Biology Student Textbook Grade 10 PDF

Biology Student Textbook Grade 10 Author: Ann Fullick Adviser: Alemu Asfaw Evaluators: Solomon Belayneh Getachew Bogale Silas Araya Federal Democratic Republic of Ethiopia Ministry of Education Biotechnology...

Biology Student Textbook Grade 10 Author: Ann Fullick Adviser: Alemu Asfaw Evaluators: Solomon Belayneh Getachew Bogale Silas Araya Federal Democratic Republic of Ethiopia Ministry of Education Biotechnology Unit 1 Contents Section Learning competencies 1.1 What is Define biotechnology. biotechnology? Discuss the significance of (page 1) biotechnology. Explain some of the traditional uses of biotechnology, for example, in preparing bread, yoghurt, cheese and beer. 1.2 New Identify new applications of applications of biotechnology in agriculture, biotechnology food production, medicine and energy (page 7) production. 1.1 What is biotechnology? By the end of this section you should be able to: Define biotechnology. KEY WORDS Discuss the significance of biotechnology. biotechnology use of Explain some of the traditional uses of biotechnology, for micro-organisms to make example, in preparing bread, yoghurt, cheese and beer. things that people want, often involving industrial production Biology, as you discovered last year, is the study of living organisms. Now, at the beginning of your grade 10 biology course, you are microbiology study of going to be studying biotechnology. micro-organisms and their effect on humans Biotechnology is the use of micro-organisms to make things that people want, often involving industrial production. bacteria unicellular micro- organisms Biotechnology has always been extremely important. It involves viruses sub-microscopic ways of making and preserving foods and making alcoholic drinks. Traditional applications of biotechnology involve brewing infectious agents that are beers, making wines, making bread, and making cheese and unable to grow or reproduce yoghurt. Modern applications of biotechnology include using outside a host cell genetic engineering to change crops and animals; producing new fungi simple organisms, medicines; and helping to provide new energy sources. It has often microscopic, that enormous significance in helping people to improve and control cannot photosynthesise their lives. and feed as parasites or Biotechnology is based on microbiology. As you know, saprophytes microbiology is the study of micro-organisms – tiny living protoctista unicellular organisms including bacteria, viruses, fungi and protoctista, organisms Grade 10 1 Unit 1: Biotechnology which are usually too small to be seen with the naked eye. Some micro-organisms cause disease; others are enormously useful to people – for example, they play a vital role in decay and the recycling of nutrients in the environment. With the arrival of new technologies such as genetic engineering, micro-organisms are becoming more useful all the time. Not all types of micro-organism are used in biotechnology. The main groups are bacteria and fungi, although viruses are being used more and more for genetic engineering. Just to remind you – bacteria are single-celled organisms that are much smaller than the smallest plant and animal cells. In ideal conditions, they can reproduce very quickly. Viruses are even smaller than bacteria. They do not carry out any of the normal functions of living things. Moulds and yeasts are both fungi – living organisms which obtain KEY WORDS their food from other dead or living organisms. Yeasts are single- anaerobically without celled organisms, while moulds are made up of thin, thread-like oxygen structures called hyphae. fermentation anaerobic Slime capsule Genetic Cell Flagellum Protein respiration in yeast that (not always present) material membrane (not always coat present) produces ethanol Genetic material Cell wall A typical bacterium A typical virus Figure 1.1 Bacteria (left) and viruses (right) are both used in biotechnology. Developing new applications of biotechnology is one of the fastest- growing industries around the world, and is beginning to grow in Ethiopia too. It is easy to think that biotechnology is very new, but much of it has been in use for thousands of years. People have used micro-organisms to make food and drink almost as far back as our Nucleus records go. Bacteria are used in the manufacture of irgo (yoghurt) Cell wall and Ayib (cheese). Yeast is used to make many traditional Ethiopian Cytoplasm fermented foods, including injera, and also to produce alcoholic drinks, such as tej and tella. Figure 1.2 Yeast cells – these Traditional technology using yeast microscopic organisms have been One of the most useful micro-organisms is yeast. The yeasts are useful to us for centuries. single-celled organisms. Each yeast cell has a nucleus, cytoplasm and a membrane surrounded by a cell wall. The main way in which yeasts reproduce is by asexual budding – splitting into two, to form new yeast cells. Just one gram of yeast contains about 25 billion cells! When yeasts have plenty of oxygen, they respire aerobically, breaking down sugar to provide energy for the cells, and producing water and carbon dioxide as waste products. But yeasts are useful because they can also respire anaerobically. When yeast cells break down sugar in the absence of oxygen, they produce ethanol (commonly referred to as alcohol) and carbon dioxide. 2 Grade 10  UNIT 1: Biotechnology Aerobic respiration provides more energy than anaerobic respiration, allowing yeast cells to grow and reproduce. However, once they exist in large numbers, yeast cells can survive for a long time in low-oxygen conditions, and will break down all the available sugar to produce ethanol. The anaerobic respiration of yeast is sometimes referred to as fermentation. We have used yeast for making bread and alcoholic drinks almost as far back as human records go. We know yeast was used to make bread in Egypt in 4000 BC, and some ancient wine found in Iran dates back to 5400–5000 BC. Here in Ethiopia yeast (known locally as ershoo) has been used to make injera (bread) possibly since even earlier times. Injera needs yeast When you make injera, grind your teff or barley and then add water. Mix it well and leave the dough at room temperature for about two days. Natural yeasts start to grow and respire in the dough. At first the yeast respires aerobically, although this may change to anaerobic respiration. The yeast produces carbon dioxide, making the mix rise a little and giving it a tangy flavour. When you cook the mixture, the bubbles of gas expand in the high temperature, giving injera its typical texture, which is so good for soaking up the food. The yeasts are killed during the cooking process. Activity 1.1: Making injera You are going to investigate the factors that affect the growth of yeast in injera mix. Yeasts are living organisms – by changing their conditions, you can change the speed at which they respire and produce carbon dioxide in your dough. 1. Mix up some teff or barley flour with water, then divide your mix into three containers. 2. Leave one container at room temperature in the normal way for making injera. 3. Put the second container in the coolest place you have available. If there is a fridge, put your mixture in it. Figure 1.3 When you make 4. Take your third mixture and heat it in a water bath to at injera, the mix needs to be left least 50 °C. Make sure the mixture itself reaches 50 °C for for at least two or three days so 5 minutes. Then allow the mixture to cool down to room the yeast can make the carbon temperature and leave it with the other sample at room dioxide gas needed to produce the temperature. holes in the bread. 5. After two to three days, observe the three mixtures very carefully. Describe their appearance and smell. 6. Then cook each of the three samples. Observe their appearance very carefully as they cook – the number of air bubbles that appear, and the texture of the bread that results. 