Genetics and Biotechnology Chapter 1 PDF

Genetics and 1 biotechnology How are traits inherited? Although children resemble their parents, grandparents and each other, no two people are exactly alike, not even identical twins. You may have noticed that man...

Genetics and 1 biotechnology How are traits inherited? Although children resemble their parents, grandparents and each other, no two people are exactly alike, not even identical twins. You may have noticed that many personal features such as hair colour, facial shape and athleticism seem to ‘run in the family’. These features may be obvious, but family traits can also be hidden and subtle. Doctors may ask their patients about family members with conditions such as asthma, diabetes or cancer. This is because physical and biological features, as well as medical conditions, can be inherited through generations of the family. Shutterstock.com/Katie Smith Photography nelson Living world – Stage 5 Key knowledge Biological understanding has advanced through scientific discoveries, technological developments and the needs of society. The organs involved in human reproduction have specialised functions. During reproduction the transmission of heritable characteristics from one generation to the next involves DNA and genes on chromosomes. The Watson–Crick model of DNA explains the exact replication of DNA and changes in genes (mutation). Developments in technology have advanced biological understanding, e.g. vaccines, biotechnology, stem-cell research and in vitro fertilisation. The application of biotechnology can have advantages for human health and society. Using biotechnology can also have disadvantages and involves balancing ethical considerations. ACTIVITY SHEET CULMINATING ASSESSMENT TASK CAT with rubric: Conference of chromosomes and patterns of heredity Conference of chromosomes and patterns of heredity Your task is to investigate a genetic condition in humans caused by a DNA mutation. You will identify symptoms and the pattern of inheritance and discuss issues related to this condition. Describe current and future applications of biotechnology for testing or treating this condition. After evaluating the need for research funding for this condition, you will present your findings in a classroom conference on chromosomes and patterns of heredity. 2 ISBN 9780170231510  Chapter 1 Genetics and biotechnology ICT What do you already know about genetics? Share your ideas With a partner: using Padlet.  define as many of the following words as you can, using your current knowledge and ideas. You don’t have to be correct. mutation, gene, DNA, chromosome, genetic testing, inherited, gamete, mitosis, recessive trait, dominant trait, genetic engineering, genome WEBLINK list five questions that you have about genetics. Padlet Share your ideas with the class, in discussion or on a shared digital space – such as Wikispaces or Padlet. 1.1 Genetics Humans have been practising genetics from at least 15  000  BCE. The first attempts at genetics genetic engineering are evident in the sealed tombs of our ancestors. Ancient pollen from the study of genes and domesticated plants found in the tombs shows that farmers were selecting and breeding plants inheritance for desired characteristics. However, the farmers did not know how these characteristics were genetic engineering passed on from one generation to the next. the technique of manipulating the Genetics is the study of genes. It looks at both inheritance and variation, and investigates genetic make-up of an organism how characteristics are passed from one generation to the next. Knowledge of genetics is essential in many areas. For example, genetics is used in agriculture to increase crop gene productivity, in biology to identify people at risk of genetic conditions, and in police work to an individual segment of DNA that solve crimes. holds the coded instructions to make a protein product To get a clear picture of why we look the way we do, we must look deep into the body and its cells to understand how one molecule controls all of our features and the way our body works. This deoxyribonucleic acid (DNA) molecule is found in the nucleus of most cells and is known as deoxyribonucleic acid (DNA). the molecule containing the inherited An understanding of the structure and functions of DNA is very important. DNA is the code of genetic information chemical that codes for an individual’s features and bodily functions. Short sequences of DNA – genes – are the units of heredity. The information in DNA is stored as a code made up INTERACTIVE of four chemical bases. It is the order, or sequence, of these bases that determines the code. Introduction to genes: assessment This is similar to the way in which letters of the alphabet appear in a certain order to form a word. For example, the word ‘BUT’ has a certain meaning, but the word ‘TUB’ has a totally different meaning even though it contains the same letters. It is the order of the letters that is important. Genes are arranged in a linear sequence on the DNA molecule and each gene codes for one protein end product. The chemical nature and structure of DNA remained a mystery until the 1940s, when scientists around the world began a frantic race to be the first to discover whether it was protein or DNA that held in coded form the hereditary information in a cell. In the section that follows, we will look at the discovery of DNA and its structure and function at a molecular level so that we can understand how genetics influences heredity and variation on a larger scale. ISBN 9780170231510 3  nelson The race to discover the structure of DNA Figure 1.1 In the early 1950s, British chemist Rosalind Franklin (1920–58) Rosalind Franklin joined Maurice Wilkins (1916–2004) and his team at King’s College, London, to further their decade-long work on DNA structure. Before Franklin arrived, the team had produced images that showed a molecular pattern that suggested a X-ray crystallography helical structure. Using X-ray crystallography, accurate a technique that shines a beam of images of individual atoms in large molecules can be X-rays through purified materials, produced. Franklin’s X-ray photograph (Figure 1.2), which revealing the patterns of atoms was taken in 1952, improved on earlier attempts and clearly showed the team that DNA had a helical structure and that the Science Photo Library two outer strands consisted of sugar and