball which is only 6 cm in diameter! A precise scaffolding made of protein molecules prevents the DNA 5.3 The cell cycle from getting tangled up into knots. The DNA is wound Mitosis is nuclear division that produces two genetically around the outside of these protein molecules. The identical daughter nuclei, each containing the same combination of DNA and proteins is called chromatin. number of chromosomes as the parent nucleus. Mitosis is Chromosomes are made of chromatin. Chemically part of a precisely controlled process called the cell cycle. speaking, most of the proteins are basic (the opposite of The cell cycle is the sequence of events that takes place acidic) and are of a type known as histones. Because they between one cell division and the next. It has three are basic, they can interact easily with DNA, which phases: interphase, nuclear division and cell division. is acidic. These are shown in Figure 5.5. Chromosomes also possess two more features essential During interphase, the cell grows to its normal size after for successful nuclear division: centromeres and cell division and carries out its normal functions. At telomeres. Centromeres are visible in Figures 5.2 and some point during interphase, a signal may be received 5.3. Telomeres are visible if chromosomes are stained that the cell should divide again. If this happens, the appropriately (Figure 5.4). Centromeres are discussed DNA in the nucleus replicates so that each chromosome with mitosis in Section 5.4 and the role of telomeres is consists of two identical chromatids. This phase of the discussed in Section 5.5. cell cycle is called the S phase – S stands for synthesis (of DNA). This is a relatively short phase. The gap after cell division and before the S phase is called the Question KEY WORDS 1 The primary structure of histone protein molecules is highly conserved during evolution, meaning there mitosis: the division of a nucleus into two so are extremely few changes over time (far fewer than that the two daughter cells have exactly the is usual for proteins). same number and type of chromosomes as the a State what is meant by the primary structure parent cell of a protein. cell cycle: the sequence of events that takes b What does the fact that histone molecules place from one cell division until the next; it is are highly conserved suggest about their made up of interphase, mitosis and cytokinesis functioning? 126  5 The mitotic cell cycle Figure 5.4: Fluorescent staining of human chromosome telomeres as seen with a light microscope. Chromosomes appear blue and telomeres appear pink (×4000). are usually repaired. Preparations are also made to begin S phase: DNA nuclear division the process of division. For example, there is a sharp replication by mitosis increase in production of the protein tubulin which is G2 needed to make microtubules for the mitotic spindle. M cell division S (cytokinesis) Nuclear division follows interphase. Nuclear division is 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 to create two genetically identical cells. In animal cells, cell division involves constriction of the cytoplasm interphase between the two new nuclei, a process called cytokinesis. G1 In plant cells, it involves the formation of a new cell wall between the two new nuclei. The length of the cell cycle is very variable, depending Figure 5.5: The mitotic cell cycle. DNA replication takes on environmental conditions and cell type. On average, place during interphase, the period between cell division root tip cells of onions divide once every 20 hours; and the next nuclear division: S = synthesis (of DNA); epithelial cells in the human intestine every 10 hours. G = gap; M = mitosis. G1 phase (G for gap). The gap after the S phase and before cell division is called the G2 phase. Interphase 5.4 Mitosis therefore consists of G1, S and G2. During G1, cells The process of mitosis is best described by annotated make the RNA, enzymes and other proteins needed for diagrams as shown in Figure 5.6. Although in reality growth. At the end of G1, the cell becomes committed to the process is continuous, it is usual to divide it into four dividing or not dividing. main stages for convenience, like four snapshots from During G2, the cell continues to grow and the new DNA a film. The four stages are called prophase, metaphase, that was made during the S phase is checked. Any errors anaphase and telophase. 