7. Write up your experiment carefully, and explain your observations. Grade 10 3 Unit 1: Biotechnology Making alcoholic drinks When fruits fall to the ground and begin to decay, wild yeasts on their skin break down the fruit sugar to form ethanol and carbon dioxide. These fermented fruits can cause animals to become drunk when they eat them – and this is probably how our ancestors discovered alcohol! We now use this same reaction in a controlled way to make drinks such as beer, tej and wine. In both cases the yeast has to be supplied with carbohydrates to act as an energy source for respiration. Tej is one of the oldest drinks known to the human race – it has been known since at least 400 BC. When you make tej, you need honey, water and gesho leaf or gesho stick. Gesho gives a bitter edge to the brew, and wild yeasts found on the plant start the fermentation going. The yeasts use the honey as a source of food. As the yeast colonies grow they start to respire anaerobically, and this produces ethanol and carbon dioxide. The alcohol content of Figure 1.4 Here in Ethiopia tej varies from about 6 to 11%. Tej and tella are the most commonly people have been drinking tej consumed alcoholic drinks in Ethiopia. for thousands of years – the fermentation process by which In contrast, winemaking uses natural sugar, found in fruit such as it is made is an example of grapes, as the energy source for the yeast. You press the fruit and biotechnology. mix the juice with yeast and water. You then let the yeast respire anaerobically until most of the sugar has been used up. At this stage, you filter the wine to remove the yeast and put it in bottles, where it will remain for some time to mature before it is sold. Most commercially sold wine is made from grapes, but wine can be made from almost any fruit or vegetable – the yeast doesn’t care where the sugar it uses comes from! Interestingly, alcohol in large amounts is poisonous to yeast as well as to people. This is why the alcohol content of wine is rarely more than 14% – once it gets much higher, it kills all the yeast and stops the fermentation. Remember: yeast can respire aerobically in bread making, but must respire anaerobically to make alcoholic drinks. Activity 1.2: Visiting a brewery/tej production One way to become more familiar with the role of yeast in brewing is to visit a place where beer is brewed, or where tej or other alcoholic drinks are made in your neighbourhood. You can discuss with the craftsmen what might affect the growth of the yeast, and how much ethanol they produce during anaerobic respiration. Discuss what you discover in class with your teacher and other students. Food production using bacteria People began to domesticate animals quite early in human history. They soon realised that the milk female animals made for their 4 Grade 10  UNIT 1: Biotechnology babies could be used as food for us too. People have used milk from many different types of animal, including cows, sheep, goats, camels and horses. However, there is one big drawback in using milk as part of the diet – it very rapidly goes off, smelling and tasting disgusting! It didn’t take people long to discover ways of changing milk, turning it into milk-based foods with a much longer life than the original milk. These changes depend on the action of micro- organisms. Yoghurt has long been a staple part of the diet in the Middle East and Africa including Ethiopia. Cheese has also been around for a very long time almost all over the world. Making yoghurt (irgo) Figure 1.5 Many different Traditionally, yoghurt is fermented whole milk. Yoghurt is formed animals, including cows, camels, by the action of bacteria on the lactose (milk sugar) in the milk. horses, sheep and goats, are used for milking. To make yoghurt, you add a starter culture of the right kind of bacteria to warm milk. Often this starter culture is just a small amount of yoghurt you have already made. The mixture needs to be warm so the bacteria begin to grow, reproduce and ferment. As the bacteria break down the lactose in the milk, they produce lactic acid, which gives the yoghurt its sharp, tangy taste. This is known as lactic fermentation. The lactic acid produced by the bacteria causes the milk to clot and solidify into yoghurt. The action of the bacteria KEY WORD also gives the yoghurt a smooth, thick texture. Once the yoghurt- forming bacteria have worked on the milk, they also help prevent lactic acid product of the growth of other bacteria that normally send the milk bad. anaerobic respiration in Yoghurt, if it is kept cool, will last almost three weeks before it goes animal cells bad. Ordinary milk lasts only a few days – and then only if it’s kept really cold. Once you have made your basic yoghurt, you can mix in flavourings, spices and fruit. In Ethiopia, we often make yoghurt (irgo) in gourds or hollowed- out wooden vessels that have had sticks burned in them. These sticks are obtained from different plants such as the olive tree, and so on. As well as giving the yoghurt a pleasant flavour, this disinfects the vessel so that only good bacteria grow in the milk. Activity 1.3: Making yoghurt Discuss different factors that might affect the such as lemon juice, and see how this making of yoghurt. Mix up a class starter culture affects the process. of milk and a small amount of live yoghurt. Discuss 2. After two or three days, make careful with your teacher what factors might affect how observations of your yoghurt culture. the bacteria work, and why. Plan your experiment carefully. 3. Each group should share their results with the class and write them up on 1. Working in groups, try different temperature the board. conditions and see how they affect the formation of yoghurt. You might also change 4. Write up your experiments carefully, the pH of the mixture by adding substances and explain your observations. Grade 10 5 Unit 1: Biotechnology Cheese making Like yoghurt making, cheese making depends on the reactions of bacteria with milk changing the texture and taste, and also preserving the milk. Cheese making is very successful in preserving milk, and some cheeses can survive for years without decay. Around 900 different types of cheese are made around the world, but the basis of the production method is the same for them all. Just as in yoghurt making, you add a starter culture of bacteria to warm milk. The difference is in the type of bacteria added. The bacteria in cheese making also convert lactose to lactic acid, but Figure 1.6 Curds are formed they make much more lactic acid. As a result, the solid part (curds) by the action of bacteria on the is much more solid than in yoghurt. Enzymes are also added to milk – this is the basis of ayib, increase the separation of the milk. These often come from the although we often add seasoning stomachs of calves or other young animals. When it has completely and flavours before we eat it. curdled, you can separate the curds from the liquid whey (known as aguat here in Ethiopia). Then you can use the curds for cheese making. The whey is often used in other dishes. The curds can be used fresh, and can be seasoned or flavoured. Did you know? This is the basis of ayib. Alternatively, you can cut and mix the No one really knows curds with salt along with other bacteria or even moulds, before when yoghurt making you press them and leave them to dry out. The bacteria and moulds first started. It seems added at this stage of the process are very important. They affect to have come from the development of the final flavour and texture of the cheese as it Turkey. Legend has it ripens – a process that may take months or years, depending on the that travellers took some type of cheese being made. This is how the majority of cheeses are milk on a journey in a made in countries such as the UK and the USA. bag made of a sheep’s Here in Ethiopia cheese is traditionally made by first making stomach. The heat of the yoghurt from fresh milk, extracting the butter by continuous sun and the bacteria in agitation, and finally boiling the remaining part to make the cheese. the stomach worked on the milk, and in the cool of the desert night they Review questions discovered the bag was 1. Which of the following statements about biotechnology is not full of yoghurt! Carrying true? milk in the stomach of A Biotechnology is the use of micro-organisms to make things a camel has a similar that people want. effect. B Biotechnology is a new, modern concept. C Biotechnology is based on microbiology. D Biotechnology is one of the fastest-growing industries in the world. 2. How many cells does one gram of yeast contain? A about 10 million B 25 million C 4000 D about 25 billion 6 Grade 10  UNIT 1: Biotechnology 3. Which two of the following are the waste products of anaerobic respiration in yeast? A sugar B carbon dioxide C water D ethanol 4. Which of the following statements about lactic fermentation is not true? A It gives yoghurt its sharp, tangy taste. B It gives yoghurt a smooth, thick texture. C It means the yoghurt will only last a few days. D It causes the milk to clot and solidify into yoghurt. 