phosphate. Franklin’s photograph enabled the structure of DNA to be WEBLINK DNA: the greatest discovery worked out within a year. In 1953, while working together in a in modern science competing laboratory at the University of Cambridge, molecular biologists James Watson (1928– ) and Francis Crick (1916–2004) realised that the two chains of nucleotides in the helix ran in opposite directions. Using simple materials such as retort INTERACTIVE stands and cardboard, they built a three-dimensional DNA model Genes: what is DNA? (Figure 1.3). Their model showed all the features necessary in hereditary material – it contained bases that could hold coded information, it allowed self-replication and it could be transferred by gametes from one generation to the next. DNA as the Science Photo Library Figure 1.2 blueprint for life is now one of the fundamental ideas in biology. Rosalind Franklin’s X-ray In 1962 Watson and Crick were awarded the Nobel Prize for this photograph taken looking discovery, along with Maurice Wilkins. down the DNA molecule The Watson–Crick model of DNA increases our understanding of the structure of DNA and how it functions. It accounts for how DNA can replicate itself and how changes in genes (mutations) can arise. Science Photo Library/A. Barrington Brown Figure 1.3 American James Watson (left) was only 25 and Briton Francis Crick a graduate student when they identified the chemistry of DNA. 4 ISBN 9780170231510  Chapter 1 Genetics and biotechnology The structure of DNA Figure 1.4 DNA – the double-helix There is approximately 3  metres of DNA in the molecule nucleus of each of our cells. The structure of the DNA molecule is like a ladder: it has two vertical nitrogenous base ‘backbones’ (the sides of the ladder) made of one of the four nitrogen-containing repeating sugars and phosphates. The horizontal bases – adenine, cytosine, ‘rungs’ are made of units called nitrogenous thymine and guanine – that make up DNA bases. The DNA molecule is not flat like a real ladder, but a spiral. Imagine holding the top and double helix bottom of a ladder and twisting it around. The the two-sided spiral shape of DNA resulting shape is known as a double helix (Figures 1.4 and 1.5). nucleotide The DNA molecule is made up of many repeating the subunit that makes up DNA units called nucleotides (Figure 1.5). Each nucleotide adenine consists of a sugar, a phosphate and one of four a base found in DNA and in different chemical bases: adenine (A), guanine (G), another nucleic acid called RNA thymine (T) and cytosine (C). The bases are always found in complementary pairs. This means that the guanine base on one strand of a DNA molecule will always pair a base found in DNA and RNA with its complementary base on the other strand. For thymine example, A will always pair with T, and G will always pair with C. This is known as the complementary a base found in DNA (but not RNA) base pairing rule. This allows the DNA molecule to cytosine replicate itself. a base found in DNA and RNA The order in which the bases occur on the DNA is important in coding genetic information, complementary base pairing much like the sequence of letters in a word determines a rule that states the chemical Getty Images its meaning. bases are always found in pairs; for example, adenine will always bond with thymine, and guanine will always bond with cytosine T Nucleotide ACTIVITY SHEET Modelling DNA G T ANIMATION Base Base pairing C A Figure 1.5 The double helix – Sugar simplified. A nucleotide unit of DNA contains a sugar, a Phosphate phosphate and a base. ISBN 9780170231510 5 nelson WORKSPACE EXPERIMENT 1.1 Extracting DNA Extracting DNA DNA is easily extracted, or isolated, from a variety of biological tissues such as banana, kiwi fruit, onion, liver and wheat germ (a concentrated source). Possible risks Safety precautions Ethanol and methylated spirits are flammable Avoid handling ethanol or methylated and toxic. spirits. Use latex gloves and/or a squeeze bottle. Do not use naked flames anywhere near ethanol. Laboratory desks and equipment may be Do not eat the fruit or fresh material contaminated with toxic chemicals. provided. Use tongs to handle these items. Aim To extract DNA from a concentrated source Materials source of DNA: 1 cm3 of bananas, kiwi fruit, onions, liver or wheat germ mortar and pestle salt dishwashing detergent buffer solution 250  mL beaker funnel filter paper meat tenderiser ethanol (ice cold) in a squeeze bottle hooked Pasteur pipette or paperclip Method 1 Place the DNA source into a mortar. Using the pestle, grind the material with salt, dishwashing detergent and the buffer solution. Note: The physical grinding increases the contact surface area. The detergent breaks down the oily cell membranes and proteins. The buffer helps the DNA come together. 6 ISBN 9780170231510  Chapter 1 Genetics and biotechnology EXPERIMENT 1.1 2 Use the beaker, funnel and filter paper to filter the ground mixture. This removes the cell debris, but the cell nuclei should still pass through. 3 Add 1  tablespoon of meat tenderiser to the filtered material. This breaks down the nuclear membranes. 4 Add the ethanol by carefully allowing small amounts to run down the side of the beaker. Water is denser than alcohol, so the alcohol sits above it. The DNA, which is not soluble in alcohol, will precipitate on the interface of the two layers after a few minutes. 5 Lift the DNA out of the beaker using the hooked Pasteur pipette or paperclip. Results 1 Identify the material from which you extracted DNA. 2 Using an annotated diagram, describe the appearance of the DNA that has been isolated. Discussion 3 Construct a flowchart to show the steps in the method. Outline the purpose of each step. 4 Discuss whether you would you expect the same results from different amounts of blending. 5 Explain the effect of the detergent on the cell membranes. 6 Explain why the alcohol remains in a layer on top of the liquid in the beaker. 7 Predict whether you would expect the results from different tissues to be similar. Compare your results with those of another group that used a different DNA source. Explain why the results are similar or different. 