127  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK Most nuclei contain many chromosomes, but the cell divides by constriction of the cytoplasm, a process diagrams in Figure 5.6 show a cell containing only called cytokinesis. As the cell changes shape, the four chromosomes for convenience. Colours are surface area of the cell increases as the two new cells used to show whether the chromosomes are from form, so new cell surface membrane has to be made. the female or male parent. An animal cell is used The behaviour of chromosomes in plant cells is identical as an example. Note that during late prophase the to that in animal cells. However, plant cells differ in nuclear envelope ‘disappears’. In fact, it breaks up two ways: into small vesicles which cannot be seen with a light microscope. It reassembles during telophase, as shown plant cells do not contain centrosomes in Figure 5.6. As a result, diagrams of metaphase and after nuclear division of a plant cell, a new cell wall anaphase do not show the nuclear envelope. At the must form between the daughter nuclei. end of telophase, after the nucleus has divided, the Early prophase Late prophase cell surface nuclear envelope ‘disappears’ membrane (it breaks up into small vesicles which are not visible cytoplasm with a light microscope) nucleolus nucleolus ‘disappears’ (forms intact nuclear part of several chromosomes) envelope chromosomes are seen to consist centromere with of two identical chromatids; two centrosomes attached kinetochores each chromatid contains one DNA produced by centrosomes moving to molecule replication of the chromosomes start to appear as the opposite ends of nucleus original centrosome chromatin coils up, becoming shorter where they form the centromere during S phase of and thicker; they are thick enough to poles of the spindle the cell cycle become visible when stained At the end of prophase a spindle is formed. Metaphase each centrosome reaches a pole; centrosomes help to organise production of the spindle each chromosome microtubules splits at the centromere spindle (made from microtubules) the chromatids chromosomes line up across start to be pulled the equator of the spindle; apart by microtubules they are attached by their centromeres to the spindle Anaphase Telophase nuclear envelope re-forming nucleolus chromatids have reached the poles of re-forming the spindle; they will now uncoil again (each chromatid contains one DNA molecule, which will replicate itself remains of during interphase before the next spindle which division) is breaking down cytokinesis – this is division of the cytoplasm and cell into two by centrosome – constriction from the edges of the cell will replicate during interphase, cell surface membrane chromatids move to opposite poles, before the next nuclear centromeres first, pulled by the microtubules division Figure 5.6: Mitosis and cytokinesis in an animal cell. 128  5 The mitotic cell cycle a Prophase. b Stage intermediate between prophase and metaphase. c Metaphase: the spindle fibres (microtubules) are d Early anaphase. now clearly visible, and the centrosomes are located at opposite ends of the spindle in the centre of a star-shaped arrangement of radiating microtubules. e Anaphase. f Telophase and cell division (cytokinesis). Figure 5.7: Stages of mitosis and cell division in an animal cell (whitefish) (×900). Chromosomes are stained darkly. 129  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK Figure 5.8: Longitudinal section (LS) of onion root tip showing stages of mitosis and cell division typical of plant cells (×400). Try to identify the stages based on information given in Figure 5.7. It is the behaviour of the chromosomes, though, that is of particular interest. Figure 5.7 (animal) microtubules and Figure 5.8 (plant) show photographs of mitosis as kinetochore kinetochore seen with a light microscope. centromere Centrosomes, centrioles and centromeres chromatid Centrosomes are located at the poles of the spindle, one chromosome at each pole. (The poles are the two ends of the spindle. The spindle gets its name from the fact that it is similar in shape to some spindles used in spinning – Sleeping microtubules Beauty pricked her finger on a spindle in the well-known shorten fairy tale.) As noted in Chapter 1, the centrosome is an organelle found in animal cells that acts as a microtubule organising centre (MTOC). Centrosomes are responsible for making the spindle, which is made of microtubules. The spindle is needed for separation of the chromatids. Each centrosome consists of a pair of centrioles surrounded by a large number of proteins. It is these Figure 5.9: Role of the centromere, kinetochores and proteins that control production of the microtubules, not microtubules during mitosis the centrioles. Plant mitosis occurs without centrosomes. The centromere holds the chromatids together (see KEY WORD Figures 5.2 and 5.3), but is also involved in the separation of chromatids during mitosis. During mitosis the centromere kinetochore: a protein structure found at the is the site of attachment of spindle microtubules. Each centromere of a chromatid to which microtubules metaphase chromosome has two kinetochores at its attach during nuclear division centromere, one on each chromatid (Figure 5.9). 