1.2 New applications of biotechnology By the end of this section you should be able to: Identify new applications of biotechnology in agriculture, food production, medicine and energy production. Around the world traditional biotechnologies – brewing, winemaking, bread making and the production of yoghurt and cheese – are extremely important. In many countries they are not only carried out in the home on a small scale, they also take place in massive industrial production processes. Some new applications of biotechnology also take place in an KEY WORDS industrial setting. Many advances in agriculture are the result of one of the most important new areas of biotechnology – genetic genetic engineering/ engineering (also known as genetic modification). Genetic genetic modification engineering is used to change an organism and give it new process of inserting new characteristics which people want to see. genetic information into existing cells in order to What is genetic engineering? modify a specific organism Genetic engineering involves changing the genetic material of for the purpose of changing an organism. Genetic material carries the instructions for a new its characteristics organism, found in the nucleus of every cell. You take a small piece of information – a gene – from one organism and transfer it to the genetic material of a completely different organism. So, for example, a gene from one of your human cells can be ‘cut out’ using enzymes, and transferred to the cell of a bacterium. Your gene carries on making a human protein, even though it is now in a bacterium. There is a limit to the types of protein that bacteria are capable of making. Scientists have found that genes from one organism can be transferred to the cells of another type of animal or plant at an early stage of their development. As the animal or plant grows, Grade 10 7 Unit 1: Biotechnology Human cell with Bacterium with ring it develops with the new, desired characteristics from the other insulin gene in of DNA called a its DNA plasmid organism. The technology A lot of new biotechnology relies on growing large numbers of Plasmid Insulin gene taken out of micro-organisms on an industrial scale in large vessels, known as cut out of bacterium DNA by an and split fermenters. If a lot of micro-organisms are grown together, they can enzyme open by an enzyme easily use up all the oxygen available and even poison each other with waste products. Industrial fermenters usually have a range Insulin gene inserted into of features to overcome the problems that stop a culture growing plasmid by another enzyme satisfactorily. They react to changes, keeping the conditions as stable Plasmid with as possible. This, in turn, means we can obtain the maximum yield. insulin gene in it taken up Industrial fermenters usually have: by bacterium Bacterium multiples an oxygen supply – to provide oxygen for respiration by the many times micro-organisms a stirrer – to keep the micro-organisms in suspension, maintain The insulin gene is switched on and the insulin an even temperature, and make sure oxygen and food are is harvested distributed evenly through the culture a water-cooled jacket – to remove the excess heat produced by Figure 1.7 The basic process of the respiring micro-organisms – any rise in temperature is used genetic engineering to heat the water, which is constantly removed and replaced with more cold water measuring instruments – for continuous monitoring of factors such as pH and temperature so that adjustments can be made if necessary Probe to measure temperature, pH, etc. Motor Warm water out Paddle stirrer Water-cooled jacket to maintain the correct temperature Figure 1.8 The design of fermenters is improving all the Cold time as new ways of keeping water in conditions inside the fermenter as Outlet for stable as possible are developed all harvesting Oxygen the time. the culture There are many different areas where new biotechnology – and in particular genetic engineering – is very important. Some of them are summarised on the following pages. 8 Grade 10  UNIT 1: Biotechnology Applications of biology in agriculture For many years, we have used selective breeding to change our livestock and crops. We select animals or plants with characteristics we want, such as big grains, resistance to disease or plenty of milk, and breed from them. Gradually, the characteristics change to what we want. But selective breeding takes time, and there are limitations to it. You will be looking at this in more detail in the next unit. By using genetic engineering, we can introduce new characteristics very rapidly. Engineered genes can be used to improve the growth rates of plants and animals. They can be used to improve the food value of crops. Genetic engineering has been used to make crop plants that are resistant to drought and to disease, and to produce plants that make their own pesticide chemicals. Glowing genes from jellyfish have even been used to produce crop plants that give off a blue light when they are attacked by insects so the farmer knows they need spraying! This means the farmer has to use less insecticide (chemicals that kill insects), which saves money and protects the environment. Much of the research into genetically engineered crops and animals has been carried out in countries like the UK and the KEY WORD USA. However, here in Ethiopia our scientists are increasingly working with these new technologies. At the Ethiopian Institute of mycoprotein fungal protein Agricultural Research and Addis Ababa University, scientists are analysing the genes of many of our most important crop plants, including teff. The Ethiopian Agricultural Research Institute is using modern biotechnology to improve teff, coffee, fruit plants and some of our forest trees for commercial cultivation. However, there are some possible problems with the new biotechnologies, so we must be careful. Insects may become pesticide-resistant if they eat a constant diet of pesticide-forming plants. Genes from genetically modified plants and animals might spread into the wildlife of the countryside, which could make difficulties. Genetically modified crops are often not fertile, which means farmers have to buy new seed each year. But if these problems can be overcome, biotechnology offers us the hope of better crops and more food, both for our own people and to sell internationally. Applications of biology in food The new biotechnology is often used in food processing. One of the biggest changes is that enzymes are produced by genetically engineered bacteria, and the enzymes are then used in the production of processed foods and drinks. Enzymes are used to clarify beer. They are used to break down starch and convert the sugars into glucose syrup. They are used to make meat more tender, and to break down the food used to make commercial baby food. Figure 1.9 Plant technologists Biotechnology plays a big part in food production. It has even at EIAR have improved different been used to create a completely new food based on fungi, which crops like this teff to ensure food has been developed in the UK. It is known as mycoprotein, which security. Grade 10 9 Unit 1: Biotechnology means ‘protein from fungus’. It is produced using the fungus Fusarium, which grows and reproduces rapidly on a relatively cheap sugar syrup in large, specialised fermenters. It needs aerobic conditions to grow successfully, and can then double its mass every five hours or so. The fungal biomass is harvested and purified. Then it is dried and processed to make mycoprotein, a pale yellow solid with a faint taste of mushrooms. On its own, it has very little flavour, but mycoprotein can be given a range of tastes and flavours to make it similar to a whole range of familiar foods. It is a high- protein, low-fat meat substitute used by vegetarians, people who want to reduce the fat in their diet, and people who just want to eat cheap protein. When mycoprotein was first developed, people thought a world food shortage was on its way. They were looking for new ways to make protein cheaply and efficiently. The food shortage never happened, but the fungus-based