8 If the average plant cell contains 3  m of DNA and we consume 50  million cells in an average meal, calculate how many kilometres of DNA we eat in a week. 9 A person cannot see a single cotton thread 30  m away, but if you wound thousands of threads together into a rope, it would be visible from much further away. Explain how this statement is relevant to your procedure. Conclusion Compare the volume of strained DNA with the original amount of tissue you 10  ground. Estimate the percentage difference. ISBN 9780170231510 7  nelson WORKSPACE QUESTIONS 1.1 What have you learnt? 1.1 What have you learnt? Understanding 1 Define ‘genetics’. 2 a What does DNA stand for? b Identify the basic subunit of the DNA molecule. c Identify the parts that make up the subunit. d  Draw a labelled diagram to represent the structure of the DNA molecule. ICT 3 Outline the complementary base-pairing rule. Use a blog or glog to Applying organise what you 4 Rosalind Franklin died of ovarian cancer at the age of 37. Propose any have learnt. occupational risks (that is, at her workplace) to which she may have been exposed. Evaluating 5 Rosalind Franklin’s work was given to James Watson without her WEBLINK knowledge. Without Franklin’s photograph, Watson and Crick may not Blogger have determined the structure of the DNA molecule for many years. Discuss whether you think it was right or wrong for Watson to use Franklin’s results without seeking her permission. 6 Rosalind Franklin was not awarded a Nobel Prize for her contribution WEBLINK to the discovery of DNA because Nobel Prizes are not awarded GlogsterEDU posthumously (that is, after a person has died). Discuss your views on this. Creating daughter cells 7 Create a personal blog or glog called ‘Genetics and biotechnology’. cells produced through mitosis or Add concepts and ideas to your personal organiser as you go through meiosis (daughter cells created through mitosis are identical to this topic to assist your final revision. the parent cell; those produced through meiosis are genetically different) replication the process in which cells copy 1.2 DNA: coding for life the DNA prior to cell division To maintain the integrity of the genetic code and pass it on from cell to cell, there must be an orderly process to replicate the DNA during cell division. enzyme a protein that acts as a catalyst in biological reactions DNA replication DNA helicase When cells divide, they first make an exact copy of the DNA so that a replica of instructions can the enzyme molecule that be distributed equally to each of the daughter cells. The complementary base-pairing rule of ‘unzips’ DNA the Watson–Crick model of DNA allows us to explain how replication occurs (Figure 1.6). DNA polymerase Before the DNA molecule can replicate itself, it is unwound and separated by a special enzyme called DNA helicase. The DNA separates down the middle, in a manner similar to a the enzyme that lays down new bases on an exposed DNA strand zipper being undone. This exposes the bases on each strand. They are used as a template by to create a new DNA strand another enzyme, DNA polymerase, which lays down new bases according to the complementary 8 ISBN 9780170231510  Chapter 1 Genetics and biotechnology base-pairing rule. This forms two new DNA strands. Each side of the DNA molecule now contains semiconservative one old strand of DNA and one new strand of DNA. This kind of replication is described as describes replication of DNA, semiconservative replication. The two molecules are twisted into the familiar double helix shape. whereby one strand is from the old DNA molecule and one strand is a new DNA molecule INTERACTIVE Genes: DNA replication Figure 1.6 DNA replication – the Replicated DNA strands Parental DNA unwinding of DNA strands and the rewinding of newly Replication fork replicated strands Protein synthesis WEBLINK Once Watson and Crick established the structure of DNA, it became clear that there must DNA replication be a relationship between DNA in the nucleus and the many different proteins found in the cytoplasm. Proteins have many functions within a living organism. For example, they can act as enzymes (such as in digestion) or provide structure to cells (the cell membrane contains protein many different proteins). All proteins are made of many subunits called amino acids. Different a complex organic molecule proteins contain different amounts of amino acids in different orders. that contains carbon, hydrogen, oxygen, nitrogen and usually Scientists realised that DNA somehow provided a plan for the production of all the possible sulfur; is composed of one or proteins a cell may need and that it was the order of the base pairs in the DNA that coded for more chains of amino acids, and the order of the amino acids in a protein. Protein synthesis from the DNA code can be broken is the fundamental component of all living cells down into two parts: transcription and translation (Figure 1.7). amino acid the subunit that makes up proteins Cytoplasm protein synthesis DNA the process within cells that builds protein molecules Transcription transcription Nucleus the process in which the DNA information is copied, nucleotide mRNA by nucleotide, into mRNA Amino acid translation the process in which the mRNA Protein tRNA Translation information is converted into an amino acid sequence mRNA Figure 1.7 Simplified diagram summarising the process Ribosome of protein synthesis. Several Cell membrane enzymes assist with each stage. ISBN 9780170231510 9  nelson Transcription Each DNA molecule has many genes made up of many bases located along its length. Each messenger RNA (mRNA) gene codes for a particular protein or part of a protein. For the information coded in a gene to a type of RNA used to transfer be transferred to a protein chain, the DNA separates into two single strands, just as when the the information from the DNA template into the cytoplasm DNA is replicating. However, this time one of the strands acts as a template for the manufacture of the molecule messenger RNA (mRNA) in a process called transcription. mRNA is a single- uracil stranded molecule and, like DNA, contains the bases A, C and G, but has the base uracil (U) a base found in RNA (but not DNA) instead of thymine (T). For example, if the DNA template code was: ATTCGAACGTCCGCC the mRNA code would be complementary but would contain uracil instead of thymine: UAAGCUUGCAGGCGG The rule of pairing is similar to that of DNA, but A pairs with U as there is no thymine in RNA. Genes are small stretches of bases in a DNA molecule. Single genes may be transcribed, so the resulting mRNA molecule is much smaller than the DNA and is able to pass through the nuclear membrane. The