130  5 The mitotic cell cycle The kinetochores are made of protein molecules which For a unicellular organism such as Amoeba, cell division connect the centromere to the spindle microtubules. inevitably results in reproduction. For multicellular Bundles of microtubules called spindle fibres extend organisms, new individuals may be produced which from the kinetochores to the poles of the spindle during bud off from the parent in various ways (Figure 5.10). mitosis. Construction of kinetochores begins before Budding is particularly common in plants. It is most nuclear division starts (during the S phase of the cell commonly a form of vegetative propagation in which a cycle) and they are lost again afterwards. bud on part of the stem simply grows a new plant. The new plant eventually becomes detached from the parent The microtubules attached to the kinetochore pull the and lives independently. The bud may be part of the stem kinetochore towards the pole of the spindle. The rest of an overwintering structure such as a bulb or tuber. The of the chromatid drags behind, giving the characteristic ability to generate whole organisms from single cells or > or < shape of chromatids during anaphase small groups of cells is important in biotechnology and (Figures 5.6−5.8). The pulling action is achieved by genetic engineering, and it is the basis of cloning. shortening of the microtubules, both from the pole end and from the kinetochore end. a Question 2 How can the microtubules be shortened? (Refer back to Chapter 1.) Importance of mitosis Growth of multicellular organisms The two daughter cells formed after mitosis have the same number of chromosomes as the parent cell and are genetically identical (that is, they are 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 the meristems (growing points) of plants. b Replacement of damaged or dead cells and repair of tissues by cell replacement This 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. The offspring are genetically identical to Figure 5.10: a Asexual reproduction by budding in Hydra the parents. Asexual reproduction can take many forms. (×60). Hydra lives in fresh water, catching its prey with the aid of its tentacles. The bud growing from its side is KEY WORD genetically identical to the parent and will eventually break free and live independently. b Asexual reproduction in asexual reproduction: the production of new Kalanchoe pinnata. The plant produces genetically identical individuals of a species by a single parent organism new individuals along the edges of its leaves. 131  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK Immune response d how many chromatids are present in the nucleus of each daughter cell after mitosis and The cloning of B- and T-lymphocytes during the cell division? immune response is dependent on mitosis (Chapter 11, Section 11.2, Cells of the immune system). e how many chromatids are present in the nucleus of a cell after replication of DNA? Questions 5 Draw a simple diagram of a cell which contains only one pair of chromosomes: 3 Outline how mitosis allows asexual reproduction to a at metaphase of mitosis take place. b at anaphase of mitosis. 4 Human cells contain 46 chromosomes. In the mitotic cell cycle of a human cell: 6 State two functions of centromeres during nuclear division. a how many chromatids are present as the cell enters mitosis? 7 Thin sections of adult mouse liver were prepared and the cells stained to show up the chromosomes. b how many DNA molecules are present? In a sample of 75 000 cells examined, 9 were found c how many kinetochores are present? to be in the process of mitosis. Calculate the length of the cell cycle in days in mouse liver cells, assuming that mitosis lasts one hour. PRACTICAL ACTIVITY 5.1 Investigating mitosis using a root tip squash Procedure Growth in plants is confined to regions known The root tips of garlic, onion, broad bean and as meristems. A convenient example to study is sunflower provide suitable material. Bulbs or seeds the root tip meristem. This lies just behind the can be grown suspended by a pin over water for a protective root cap. In this meristem there is a zone period of a week or two. The tips of the roots (about of cell division containing small cells in the process 1 cm) are removed and placed in a suitable stain of mitosis. such as warm, acidified acetic orcein. This stains chromosomes a deep purple. The stained root tip You may be able to study commercially prepared can be squashed into a sheet of cells on a glass permanent slides of root tips. You can also make slide, using a blunt instrument such as the end of the your own temporary slides. Cutting thin sections handle