food continued. It is versatile, high in protein and fibre, and low in fat and calories, and so has found a secure and healthy place on the meal tables of the developed world. Scientists in Ethiopia and elsewhere are trying to develop a local equivalent of mycoprotein, looking at different plants and fungi that have a relatively high protein content. Applications of biology in medicine Biotechnology is extremely important in modern medicine. It is used to develop vaccines and to create new medicines. The first medicine that really relied on microbiology was penicillin. This antibiotic is one of the best-known medicines in the world, and has revolutionised medicine in the time since it was first manufactured. We are going to look more closely at this story because it shows clearly how changes in biotechnology make life easier. In 1928 Alexander Fleming, a young researcher at St Mary’s Hospital in the UK, left some plates on which he was culturing bacteria uncovered near an open window. When he remembered to look at them, he found bacteria were growing on the surface of his dishes, as he expected. But Fleming also noticed spots of mould growing, and around these were clear areas of agar. The bacteria Figure 1.10 The keen eyes of were no longer growing there. Whatever had blown in through the Alexander Fleming noticed the window and started growing on his plates was producing a chemical clear areas on his plates, and he that killed the bacteria. realised he had made a discovery Fleming found that the micro-organism which had invaded his of enormous potential. Petri dishes was a common mould called Penicillium notatum. He managed to extract a tiny amount of the chemical that killed the bacteria, and used it to treat an infected wound. He called his extract penicillin. But it was very hard to extract, and very unstable once extracted, so Fleming decided he wouldn’t be able to obtain useful amounts of penicillin from his mould. Howard Florey and Ernst Chain were working at Oxford University in the UK in a desperate search to find a drug to kill the bacteria 10 Grade 10  UNIT 1: Biotechnology that infected wounds suffered by soldiers in the Second World War. They used Fleming’s mould and finally managed to extract enough penicillin to show what it could do. They wanted to manufacture it in large amounts, but the yield of drug was very poor. Fleming’s original mould, Penicillium notatum, was extremely difficult to grow in large cultures, yielding only one part penicillin for every million parts of fermentation broth. Then a mould growing on a melon in a market was found to yield 200 times more penicillin than the original. What is more, it grew relatively easily in deep tanks, making large-scale production possible. By 1945, enough penicillin was made each year to treat seven million people. Modern strains of Penicillium mould give even higher yields. We grow the mould in a sterilised medium, containing sugar, amino acids, mineral salts and other nutrients, which is made from soaking corn in water. It is grown in huge 10 000 dm3 fermenters, and still saves many thousands of lives every year. When genetically engineered bacteria are cultured on a large scale, they can make huge quantities of protein. We now use them to make a number of drugs and hormones used as medicines. These genetically engineered bacteria make exactly the protein needed, in exactly the amounts needed, and in a very pure form. For example, people with diabetes need supplies of the hormone insulin. It used to be extracted from the pancreas of pigs and cattle, but it wasn’t quite the same as human insulin, and the supply was quite variable. Both problems have now been solved by the introduction of genetically engineered bacteria that can make human insulin. Biotechnology also makes it possible to develop vaccines more easily. A number of sheep and other mammals have been engineered to produce life-saving human proteins in their milk. These are much more complex proteins than those produced by bacteria, and have the potential to save many lives. For example, genetically modified sheep can make special blood-clotting proteins in their milk. These can be used for people with haemophilia, so they are no longer at risk from receiving contaminated blood. Applications of biology in energy production Everyone needs fuel of some sort to provide them with energy. It might be direct energy such as heat to cook on, or it might be indirect energy – heat being used to make electricity, for example. However, there is only a limited amount of fossil fuels such as coal, Figure 1.11 Diabetes is a oil and gas for us to use. Even wood and peat are becoming scarce. dangerous condition if it is Around the world, we all need other, renewable forms of fuel. The not controlled with insulin. generation of biogas from human and animal waste is becoming Biotechnology is making pure increasingly important in both the developing and the developed human insulin much more easily world. This depends on biotechnology. available. Grade 10 11 Unit 1: Biotechnology What is biogas? Biogas is a flammable mixture of gases, formed when bacteria break down plant material, or the waste products of animals, in anaerobic conditions. It is mainly methane, but the composition of the mixture varies depending on what is put into the generator and which bacteria are present. Table 1.1 The components of biogas Components Percentage in the mixture by volume Figure 1.12 Biogas generators Methane 50–80 like this have made an enormous difference to many families by producing cheap and readily Carbon dioxide 15–45 available fuel. Water 5 Hydrogen sulphide 0–3 Other gases including 0–1 hydrogen Around the world, millions of tonnes of faeces and urine are produced by animals like cows, pigs, sheep and chickens. We produce our fair share of waste materials too! Also, in many parts of the world, plant material grows very rapidly. Both the plant material and the animal waste make up a potentially enormous energy KEY WORDS resource – but how can we use it? biogas generator/ To produce biogas, you collect dung or plant material, which digester takes in waste contains a high level of carbohydrates, and put it into a biogas material or plants, and generator or digester. Then you add a mixed population of many different types of bacteria which are needed to digest the biogas and useful fertilisers carbohydrate. The bacteria you use are similar to those in the come out the other end stomachs of ruminants such as cows or sheep. Some of the bacteria exothermic reaction that break down the cellulose in plant cell walls. Others break down the produces heat energy sugars formed, to produce methane and other gases. The biogas produced is passed along a pipe into your home, where you burn it to produce heat, light or refrigeration. The bacteria involved in biogas production work best at a temperature of around 30 °C, so biogas generators tend to work best in hot countries. However, the process generates heat (the reactions are exothermic). This means that if you put some heat energy in at the beginning to start things off, and have your generator well insulated to prevent heat loss, biogas generators will work anywhere. Some generators are so simple, they are little more than a big plastic bag and some pipes. Yet they can make a big difference to our lives. 