mRNA leaves the nucleus and moves into the ribosome cytoplasm for translation into a protein. the structure in the cytoplasm in which protein synthesis occurs transfer RNA (tRNA) Translation the molecule that delivers the The mRNA now moves across the cytoplasm to the site of protein synthesis – the ribosome. amino acid to the ribosome Transfer RNA (tRNA) brings the amino acids to the ribosome according to the sequence of during protein synthesis the bases within the mRNA. The mRNA bases are read in groups of three. These are called codon codons and each set of three bases directs one amino acid to be placed in a growing protein a sequence of three bases on chain. This is called translation. Once the mRNA molecule has been completely read from start mRNA that codes for one to finish, the protein chain detaches from the ribosome. amino acid WEBLINK ACTIVITY 1.1 Translation Think, pair, share Imagine that you have to explain transcription and translation to someone else. INTERACTIVE Genes: protein synthesis 1 Think. How would you do this? Plan your thoughts and ideas in detail. Who is your audience? ICT 2 Pair up with another student. Share your plans. Create a new plan combining your thoughts for an agreed audience, such as a member of your family. Upload your plans and any resources 3 Share. Join with another pair of students. As a group, combine your ideas you have created to and create a sequence of diagrams or a model to explain this for your your blog. specific audience. 10 ISBN 9780170231510  Chapter 1 Genetics and biotechnology The genetic code The genetic code shows the relationship between the information on the mRNA and the amino acids that eventually build a protein. The sequence of bases in mRNA determines the sequence of amino acids in a protein chain. Changing the order or number of amino acids in a protein affects the functioning of a protein, just as changing the order or number of words in a sentence may give it a new meaning. Therefore, it is important that the order of bases in DNA is conserved, as any change will lead to a change in the sequence of bases in mRNA. A change in the sequence of bases in DNA is known as a mutation. It will result in a different mRNA, which will lead to a change in the amino acids in the protein. This may affect the ability of the protein to carry out its function in cells. QUESTIONS 1.2 WORKSPACE What have you learnt? 1.2 What have you learnt? Understanding 1 Describe the process and outcome of semiconservative replication. 2 Copy and complete the following table for different stages in protein synthesis. Name of molecule Where activity takes place Name of process Cytoplasm Protein synthesis Applying 3 This is a template sequence of DNA nucleotides. ATATTGGGCGCCAAGACT a Deduce the sequence of nucleotides on the complementary strand. b Outline the sequence of bases that would be transcribed as mRNA. ACTIVITY SHEET Translating your name into c How many different types of amino acids would there be in the genetic code resulting protein chain? Explain your answer. Analysing 4 Complete the extension activity on the activity sheet ‘Translating your name into genetic code’. It demonstrates in detail how mRNA is translated into a protein chain. Reflecting 5 Do appearances matter? Discuss how this section contributed to your understanding of what you look like. ISBN 9780170231510 11  nelson 1.3 Human reproduction and genes Before we can understand how traits are passed on to each generation, we must first understand reproduction. Sexual reproduction is a method by which a male and a female parent produce offspring that are genetically different. It is one of the ways in which genetic variation is maintained in populations. This is how humans and many other species reproduce. In iScience 8 for NSW you studied the structure and functioning of the male and female reproductive systems. Specialised reproductive organs in males and females carry out specific functions to ensure the successful production of viable gametes during sexual reproduction. Activity sheet ACTIVITY SHEET Male and female ‘Male and female reproductive systems’ will help you to remember what you have learnt about reproductive systems sexual reproduction and add some new knowledge to expand your understanding. Activity sheet ‘Hormonal control of reproduction’ gives a brief overview of hormonal control of reproduction. It is optional, additional content in the syllabus. ACTIVITY SHEET Hormonal control of reproduction How parents pass on genes Sexual reproduction involves the fusion of male and female gametes (sex cells) to form a gamete zygote (fertilised egg). Gametes carry family characteristics for features such as eye colour, hair colour and height from one generation to the next. Up until the early 1900s, biologists were a sex cell – sperm in males and ova in females puzzled about how tiny, microscopic, single-celled gametes can store so much information. The best explanation seemed to be that gametes carry information for the thousands of zygote characteristics in a coded chemical form. a fertilised egg cell containing Improvements in technology and scientific procedures allowed scientists to conduct valid chromosomes from a sperm and experiments to test this hypothesis. In 1902, biologists showed that a fertilised egg contains a an egg that have fused combination of genetic material from both parents, with 50% of inherited characteristics coming maternal from the maternal (mother’s) side of the family and the other 50% from the paternal (father’s) coming from the mother side of the family. However, by midway through last century, scientists had still not discovered which chemical paternal in cells holds the instructions in coded form. In the early 1950s, researchers were as competitive coming from the father as elite sportsmen and sportswomen in their endeavours to be the first to make this discovery. Why do our cells contain genes from our parents? The answers involve understanding how genes are coded in DNA and carried on chromosomes, and how they are sorted and passed on from one generation to another during a process called meiosis. meiosis a type of cell division that halves the number of chromosomes, Packaging DNA for gametes necessary to produce four haploid It would not be possible for a cell to function or divide with 3  metres of DNA floating in the sex cells nucleus. Each DNA molecule is tightly coiled up with proteins called