of a mounted needle. of plant material is tricky, but this is not needed if the squash technique is used. This involves staining You should be able to see and draw cells similar to the root tip, then gently squashing it. This spreads those shown in Figure 5.8 (but note that Figure 5.8 the cells out into a thin sheet in which individual shows a longitudinal section of a root tip, not a dividing cells can be clearly seen. squash). You could also use Figure 5.8 to make some annotated drawings of the different stages of mitosis. (See Practical Investigation 5.1 in the Practical Workbook for additional information.) possible to understand the reason for this without 5.5 The role of telomeres a detailed knowledge of replication.) If part of the DNA is not copied, that piece of information is lost. You have seen that DNA is replicated (copied) during At each subsequent division, another small section of the S phase of the cell cycle. The copying enzyme cannot information from the end of the DNA strand would be run to the end of a strand of DNA and complete the lost. Eventually, the loss of vital genes would result in replication – it stops a little short of the end. (It is not cell death. 132  5 The mitotic cell cycle The main function of telomeres is to ensure that the adding bases to telomeres is called telomerase. The main ends of the molecule are included in the replication function of telomeres is therefore to prevent the loss and not left out when DNA is replicated. Telomeres of genes during cell division and to allow continued are found at the ends of chromosomes (see Figure 5.11 replication of a cell. and also Figure 5.4). They have been compared with Some cells do not ‘top up’ their telomeres at each the plastic tips on the ends of shoe laces. Telomeres division. These tend to be fully differentiated are made of DNA with short base sequences that are (specialised) cells. With each division, their telomeres get repeated many times (‘multiple repeat sequences’). a little shorter until the vital DNA is no longer protected Telomeres work by making the DNA a bit longer. They and the cell dies. This could be one of the mechanisms have no useful information, but allow the copying of ageing, by which humans grow old and die. This, of enzyme to complete copying all the meaningful DNA. course, suggests that by somehow preventing the loss of As long as extra bases are added to the telomere during telomeres scientists might be able to slow down or even each cell cycle to replace those that are not copied, no prevent the process of ageing (see ‘Why grow old?’ at the vital information will be lost from the non-telomere beginning of the chapter). DNA and the cell will be able to continue dividing successfully. The enzyme that performs the role of 5.6 The role of stem cells A stem cell is a cell that can divide an unlimited number of times (by mitosis). When it divides, each new cell has the potential to remain a stem cell or to develop (differentiate) into a specialised cell such as a blood cell or a muscle cell. The power of a stem cell to produce different types of cell is variable and is referred to as its potency. Stem cells that can produce any type of cell are described as totipotent. The zygote formed by the fusion of a sperm with an egg at fertilisation is totipotent, as are all the cells up to the 16-cell stage of development in humans. After that, some cells become specialised to form the placenta, while others lose this ability but can form all the cells that will lead to the development of the embryo and later the adult. These embryonic stem cells are described as pluripotent. As tissues, organs and systems develop, cells become more and more specialised. There are more than 200 different types of cell in an adult human body. KEY WORDS telomere: repetitive sequence of DNA at the end of a chromosome that protects genes from the chromosome shortening that happens at Figure 5.11: Coloured scanning electron micrographs of each cell division human chromosomes showing the location of telomeres stem cell: a relatively unspecialised cell that at the ends of the chromosomes. Chromatids and retains the ability to divide an unlimited number centromeres are also clearly visible. Telomeres contain of times, and which has the potential to become a short repeated sequences of DNA. As cells replicate and specialised cell (such as a blood cell or muscle cell) age, the telomeres gradually get shorter. Stem cells are an exception. 