12 Grade 10  UNIT 1: Biotechnology Methane for use in cooking, heating or refrigeration Dung from animals or people Farm waste Slurry, which can be used as fertiliser Garden for gardens rubbish or farms Biogas generator Figure 1.13 Biogas generators take in waste material or plants, and biogas and useful fertilisers come out the other end. This simple generator, involving a big plastic bag, is being tried in Addis Ababa and the surrounding area. Scaling up the process Did you know? At the moment, most biogas generators around the world operate on a relatively small scale, supplying the energy needs of one family, Under ideal conditions, a farm, or at most a village. What you put into your small generator 10 kg of dry dung can has a big effect on what comes out. produce 3 m3 of biogas. Biogas units are widely used in China, where there are well over That will give you three 7 million biogas units, producing as much energy as 22 million hours’ cooking, three tonnes of coal. Waste vegetables, animal dung and human waste are hours’ lighting or 24 the main raw materials. These Chinese digesters produce excellent hours of running a fertiliser, but relatively low-quality biogas. refrigerator. Not only that, but you can use In India, there are religious and social taboos against using human the waste from your waste in biodigesters. As a result, only cattle and buffalo dung is put generator as a fertiliser. into the biogas generators. This produces very high-quality gas, but much less fertiliser. There are also different sizes and designs of biogas generator. The type chosen will depend on local conditions. For example, many fermenters are sunk into the ground, which provides very good insulation. Others are built above ground, which may be easier and cheaper, but offers less insulation. If night-time temperatures fall too low, it could cause problems. Many countries are now looking at biogas generators, and experimenting with using them on a larger scale. The waste material we produce from sugar factories, sewage farms and rubbish tips all has the potential to act as a starting point for the production of biogas. We have some problems to overcome with scaling the process up, but the early progress looks promising. Biogas could Grade 10 13 Unit 1: Biotechnology well be an important fuel for the future for all of us. It would help us to get rid of much of the waste we produce, as well as providing a clean and renewable energy supply. More biofuels In countries such as Ethiopia, plants grow quickly. Sugar cane grows about 4–5 metres in a year, and has a juice which is very high in carbohydrates, particularly sucrose. Maize and sweet potatoes also grow fast. We can break down the starch in maize kernels or potato tubers into glucose, using the enzyme carbohydrase. We can convert the carbohydrates we grow into clean and efficient fuels. Ethanol-based fuels If sugar-rich products from cane and maize are fermented anaerobically with yeast, the sugars are broken down incompletely to give ethanol and water. You can extract the ethanol from the Figure 1.14 Conditions in products of fermentation by distillation, and you can then use it in Ethiopia allow plants like this cars and other vehicles as a fuel. sugar cane to photosynthesise Car engines need special modification to be able to use pure ethanol and grow very rapidly – the next as a fuel, but it is not a major job. Many cars can run on a mixture of step is to turn them into usable petrol and ethanol without any problems at all. fuel. Advantages and disadvantages of ethanol as a fuel In many ways, ethanol is an ideal fuel. It is efficient, and it does not produce toxic gases when you burn it. It is much less polluting than conventional fuels, which produce carbon monoxide, sulphur KEY WORDS dioxide and nitrogen oxides. In addition, you can mix ethanol carbohydrase enzyme with conventional petrol to make a fuel known as gasohol. This is increasingly being done, and reduces pollution levels considerably, which breaks down although there is still some pollution from the petrol part of the carbohydrates mix. distillation process of purifying a liquid by boiling it and condensing its vapours Sugar cane Maize Fermentation Enzyme Ethanol breakdown of starch g sugars Fermentation Ethanol Figure 1.15 The starch in maize needs to be broken down by enzymes before yeast can use it as fuel for anaerobic respiration. Although it takes more steps to produce ethanol from maize than from sugar cane, maize can be grown in many more countries around the world. 14 Grade 10  UNIT 1: Biotechnology Using ethanol as a fuel is a carbon-neutral activity. This means KEY WORDS there is no overall increase in carbon dioxide in the atmosphere when you burn ethanol. The original plants removed carbon dioxide carbon-neutral process from the air during photosynthesis. When you burn the ethanol, whereby the amount of you simply return it. carbon emitted is matched The biggest difficulty with using plant-based fuels for our cars is that by the amount absorbed it takes a lot of plant material to produce the ethanol. As a result, the use of ethanol as a fuel has largely been limited to countries with enough space, and a suitable climate, to grow a lot of plant material as quickly as possible. Here in Ethiopia, we have that capability. The main problem for many countries is finding enough ethanol. If people in Europe added 5% ethanol to their fuel, it would reduce carbon dioxide emissions – but they would need 7.5 billion litres of ethanol a year, which they cannot produce themselves. The methods of ethanol production we use at the moment leave large quantities of unused cellulose from the plant material. To make ethanol production work financially in the long term, we need to find a way to use this cellulose. We might develop biogas generators, which can break down the excess cellulose into methane, another useful fuel. Genetically engineered bacteria or enzymes may be able to break down the cellulose in straw and hay and make it available for yeast to make more ethanol. We don’t know exactly what the future will hold, but it seems likely that ethanol-based fuel mixes will be part of it. Here in Ethiopia the Ministry of Mines and Energy has already started mixing ethanol with petrol to provide fuel for cars. Along with the production of biodiesel from plants such as castor oil beans and jatropha, which grows in dry climatic conditions that do not suit crop production, Ethiopia is making great strides in the use of biofuels. As long as we maintain a balance between the use of land to provide food and the use of land to provide us with fuel, the use of biotechnology in this way has great potential for us in the future. Review questions 1. Which of the following statements about genetic engineering is not true? A It is used to change an organism and give it new characteristics that people want. B It involves changing the genetic material of an organism. C It can be used to produce crops that are resistant to disease. D It does not allow genes to be transferred from one type of organism to another. 2. Which of the following is not a component of biogas? A carbon dioxide B ethanol C methane D water Grade 10 15 UNIT 1: Biotechnology 3. Put the following stages of the process of making and using biogas into the correct order: A Some of the bacteria break down the plant cell walls while others break down the sugars formed, producing methane and other gases. B Dung or plant material is collected and put into a biogas generator, or digester. C The biogas produced is piped into homes, where it is burned to produce light, heat or refrigeration. D A mixed population of different types of bacteria is added. 4. Which of the following are advantages of using ethanol as a fuel, and which are disadvantages? Can you explain why? A It is a carbon-neutral activity. B It takes a lot of plant material to produce the ethanol. C It does not produce toxic gases when burnt. D It can be mixed with conventional petrol to make gasohol. Summary In this unit you have learnt that: Biotechnology is the use of micro-organisms to make things that people want, often involving industrial production. Biotechnology has been used for thousands of years to make bread, alcoholic drinks and fermented food products such as yoghurt and cheese. Yeast is a single-celled organism that can respire aerobically, producing carbon dioxide and water; this reaction is used in bread making to make the dough rise. Yeast can also respire anaerobically, producing ethanol and carbon dioxide in a process known as fermentation – the fermentation reaction of yeast is used to produce ethanol in the production of beer and wine. Bacteria are used in making both yoghurt and cheese. In the production of both, a starter culture of bacteria acts on warm milk. Lactose is converted to lactic acid in a lactic fermentation reaction. This changes the texture and taste of the milk to make yoghurt. In cheese making, a different starter culture is added to warm milk, giving a lactic fermentation which results in solid curds and liquid whey. The curds are often mixed with other bacteria or moulds before they are left to ripen. Modern biotechnology often involves genetic engineering and large-scale fermenters. 