histone proteins to form chromosome chromosomes (Figure 1.8), much like cotton thread is coiled around plastic spools to keep it a densely coiled structure from getting tangled. Each chromosome contains one coiled molecule of DNA and each DNA that holds the DNA (with molecule contains many genes placed end to end. coded instructions for our characteristics) 12 ISBN 9780170231510  Chapter 1 Genetics and biotechnology Chromosome Chromatid Centromere ACTIVITY SHEET Looking at chromosomes WEBLINK How DNA is packaged DNA double helix Histone proteins A A G T T T C Chemical bases A G T C A G C A C G Figure 1.8 G C DNA is wound tightly around histone proteins to form A T chromosomes. ISBN 9780170231510 13  nelson We know that cells have special structures called chromosomes and that chromosomes are made up of genes that code for proteins. How do these genes get into gametes to then be passed on to the next generation? somatic cell Prior to sexual reproduction, cells divide by meiosis. This process allows the DNA stored in the nucleus of cells to be distributed in sperm or ova so that the correct number of a body cell (not gamete) that contains (in humans) a diploid chromosomes (and genes) can be passed on to new generations. number of chromosomes In body cells, or somatic cells, each cell has two copies of every chromosome. These cells are said to have the diploid (2n) number since they contain two full sets of chromosomes. Each pair homologous chromosomes of chromosomes consists of a maternal chromosome (inherited from the mother) and a paternal a pair of chromosomes with chromosome (inherited from the father). These pairs are called homologous chromosomes. matching genes Belgian embryologist Edouard Van Beneden (1846–1910) determined that all sex cells haploid (n) had half the number of chromosomes of other cells in the body. He counted exactly eight a cell with one of each chromosomes in the body cells of horse roundworm but found only four in its gametes. chromosome (in humans, one from The process of meiosis produces gametes with half the total number of chromosomes – they each pair); a sex cell is haploid only have one copy of each chromosome pair in the cell. This is called the haploid (n) number. diploid (2n) So when two gametes – the sperm and ovum – come together at fertilisation, the total having two copies of each complement of chromosomes for the organism is restored. The resulting fertilised egg or chromosome in a body cell zygote has the diploid (2n) number. Gigabytes of gene files? If all the information in the DNA of a single diploid cell of your body were to be stored in binary form like computer data, it would take up 1.5 gigabytes of computer space! autosome Each species, from plants to animals, has a different number of chromosomes in their a chromosome that does not body cells. Humans have 46 chromosomes – that is, 23 pairs of chromosomes. Twenty-two of determine sex (in humans, these pairs are known as autosomes; the remaining pair determines the organism’s sex and pairs 1–22) they are known as the sex chromosomes. Females have two X chromosomes, whereas males sex chromosomes have one X and one Y chromosome. The full set of human chromosomes is shown in Figure 1.9. chromosomes that determine the sex of an organism (X and Y) Human chromosomes (a total of 46) 1 2 3 4 5 6 7 8 9 10 11 12 Figure 1.9 Humans have 23 pairs Shutterstock.com/Blamb of chromosomes – 46 in 13 14 15 16 17 18 total. Shown here are the chromosomes of a male – there is one X chromosome and one Y chromosome. 19 20 21 22 14 ISBN 9780170231510  Chapter 1 Genetics and biotechnology Table 1.1 shows the different haploid and diploid numbers of some species. Table 1.1 Chromosome numbers in several species Chromosome number Species Somatic cells (diploid, 2n) Gametes (haploid, n) Garden pea (Pisum sativum) 14 7 Plains lubber grasshopper 24 12 (Brachystola magna) Fire salamander 24 12 (Salamandra salamandra) European sea urchin 38 19 (Echinus esculentus) Human (Homo sapiens) 46 23 Dog (Canis lupus familiaris) 78 39 Black rhinoceros (Diceros bicornis) 84 42 Adder’s tongue A small herb, adder’s tongue (Ophioglossum reticulatum), has 1262 chromosomes in each body cell. This is the gonad highest known chromosome count for any an organ that produces sex cells; Science Photo Library/Vaughan Fleming living species. in humans, ovaries in females and testes in males crossing over an exchange of genetic material in maternal and paternal homologues prophase the first stage of cell division Making gametes: meiosis homologue a pair of chromosomes with In order for humans to pass on genes, there has to be a process to divide a parent’s 46 matching genes chromosomes down to the haploid number of 23. This process in called meiosis. Meiosis is the sister chromatids division of a cell nucleus. In multicellular organisms such as humans, meiosis occurs in specialised two identical chromosomes joined reproductive structures called the gonads (in humans, these are the ovaries of females and testes together, after the chromosome of males). Meiosis occurs in two stages. During meiosis I, one cell divides into two; during meiosis II, has replicated and before it these two cells further divide into four cells. Each stage consists of a number of phases (Figure 1.10 divides on page 16). Meiosis I includes two crucial events. First, crossing over (during prophase I) swaps interphase sections of DNA between homologues and results in four unique chromatids (Figure 1.12 on page 18). the period in which the cell is not Second, homologous chromosomes are separated (during anaphase I). Meiosis II divides the sister dividing, and the cytoplasm is chromatids of the haploid cells. Meiosis I is preceded by interphase. active ISBN 9780170231510 15  nelson MEIOSIS I Cell eventually divides into Nuclear Centriole pair Chromosomes two after cytokinesis occurs. membrane line up along Pairs of disappearing midline of centrioles* cells. Chromosomes separate and uncoil and become give rise to Cell less visible. the spindle fibres. Homologous membrane Spindle fibres Homologous chromosomes Nuclear chromosomes separate and move to membrane opposite ends of the cell. may re-form. Prophase I Metaphase I Anaphase I Telophase I Daughter cells Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Daughter cells Chromosomes coil and Homologous Homologous chromosomes One chromosome of each The two daughter become visible. chromosomes