133  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK Question Cancers illustrate the importance of controlling cell division precisely, because cancers are a result of 8 As a result of mitosis, all 200+ different types of uncontrolled mitosis. Cancerous cells divide repeatedly cell contain the same set of genes as the zygote. and form a tumour, which is an irregular mass of cells. Genes control the activities of cells. What does this Figure 5.12 shows a tumour in the lung of a patient who suggest about the mechanism by which cells become died of lung cancer compared to a healthy lung (from different? a patient who died from some other cause). Worldwide, lung cancer kills more people than any other cancer. The more ‘committed’ cells become to particular roles, Cancer cells usually show abnormal changes in shape the more they lose the ability to divide until, in the (Figure 5.13). adult, most cells do not divide. However, for growth and repair it is essential that small populations of stem cells Cancers start when changes occur in the genes that remain which can produce new cells. Adult stem cells control cell division. A change in any gene is called have already lost some of the potency associated with a mutation. The term for a mutated gene that causes embryonic stem cells and are no longer pluripotent. cancer is an oncogene, from the Greek word ‘onkos’ They are only able to produce a few types of cell and meaning bulk or mass. Mutations causing cancer can be may be described as multipotent. For example, the inherited but most of the mutations that cause cancers stem cells found in bone marrow are of this type. They occur over the course of the lifetime of an individual. can replicate any number of times, but can produce Mutations are not unusual events, and most of the only blood cells, such as red blood cells, monocytes, time they do not lead to cancer. Most mutated cells are neutrophils and lymphocytes. Mature blood cells have affected in some way that results in their early death a relatively short lifespan, so the existence of these stem or their destruction by the body’s immune system. cells is essential. For example, around 250 billion red Most cells can be replaced, so mutation usually has no blood cells and 20 billion white blood cells are lost and harmful effect on the body. Unfortunately, cancer cells must be replaced each day. manage to escape both cell death and destruction so, although the mutation may originally occur in only one In the adult, stem cells are found throughout the body – cell, it is passed on to all that cell’s descendants. By the for example, in the bone marrow, skin, gut, heart and time it is detected, a typical tumour usually contains brain. Research into stem cells has opened up some about a billion cancer cells. Any agent, such as asbestos, exciting medical applications. Stem cell therapy is the that causes cancer is called a carcinogen and is described introduction of new adult stem cells into damaged tissue as carcinogenic. to treat disease or injury. Bone marrow transplantation is an example of this therapy that has progressed beyond Although you do not need to know about different the experimental stage into routine medical practice. It types of tumour, you may be interested to know that is used to treat blood and bone marrow diseases, and not all tumours are cancerous. Some tumours do not blood cancers such as leukaemia. In the future, it is spread from their site of origin – these are known as hoped to be able to treat conditions such as diabetes, muscle and nerve damage, and brain disorders such as KEY WORDS Parkinson’s and Huntington’s diseases. Experiments with growing new tissues, or even organs, from isolated cancers: a group of diseases that result from stem cells in the laboratory have also been conducted. a breakdown in the usual control mechanisms that regulate cell division; certain cells divide uncontrollably and form tumours, from which cells may break away and form secondary 5.7 Cancers tumours in other areas of the body (metastasis) In high-income countries, cancers cause roughly one in mutation: a random change in the base four deaths. Globally, cancers account for about one sequence (structure) of DNA (a gene in six deaths (9.6 million people in 2018). This makes mutation), or in the structure and/or number of cancers second only to cardiovascular disease as a cause chromosomes (a chromosome mutation) of death. There are more than 200 different forms of carcinogen: a substance or environmental factor cancer, and the medical profession no longer thinks of that can cause cancer cancers as a single disease. 