16 Grade 10  UNIT 1: Biotechnology In genetic engineering, desirable genes from one organism can be ‘cut out’ using enzymes and transferred to the cells of bacteria, animals and plants. Micro-organisms can be grown on a large scale in vessels known as fermenters, to make useful products such as antibiotics. Industrial fermenters have a range of features to make sure fermentation takes place in the best possible conditions. Modern biotechnology has many applications. In agriculture biotechnology is used to develop better crops and livestock, and to develop plants that contain their own pesticide. In food it is used in many ways: including to break down starch to form useful sugar syrups; to improve the production of beers and other fermented products; and to develop new foodstuffs. In medicine biotechnology is used to make drugs and medicines. Biotechnology is important in producing new forms of fuel to provide us with energy. Biogas – mainly methane – can be produced by anaerobic fermentation of a wide range of plant products and waste materials that contain carbohydrates. Ethanol-based fuels can be produced by the anaerobic fermentation of sugar cane juices and from glucose derived from maize starch by the action of the enzyme carbohydrase. End of unit questions 1. What is biotechnology and why is it so important? 2. Name three different foodstuffs or drinks that are used by your family, and explain how biotechnology is involved in producing them. 3. a) You can leave injera mix in a fridge for hours without any gas bubbles being formed. Put it somewhere warm, and after a time bubbles start to appear again. Injera is usually ready to cook after two days. In a cool place it may take longer. Explain how these differences come about. b) Temperature is vital for successful beer and wine making. Why is it so important? 4. a) Write a brief report on ‘Bacteria and fermented milk products’. b) Find out how fermented bean pastes are made and write about the biotechnology of this useful food. Grade 10 17 UNIT 1: Biotechnology 5. Why are micro-organisms so important in the production of medicines? Describe two different medicines that rely on biotechnology in their production. 6. a) Using the data in the table on page 12, produce bar charts to compare high-quality biogas (high methane, low carbon dioxide) with poor-quality biogas (low methane, high carbon dioxide). b) What effect do you think differences in composition like this will have on the use of this gas as a fuel? c) Suggest ways in which people might improve the quality of the gas produced in their fermenter. 7. Write a letter to your head teacher or school administration explaining why you think they should look into the idea of providing energy for the school from biogas, and how they might do it. Copy the crossword puzzle below into your exercise book (or your teacher may give you a photocopy) and solve the numbered clues to complete it. 1 ACROSS 2 3 4 Flammable gas made in biogas generator (7) 4 7 The study of living things used to perform industrial processes (13) 5 6 8 Microscopic fungi used to make alcoholic drinks and injera (5) DOWN 7 1 Fermented whole milk (7) 2 The study of micro-organisms and their effect on humans (12) 3 Anaerobic respiration in yeast that produces ethanol (12) 5 Single-celled microscopic organisms which can reproduce very quickly (8) 8 6 Flammable mixture of gases formed when bacteria break down plant and animal material in anaerobic conditions (6) 18 Grade 10 Heredity Unit 2 Contents Section Learning competencies 2.1 M  itosis and Define heredity and compare mitosis and meiosis. meiosis Define chromosome, DNA and genes. (page 19) Describe the structure of chromosomes and list the components of DNA. 2.2 Mendelian Describe the work of Gregor Mendel on garden peas and relate his inheritance experiments to the principle of inheritance. (page 30) Demonstrate the principle of inheritance using beads. Describe the methods, importance and examples of breeding farm animals and crops. 2.3 Heredity and Describe methods of breeding farm animals and crops. breeding Explain the importance of selective breeding for society. (page 45) Explain the difference between selective breeding and cross-breeding. Give examples of selective breeding from your own experience. 2.1 Mitosis and meiosis By the end of this section you should be able to: Define a chromosome. Define DNA as the genetic material. Define genes. Describe the structure of the chromosomes. Describe the components of DNA. Define mitosis and describe its stages. Define meiosis and describe its stages. Relate the events of meiosis to the formation of the sex cells. Compare mitosis and meiosis. Almost all the cells of your body – with the exception of your mature red blood cells – contain a nucleus, the ‘control room’ of the KEY WORDS cell. The nucleus contains all the plans for making a new cell, and for making a whole new you. red blood cell type of blood cell that carries Think of the plans for building a car. They would cover many oxygen around the body different sheets of paper. Yet in every living organism, the nucleus Grade 10 Incomplete advance copy 19 UNIT 2: Heredity of the cells contains the information needed to build a whole new animal, plant, bacterium or fungus. A human being is far more complicated than a car – so where does all the information fit in? Inside the nucleus of every cell there are thread-like structures called chromosomes. This is where the genetic information passed on from parent to child is stored. The chromosomes are made up of DNA (deoxyribonucleic acid). This amazing chemical carries the instructions needed to make all the proteins in your cells. Many of these proteins are actually enzymes. These control the production of all the other chemicals that make up your body, and affect what you look like and who you are. A chromosome is a structure in the nucleus of a cell consisting of genes. Chromosomes are made up of the genetic material DNA in a DNA–protein complex. DNA is the genetic material contained in the nucleus. Each different type of organism has a different number of chromosomes in the cells – humans have 46 chromosomes and tomatoes have 24, while elephants have 56. You inherit half your chromosomes from your mother and half from your father. Chromosomes come in pairs known as homologous pairs. So people have 23 pairs, tomatoes have 12 pairs and elephants have 28 pairs of chromosomes. Scientists can photograph the chromosomes in human cells when Figure 2.1 Karyotypes, like these of they are dividing and arrange them in pairs to make a special a healthy man and woman, have picture known as a karyotype. helped scientists find out more about the mysteries of inheritance. Human karyotypes show 23 pairs of chromosomes. In 22 of the pairs, both chromosomes are the same size and shape, regardless of whether you are a boy or a girl. These 22 pairs of chromosomes Activity 2.1: Making a are known as the autosomes. They control almost everything about karyotype the way you look and the way your body works. The remaining pair You are going to make a of chromosomes is different for boys and girls. A girl has a pair of human karyotype. You will be two similar X chromosomes, but a boy has one X chromosome and provided with a photograph another, much smaller, Y chromosome. These are known as the sex or worksheet showing a chromosomes because they determine whether you are male or photograph of unordered female. Everyone inherits an X chromosome from their mother. If human chromosomes, exactly this joins with a sperm carrying another X chromosome, you will as a real scientist would take. be a girl. If it is fertilised by a sperm carrying a Y chromosome, Your task is to cut out each of you will be a boy. X chromosomes carry information about being the chromosomes and arrange female, but they also carry information about many other things – them in pairs as you have like the way your blood clots, and the formation of your teeth, body seen in figure 2.1. Try and hair and sweat glands. Y chromosomes mainly carry information identify each pair and stick about maleness. them onto a sheet of paper, labelling them carefully. Is your person male or female? 