move apart. The side to pair reaches the opposite cells are haploid. Nuclear membrane disappears. align in the middle which the maternal and poles of the cell. Note that each Spindle forms and attaches to of the cell. paternal chromosomes go is In some organisms sister chromatid is the centromeres. random. the nuclear membrane unique. Homologous chromosomes Note that, in each may re-form, and the move towards one another. chromosome, sister chromosomes become Crossing over occurs. chromatids are still attached. less visible. *Centrioles are minute rod-like structures that form the poles of the spindle. MEIOSIS II Prophase II Metaphase II Anaphase II Telophase II Daughter cells Prophase II Metaphase II Anaphase II Telophase II Daughter cells Chromosomes condense. Chromosomes Sister chromatids Chromatids reach the After the second division, Spindle forms and attaches align in the middle separate at the opposite poles of the there are four genetically to the centromere. of the cell. centromere and move cell. unique daughter cells. Nuclear membrane to opposite poles of the Cytokinesis begins. disappears. cell. Figure 1.10 The stages of meiosis 16 ISBN 9780170231510  Chapter 1 Genetics and biotechnology Interphase Most of a cell’s life takes place in interphase. The cell replicates the DNA so that it has an extra centromere copy of each chromosome. This replication occurs in the same way as described on page 15. the part of the chromosome that The replicated DNA is now joined together at the centromere (Figure 1.11). Each vertical half joins sister chromatids and where of the chromosome is known as a chromatid. the spindle fibres attach Figure 1.11 A model of a replicated Centromere chromosome. The section where the two sister chromatids join is called the centromere. Shutterstock.com/sgame chromatid each vertical strand of a chromosome Meiosis I: shuffling and mixing DNA spindle (fibres) protein fibres in the cytoplasm that control chromosome Prophase I movement A new structure called a spindle starts to form. This is a network of thin protein fibres equator that stretches across the cell and is used to move chromosomes. The spindle moves the homologous chromosomes towards one another and starts to pair them up at the equator. the centre of the cell where the chromosomes line up during This is when crossing over occurs, resulting in unique combinations of DNA information in the metaphase and anaphase four sister chromatids (Figure 1.12 on page 18). Most homologues will cross over at least once. This is an important source of variation in sexually reproducing organisms. pole one end of a dividing cell Metaphase I independent assortment The homologues line up along the equator of the cell in their pairs so that they face opposite random separation of pairs of sides of the cell. The spindle fibres are still attached to the centromere of each chromosome. chromosomes during meiosis, so that a gamete contains a mix of Anaphase I paternal and maternal genes It is during this phase of division that the number of chromosomes halves. The spindle fibres contract cytokinesis and pull one chromosome of each homologous pair to either pole of the cell. Each pole receives a the process of dividing the cell mixture of maternal and paternal chromosomes. This is called independent assortment. in two, at the end of the division of DNA Telophase I organelle During telophase 1, nuclear membranes may form and cytokinesis occurs. Cytokinesis a small structure inside the cell is the pinching of the cell membrane and the separation of organelles into the two cells. that helps maintain its function ISBN 9780170231510 17  nelson A B A B 4 chromosomes with No 2 types of information: A B A B genes AB and ab crossing over a b a b a b a b A B A B 4 chromosomes with Crossing over 4 types of information: B in one place A b A genes AB, Ab, aB and ab a b a B a b a b Figure 1.12 A schematic diagram Meiosis II: separating sister chromatids explaining why crossing over The two cells formed during meiosis I contain a chromosome from each homologous pair. during metaphase I results These chromosomes still consist of chromatids joined by a centromere. During meiosis II, each in four unique chromatids chromosome splits at the centromere, allowing the two chromatids to move into separate cells. During telophase II, the nucleus re-forms and cytokinesis occurs, creating four haploid sex cells. Each cell contains different combinations of genes and only half the original chromosome number. This overall process and order of the phases can be remembered by a simple acronym: IPMAT PMAT. WEBLINK ACTIVITY 1.2 Meiosis The meiosis dance In a group of 8–10, stretch your legs and dance your way through the stages of WEBLINK Meiosis: the chromosomal meiosis. Each person has a role to play. Use your textbook to follow the stages. wonderdance Roles: centrioles (you will need to use long string for spindle fibres), chromosome pairs/chromatids – work out who will take which roles and how many dancers you need. Music is optional, but will help you remember the stages. Watch the award- winning student video, ‘Meiosis: the chromosomal wonderdance’ for inspiration. WEBLINK Fertilisation and what happens next Fertilisation in sea urchins Humans carry out sexual reproduction. At the moment of fertilisation, the haploid sperm and ovum fuse, pairing 23 chromosomes from the father and 23 chromosomes from the mother. There are now 46 chromosomes in the new cell or zygote. This zygote grows and divides by the mitosis process of mitosis. Each time a cell divides, two identical daughter cells result. Each daughter a type of cell division that cell has the same number and type of chromosomes as the initial cell (also called the parent reproduces the DNA information cell). If the zygote inherits a Y chromosome from the sperm cell and an X chromosome from the exactly; it is used in body cells egg cell, it will develop into a male. If it inherits two X chromosomes, one from the sperm and one from the egg, the zygote will develop into a female. 