134  5 The mitotic cell cycle a a b b Figure 5.13: a False-colour scanning electron micrograph of a cancer cell (red) and white blood cells (orange and yellow). White blood cells gather at cancer sites as an immune response. They are beginning to flow around the cancer cell, which they will kill using toxic chemicals (×4500). b False-colour transmission electron micrograph (TEM) of abnormal white blood cells isolated from the blood of a person with hairy-cell leukaemia. The white blood cells are covered with characteristic hair- like cytoplasmic projections. Leukaemia is a disease in which the bone marrow and other blood-forming organs produce too many of certain types of white blood cells. These immature or abnormal cells suppress the normal Figure 5.12: a Lung of a patient who died of lung cancer, production of white and red blood cells, and increase the showing rounded deposits of tumour (white area at bottom patient’s susceptibility to infection (×6400). of picture). Black tarry deposits throughout the lung show the patient was a heavy smoker. b Section of a healthy human lung. No black tar deposits are visible. 135  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK benign tumours; warts are a good example. It is only it can be very hard to find the secondary cancers and tumours that spread through the body, invading and remove them. destroying other tissues, that cause cancer. These are The steps involved in the development of cancer are known as malignant tumours. Malignant tumours shown in Figure 5.14. 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 Question to other parts of the body to form secondary growths. 9 Research is being carried out into ways of The spread of cancers in this way is called metastasis. It inactivating the enzyme telomerase in cancer cells. is the most dangerous characteristic of cancer because Explain the reason for this. Carcinogens cause mutations. e.g. UV light 2 Cancerous cell 1 Oncogenes does not respond tar in tobacco transformed to signals from smoke by carcinogens. other cells so asbestos continues to divide. X-rays 3 Mitosis 6 Tumour gets bigger. Cells change their 4 Cancerous cells characteristics and not removed by look different under immune system. 5 Rapid mitosis the microscope. absorption of nutrients 7 Tumour supplied 8 Metastasis. Tumour with blood and cells invade other lymph vessels. tissues. Secondary Tumour cells spread cancers form in blood and lymph throughout the to other parts of the body. body. Figure 5.14: Stages in the development of cancer. REFLECTION Make a set of two pairs of chromosomes (one short pair, one long pair so they can easily be distinguished) as in Figure 5.6 late prophase. Use pipe-cleaners or Pop or Poppit beads which can be joined together to represent chromatids. Use two different colours if possible (to represent their origins from male and female parents), though this is not essential. Use these model chromosomes to test your understanding of the stages of mitosis. It is useful to draw a large spindle on a large sheet of paper on which the model chromosomes can be moved appropriately. 136  5 The mitotic cell cycle CONTINUED Personal reflection question What did you enjoy about this activity? What parts of it did you particularly like or dislike? Why? Will it help you to remember the process of mitosis? Final reflection Discuss with a friend which, if any, parts of Chapter 5 you need to: read through again to make sure you really understand seek more guidance on, even after going over it again. SUMMARY Chromosomes are made of chromatin. Chromatin consists mainly of DNA wrapped around basic protein molecules called histones. During nuclear division chromosomes become visible and are seen to consist of two chromatids held together by a centromere. Each chromatid contains one DNA molecule. Growth of a multicellular organism is a result of cells dividing to produce genetically identical daughter cells. During cell division, the nucleus divides first, followed by division of the whole cell. Division of a nucleus to produce two genetically identical nuclei is achieved by the process of mitosis. Mitosis is divided into four phases: prophase, metaphase, anaphase and telophase. Mitosis is used in growth, repair, asexual reproduction and cloning of cells during an immune response. The period from one cell division to the next is called the cell cycle. It has four phases: G1 is the first growth phase after cell division; S phase is when the DNA replicates; G2 is a second growth phase; M phase is when nuclear division takes place (followed by cell division). The ends of chromosomes are capped with special regions of DNA known as telomeres. Telomeres are needed to prevent the loss of genes from the ends of chromosomes during replication of DNA. Many specialised cells lose the ability to divide, but certain cells known as stem cells retain this ability. Stem cells are essential for growth from zygote to adult and for cell replacement and tissue repair in the adult. The behaviour of chromosomes during mitosis can be observed in stained preparations of root tips, either in section or in squashes of whole root tips. Cancers are tumours resulting from repeated and uncontrolled mitosis. They are thought to start as the result of mutation. 137