20 Grade 10  UNIT 2: Heredity Chromosomes, genes and DNA Cell Nucleus The chromosomes you inherit from your parents carry all the information needed to make a new you. The information is kept in the form of genes. Each gene is a small section of DNA. Life as we know it depends on the properties of this complicated chemical, so it seems amazing to think we have only understood it for about 50 Nucleus years! A gene is a unit of hereditary material located on the Chromosomes found in pairs, one inherited chromosomes. from your father and one from your mother DNA is a long molecule, made up of two strands twisted together to make a spiral known as a double helix – imagine a ladder that has been twisted round. The big DNA molecule is actually made up of Gene lots of smaller molecules (nucleotides) joined together. A nucleotide Each chromosome Chromosome in a pair carries consists of a phosphate group, a sugar and a base. In DNA there are genes which code four different bases that appear time after time in different orders, for the same characteristic but always paired up in the same way. The bases link the two strands of the DNA molecule together. Genes are made up of repeating Figure 2.2 The nuclei of our cells patterns of bases in the DNA. (See page 22.) contain the chromosomes that carry the genes that control the By the 1940s, most scientists had decided that DNA was probably characteristics of our whole body. the molecule that carried inherited information from one generation to the next. But how did it work? Repeating patterns of 4 different bases KEY WORDS make up the structure of the DNA molecule chromosome strand of DNA carrying genetic information DNA nucleic acid containing the genetic instructions used in the development base pairs and functioning of all known living organisms and double some viruses helix phosphate homologous chromosomes base nucleotide sugar a pair of chromosomes having the same gene Figure 2.3 The double helix structure of the DNA molecule takes you sequences, each derived deep into the chemistry of life. A small change in the arrangement of from one parent bases in your DNA would have meant a very different you. karyotype map of the By the 1950s, two teams in the UK were getting close to chromosomes in the nucleus understanding the structure of this amazing molecule. Maurice of a single cell Wilkins and Rosalind Franklin in London were taking special autosomes chromosomes X-ray photographs of DNA and looking at the patterns in the that are not sex X-rays in the hope that they would show them the structure of the chromosomes molecule. At the same time, James Watson (a young American) and Francis Crick (from the UK) were working on the DNA problem double helix pair of at Cambridge. They took all the information they could find on parallel helices intertwined DNA – including the X-ray crystallography from London – and about a common axis kept trying to build a model of the molecule that would explain Grade 10 21 UNIT 2: Heredity everything they knew. When they finally realised that the bases KEY WORDS always paired up in the same way, they had cracked the code. The adenine one of the four now famous double helix was seen for the first time. Watson, Crick bases that comprise DNA and Wilkins all received the Nobel Prize for their work. Rosalind which pairs with thymine Franklin died of cancer before the prizes were awarded. thymine one of the four Since the structure of DNA was revealed, there has been an bases that comprise DNA enormous explosion in the amount of research done on genetics. which pairs with adenine We now know that the bases that make up DNA are called adenine, guanine one of the four thymine, guanine and cytosine. The two DNA strands are linked by these bases, where adenine pairs with thymine and cytosine pairs bases that comprise DNA with guanine. The upright strands are made of deoxyribose sugar which pairs with cytosine and phosphate. Each base, sugar and phosphate together form a cytosine one of the four nucleotide. DNA is therefore a polynucleotide chain. bases that comprise DNA The genes found on the chromosomes control everything that goes which pairs with guanine on in your cells by organising all the proteins that are made. As nucleotide a building you learned in grade 9, a protein is a long chain of amino acids. block of DNA or RNA which Different combinations of amino acids can be joined together to consists of a sugar, a make different proteins. So the order of the bases in the DNA acts phosphate, and one of the as a code to instruct the cell about the order in which to join up four bases the amino acids to make a particular protein. This is how the genes polynucleotide long chains control what goes on in the cells and in the whole organism. of linked nucleotides The Human Genome Project has been a massive international effort by scientists from many countries who set out to read the DNA of the entire human genome. This work is showing us exactly what Did you know? genes we all have in common, and which characteristics they code for. The Human Genome Project has cost around Mitosis 3 billion US dollars so far. Scientists have worked New cells are needed for an organism, or part of an organism, out the 3 billion base to grow. They are also needed to replace cells that become worn pairs that make up human out and repair damaged tissue. However, the new cells that are DNA – and have shown produced must contain the same genetic information as the that everyone shares originals, so that they can do the same job. around 99.99% of their In animals and plants that have asexual reproduction it is necessary DNA. It looks as if human for one cell to split into two genetically identical cells for the beings have only between organism to reproduce. 20 000 and 25 000 genes, far fewer than scientists originally predicted. 22 Grade 10  UNIT 2: Heredity KEY WORDS somatic cells any of the cells of a plant or animal except the reproductive cells mitosis cell division in which the nucleus divides into nuclei containing the same number of identical chromosomes chromatids the two strands of a chromosome that separate during mitosis daughter cells the two identical cells that Figure 2.4 As we grow, it is important that we can make new cells are formed when a cell which are just the same as the old ones, so we can grow and repair reproduces itself by splitting any damage that occurs during our life. into two Body cells (also known as somatic cells) divide to make new cells. The cell division that takes place in the normal body cells and produces identical cells is known as mitosis. As a result of mitosis, every body cell has the same genetic information. In asexual reproduction, the cells of the offspring are produced by mitosis from the cells of their parent. This is why they contain exactly the same genes with no variety. Mitosis is division of the somatic cells to make identical daughter cells. How does mitosis work? Before a cell divides, it produces new copies of the homologous pairs of chromosomes in the nucleus. Each chromosome forms two identical chromatids. Then the chromatids divide into two identical packages, and the rest of the cytoplasm divides as well to form two genetically identical daughter Did you know? cells. Once the new cells have formed, the chromatids are again Your red blood cells referred to as chromosomes. The daughter cells each have exactly have a finite life because the same number of chromosomes as the original cell. To make it they lose their nuclei easier to understand what is going on, we divide this process into as they mature. Worn- stages – interphase, prophase, metaphase, anaphase and telophase out red blood cells are (figure 2.5) – but in fact mitosis is one continuous process. destroyed, at a rate of In some areas of the body of an animal or plant, cell division like around 100 billion per this carries on rapidly all the time. Your skin is a good example – day, by your spleen and thousands of cells are constantly being lost from the surface, and liver. Fortunately, mitosis new cells are constantly being formed by cell division to replace takes place in your bone them. As food passes along your gut (grade 9 biology), cells are marrow just as quickly to scraped off the gut lining. Fortunately, there is a layer of cells make the new red blood underneath that is constantly dividing by mitosis to replace those cells you need. that are lost. Grade 10 23 UNIT 2: Heredity Interphase Prophase End of Prophase 1 Chromosomes are 2 copied as the DNA Two sister chromatids make replicates up each chromosome, they are joined by a Parent cell has four centromere chromosomes (two homologous pairs) Metaphase Anaphase 3 A structure called the 4 spindle forms and the The spindle fibres chromatids attach to it shorten and pull the chromatids to opposite Chromatids attached poles (ends) of the cell to the spindle by the centromere Spindle fibres Telophase 5 Two new nuclei form at the poles of the cell. Each has a copy of each chromosome from the parent cell. The two new cells seperate and the chromosomes unravel as the cell goes back into interphase. Figure 2.5 The formation of identical daughter cells by simple division takes place during mitosis. It supplies all the new cells needed in your body for growth, replacement and repair. (Your cells really have 23 pairs of chromosomes – but for simplicity this cell is shown with only Figure 2.6 These cells are in the two pairs.) growing root tip of an onion, so Most of the time, you can’t see the chromosomes in the nucleus of a they are dividing rapidly and cell, even under the microscope. However, when a cell is splitting in the chromosomes have taken up two, the chromosomes become much shorter and denser, and will a red stain. You can see mitosis take up special colours called stains. At this stage you can see them taking place, with the chromatids under the microscope. The name ‘chromosome’ means ‘coloured in different positions as the cells body’, referring to what the chromosomes look like when they have divide. taken up the stain. Activity 2.2: Seeing chromosomes This experiment was done for the first time or by Walther Fleming, a German scientist, and a prepared longitudinal section of an is now carried out regularly in school labs actively growing onion root tip stained across the world. It allows you to see mitosis with acidified ethanoic orcein stain in action in the actively dividing cell in an onion root tip. Method You will need: If you are preparing your own slide: a light microscope 1. Cut off the end of a growing root tip about and either 5 mm from the end of the root. actively growing root from an onion 2. Pour a little acidified ethanoic orcein stain watch glass into the watch glass and add the root tip. acidified ethanoic orcein stain 3. Place the watch glass, stain and root on a hot plate warm hot plate for five minutes. tweezers 4. Remove the watch glass from the hot plate mounted needle and, using the tweezers, place the root tip on the slide with a drop of ethanoic orcein microscope slide and coverslip stain. blotting paper 24 Grade 10  UNIT 2: Heredity 5. Break up the root tip with the needles to 7. Look at your slide under the microscope, spread out the cells as much as possible. first using the low-power lens and then 6. Place a coverslip over the crushed root tip, moving to higher magnifications. place the blotting paper over it and press 8. Make careful observations of the down gently – this will crush the root tip chromosomes and the ways they are further. The slide is now ready to use. arranged in the cells. Make drawings Whether you have prepared your own root tip of your observations. On your slide, try slide or have a ready made one, you are now to find cells that are: resting, about to going to make observations and drawings: divide, in the middle of dividing, or just completing a division. The cells of early animal and plant embryos (known as stem cells) KEY WORDS are unspecialised. Each one of them can become any type of cell that is needed. In many animals, the cells become specialised very stem cells cells that have early in life. By the time a human baby is born, most of its cells have the ability to grow into become specialised for a particular job, such as liver cells, skin cells other kinds of cells and muscle cells. They have differentiated. Some of their genes have differentiated made been switched on and others have been switched off. This means different that when a muscle cell divides by mitosis, it can only form more ovary the female sex organ muscle cells. Liver cells can only produce more liver cells. So in that produces ova adult animals, cell division is restricted because differentiation has occurred. Some specialised cells can divide by mitosis, but this can testes the male sex organ be used only to repair damaged tissue and replace worn-out cells. that produces sperm Each cell can only produce identical copies of itself. ova egg cells (reproductive cells) produced by the ovary The cell cycle The cells in your body divide on a regular basis to bring about Did you know? growth. They divide in a set sequence, known as the cell cycle, which involves several different stages. Your body cells are lost at an amazing rate – 300 A period of active cell division – this is when mitosis takes place million cells die every and the number of cells increases. minute. Fortunately, A long period of non-division – when the cells get bigger, mitosis takes place all increase their mass, carry out normal cell activities and replicate the time to replace them. their DNA ready for the next division. Time The length of the cell cycle varies considerably. It can take less than 24 hours, or it can take several years, depending on which cells are involved and at which stage of life. There are many cycles during the Interphase years of growth and development, but it slows down once puberty is Cytokinesis over in the adult. Telophase Anaphase Cell division Mitosis is taking place all the time in tissues all over your body. But Metaphase Prophase mitosis is not the only type of cell division. There is another type that takes place only in your reproductive organs. Nuclear division (mitosis) Meiosis Figure 2.7 The cell cycle. In rapidly dividing tissue and in The reproductive organs in humans, as in most animals, are the cancer cells, interphase may only ovaries and the testes. This is where the sex cells (the gametes) are be a few hours. In other tissues, made. The female gametes, or ova, are made in the ovaries; the male or in an adult animal, interphase may last for years. Grade 10 25 UNIT 2: Heredity Original cell Four chromosomes gametes, or sperm, are made in the testes. In plants, the sex cells are in the nucleus; two homologous pairs the pollen and the ovules. The cells in the reproductive organs (also of chromosomes known as germ cells) divide to make sex cells. The cell division that Each chromosome takes place in the reproductive organ cells and produces gametes is duplicates itself; now there are eight known as meiosis. chromatids in the nucleus Meiosis is a special form of cell division where the chromosome The cell number is reduced by half. When a cell divides to form gametes, the divides into two; each new chromosomes are copied so there are four sets of chromatids. The cell has four chromatids cell then divides to form two identical daughter cells. These cells then divide again immediately, without the chromatids doubling again, in the second meiotic division. This forms four gametes, each with a single set of chromosomes. The details of this process are shown in figures 2.8 and 2.9. As in mitosis, the process is shown as being broken up into different stages, but in real life it is a single, Figure 2.8 This simple diagram flowing process that has been described, rather poetically, as the sums up the main stages of ‘dance of the chromosomes’. meiosis – see figure 2.9 for the Meiosis is the division of the sex cells resulting in daughter cells details. with half the original number of chromosomes. Why is meiosis so important? Your normal body cells have 46 KEY WORDS chromosomes in two matching sets, 23 from your mother and sperm male reproductive 23 from your father. If two body cells joined together in sexual cells produced by the testes reproduction, the new cell would have 92 chromosomes, which simply wouldn’t work. As a result of meiosis, your sex cells contain meiosis the type of cell only one set of chromosomes, exactly half the full chromosome division that creates egg number. So when the gametes join together at fertilisation, the new and sperm cells cell that is formed contains the normal number of 46 chromosomes. Gametogenesis Did you know? Meiosis occurs as part of a process known as gametogenesis, or One testis can produce gamete formation. In females this is called oogenesis. In a baby girl, over 200 million sperm the first stage of meiosis is completed before she is even born. The each day by meiosis. As tiny ovaries of a baby girl contain all the ova she will ever have. The most boys and men have second meiotic division begins as the eggs mature in the ovaries two working testes, that during the monthly cycle. gives a total of 400 million In males, meiosis doesn’t start until p