18 ISBN 9780170231510  Chapter 1 Genetics and biotechnology ACTIVITY SHEET A different take on hotting things up? Sex as a game of ‘chance’ In some reptilian species, sex is determined by environmental temperature. Cooler temperatures during a critical stage of some turtle egg development will result in all male offspring. Warmer nest conditions will result in all female offspring. ACTIVITY SHEET Values and issues Mitosis ACTIVITY SHEET Modelling mitosis Mitosis is the process in which somatic (body) cells undergo a single nuclear division, giving rise to two genetically identical daughter cells. Mitosis is important for growth, for replacing damaged cells and for the maintenance of an organism. Figure 1.13 German biologist Walther Flemming (1843–1905) stained cells taken from an amphibian. In The stages of mitosis were the nuclei, he saw thread-like structures he called chromosomes (from the Greek word chroma, first described in detail meaning ‘colour’). The chromosomes divided lengthwise and separated equally between the two by Walther Flemming in daughter cells. Flemming named this process mitosis (from the Greek mitos meaning ‘thread’), 1882. By the end of this cell and showed how the cell nuclei of the two daughter cells would be exact copies of the original division, the two daughter nucleus (Figure 1.13). cells have the same number of chromosomes as the original parent cell. Pairs of centrioles* Chromosomes separate and give rise Centriole pair uncoil and become to the spindle fibres. Spindle fibres less visible. Cell eventually divides into two Cell membrane after cytokinesis Cytoplasm Chromosomes occurs. Nuclear membrane line up along Chromatids disappearing midline of cells. separate and move Nuclear to opposite ends of the cell. membrane re-forms. Prophase Metaphase Anaphase Telophase *Centrioles are minute rod-like structures that form the poles of the spindle. Prophase (Greek pro Metaphase (Greek meta Telophase (Greek telos meaning ‘beginning’) meaning ‘after’) Anaphase meaning ‘end’) Chromosomes coil and Chromosomes are The spindle fibres contract The chromosomes reach the become shorter and visible. positioned around the towards the poles of the cell. opposite poles of the cell. The nuclear membrane equator, forming a circle. The centromeres divide – the The nuclear membrane re-forms, disappears. chromosomes are now two and the chromosomes become A network of spindle fibres sister chromatids. less visible. forms. Sister chromatids move Cytokinesis begins – animal cells Spindle fibres attach to apart. ‘pinch in’ and plant cells grow new the centromere of each cell membranes and cell walls chromosome. between the two nuclei. ISBN 9780170231510 19 nelson WORKSPACE QUESTIONS 1.3 What have you learnt? 1.3 What have you learnt? Understanding 1 Distinguish between ‘gamete’ and ‘zygote’. 2 Explain how gametes, DNA and genes each play a part in passing on heritable characteristics during reproduction. 3 Describe where genes are located in a cell. 4 Explain where the chromosomes in our own cells came from. 5 A kangaroo has 12 paired chromosomes. a Explain what is meant by the diploid (2n) number of chromosomes. b State how many autosomes there are in each cell. c If this kangaroo is male, propose how many sex chromosomes are in each sperm. d Calculate what number n is in a kangaroo. 6 A platypus has 52 chromosomes. a Explain how many chromosomes came from its mother. b Calculate what the haploid number is in a platypus. c Determine how many homologous pairs are present. d State the number of chromosomes that will be present in the sperm of a male platypus. e Explain why the sperm must have this number of chromosomes. f Discuss whether all the sperm produced by a particular platypus will be identical in their genetic make-up. g Calculate how many chromosomes there would be in the daughter cells formed when a platypus body cell divides by mitosis. h Explain what the genetic difference, if any, will be between these daughter cells. Applying 7 A man and a woman plan to have a family. Calculate the probability that: a their first child will be a son b their second child will be a son c they will have a daughter, a son and then another daughter d they will have a son and a daughter if they have two children. Analysing 8 Figure 1.14 shows a cell dividing by mitosis. Suggest the evidence it shows to support this. Figure 1.14 A dividing cell 20 ISBN 9780170231510  Chapter 1 Genetics and biotechnology QUESTIONS 1.3 9 Compare the two types of cell division by completing the table below. Mitosis Meiosis a What happens to the number of chromosomes? b How many cell divisions are involved? c How many daughter cells are produced? d What is the purpose of this kind of division? Evaluating 10 Identical twins are the result of the newly formed zygote splitting in two. a Explain why identical twins look alike. b Explain whether it is possible to have identical twins that are a boy and a girl. 1.4 Sources of variation Sexual reproduction In meiosis, crossing over and independent assortment of paternal and maternal chromosomes shuffle and rearrange genes. The fertilisation of cells from two parents further shuffles genes. ACTIVITY SHEET Together, these processes produce variation that shows up in the offspring and contributes to the Hypotheses for the origins of sex biodiversity that we see on Earth. Some of these offspring are likely to have gene combinations that result in them being less well adapted to the current environment than their parents and some gene combinations will give them an advantage. For example, they may be smaller or larger, slower or faster growing or more sensitive to disease. For a species, this diversity is essential because if the environment changes, some individuals may be able to survive. Shutterstock.com/Celeste Cota Photography Figure 1.15 Offspring in the same litter can display a large range of genetic variation. ISBN 9780170231510 21 nelson WORKSPACE ACTIVITY 1.3 Variable offspring – ‘If... then’ statements Variable offspring – ‘If... then’ statements 1 Working in a group, brainstorm at least five ideas for each of the following ‘If … then’ statements. If only poorly adapted organisms were produced, then... If only well-adapted organisms were produced, then... 2 Using your brainstormed points for the ‘If …   then’ statements, justify or refute the following statement: ‘Variation in offspring is vital for the survival of a species’. Mutation mutation Mistakes or mutations can happen during DNA replication preceding cell division, or during a change in the cell DNA code or the process of meiosis, giving rise to a change in the DNA code or chromosome number. Only chromosome number mutations that occur during meiosis can be passed on to offspring. Mutations are an important source of variation within a species. There are many kinds of mutagen mutations. Some mutations are very harmful. Changes to amino acid order can impede or stop an environmental factor that cell function. Damage to the cell division processes can cause tumours and cancer. causes mutations Occasionally, mutations produce a variation that is neither harmful nor beneficial to the organism. For example, hair colour or skin tone can vary from harmless mutations. They are not life-threatening and so they are not eliminated from the population by natural selection. Some mutations are ‘silent’ and do not produce a noticeable effect in the organism. For WEBLINK example, sometimes a nucleotide is replaced in DNA with a different one, yet the amino acid Inside cancer that it translates to remains unchanged. Mutations may arise spontaneously during DNA replication. Alternatively, certain environmental factors (mutagens) may cause mutations to occur during replication. Exposure to excessive ultraviolet (UV) rays can cause mutations in dividing skin cells. Exposure to WEBLINK cigarette smoke, which contains harmful chemicals, can cause mutations in dividing lung cells. Gene mutation Excessive exposure to X-rays can also result in mutations. QUESTIONS 1.4 WORKSPACE What have you learnt? 1.4 What have you learnt? Understanding 1 Define ‘mutation’. 2 Identify the processes during which mutations can occur. 3 Outline some of the changes that can result because of mutations in DNA. 22 ISBN 9780170231510  Chapter 1 Genetics and biotechnology QUESTIONS 1.4 4 Explain why some mutations are ‘silent’. 5 Identify three factors that can increase the rate of mutations. Applying 6 The answer is ‘mutation’. Think of 10 possible questions. List these on your blog. 1.5 Mendel: the father of genetics Our understanding of genetics increased dramatically in the 20th century, once the structure of DNA was discovered. We can now use this knowledge to predict the likelihood of an individual passing on certain inherited conditions to the next generation. The path for this understanding was first laid in the 19th century by Austrian Augustinian monk Gregor Mendel (1822–84). Mendel studied physics and mathematics at universities in Olmütz and Vienna between 1840 and 1850. While other natural scientists were describing varieties of species, Mendel performed pure-breeding breeding experiments with a species of self-fertilising garden peas, Pisum sativum (Figure 1.16). individuals who carry the same He was looking for numerical patterns in the results. He noticed in some plants that certain information for a particular trait varieties with the same characteristics appeared generation after generation. Mendel considered parental generation (P) these plants to be pure-breeding. the parents in a cross Mendel investigated what would (pure-breeding in Mendel’s happen if two different pure-breeding experiments) varieties of peas were crossed, for first filial (F1) generation example a round-seeded pea plant with a wrinkled-seeded pea plant. Mendel’s the generation that results after breeding parent organisms investigations were scientifically valid, as he was able to control plant crosses. He dominant trait removed the anthers of unopened flowers a trait that masks the other trait of a round-seeded pure-breeding plant and of a pair, if present brushed its pollen onto the stigma of a wrinkled seeded pure-breeding plant. These first two plants became known as the parental generation (P). In these experiments, Mendel observed VIDEO Mendel’s experiments that the resulting offspring plants, or first on inheritance Shutterstock.com/Susan Law Cain filial (F1) generation, always resembled only one of the parent plants – in this case they all had round seeds. He therefore called the feature they exhibited the Figure 1.16 Mendel used the garden dominant trait. pea Pisum sativum for his experiments. ISBN 9780170231510 23  nelson The Habsburg chin The imperial House of Habsburg is nearly as well known for its chin as for its important political connections across Europe over six centuries. Famous Habsburgs include Queen Marie-Antoinette (1755–93) (left), who died during the French revolution, and Spain’s King Charles II (1661–1700) (right), whose prominent jaw prevented him from chewing. Alamy/Masterpics Getty Images second filial (F2) generation However, when the F1 plants self-fertilised, the next generation, or second filial (F2) the generation produced from generation, always had most plants resembling the dominant parent (round seeds) and some breeding the first filial (F1) resembling the non-dominant parent (wrinkled seeds). Mendel inferred the non-dominant trait generation must have been carried, but hidden, within the F1 generation plants. He called these plants hybrid hybrids (mixed) and said the hidden trait was recessive. Mendel investigated seven pea plant features in thousands of P, F1 and F2 plants. Activity offspring from the cross of two different pure-breeding parents sheet ‘Mendel’s original data’ has a table that looks at some of his original data. Mendel recognised that each F2 generation showed the dominant and recessive features recessive in an approximate 3:1 ratio, respectively. This is now known as the Mendelian ratio. By a trait that is only visible if no mathematically analysing his results, Mendel concluded that the information for traits was dominant trait is present carried in pairs, only half of the information was passed on from each parent and fertilisation Mendelian ratio combined the halves back into a pair (as described on page 18). The appearance of the the 3:1 pattern of dominant to offspring was determined by the information that was paired together. That he was able to recessive individuals in the F2 reach these conclusions is amazing, because Mendel had no knowledge of the structure of generation DNA, the existence of genes or the process of meiosis. allele With our understanding of chromosomes, genes and meiosis, we can see how Mendel’s discoveries match what actually happens in cells. For every gene (such as the one controlling an alternative form of a particular gene seed shape) on one chromosome, there is a corresponding gene on the homologous chromosome. However, Mendel did not know the word ‘gene’. He referred to ‘factors’. He discovered that in pea plants, there are alternative forms of these factors (genes), such as round or wrinkled seeds. In modern genetics, these different forms of the same gene are called