HAND-ME-DOWN GENES an introduction to genetics

Transcription

1 HAND-ME-DOWN GENES an introduction to genetics program 1: how genes work Written by Jan Rosenberg B.Ed. Edited by David Brown and Stewart Jackel Teacher Notes Following many requests from teachers, these notes have been printed on A4 size sheets for easy photocopying. 111A MITCHELL STREET BENDIGO VIC 3550 TELEPHONE (054) FAX (054) Produced by ROD REES for VEA,1997 Duration: 24 minutes This program is protected by copyright laws both in Australia and overseas. The penalties for unauthorised copying of this program include a $50,000 fine

2 for individuals and a $250,000 fine for institutions. Program 1: How Genes Work The two part program HAND-ME-DOWN GENES introduces basic genetic concepts for years 9 & 10 science. In particular, the programs support the following curricula: National Curriculum - Middle school science Victorian Curriculum - CSF Life and Living strand Level 6 NSW Curriculum - Middle school science - 9 & 10 GENERAL INTRODUCTION Almost every week there are news items of discoveries or new applications of genetic science. Genetically engineered crops and food plants, methods for detecting genetic disorders in embryos, cloned animals and plants, trials of genetic based treatments for life threatening conditions, and the production of many important drugs are just a few. It is important for students to gain a sound understanding of genetics so they can appreciate the impact of these developments and the science behind them. These programs are designed for middle school science students and cover the fundamental ideas needed to understand how genes work and how human characteristics are inherited. They are based on the latest research in genetics and presented in such a way that students will find interesting and comprehensible. The latest in computer generated graphics and film footage of young people discussing their genetic conditions provide engaging examples of the ideas that underpin the programs. USING THE PROGRAMS The programs offer explanations of the fundamental principles of genetics. The material is logical and dense and therefore lends itself to use in contained segments rather than in a single viewing. The programs could provide the basis for designing a whole unit of work around each of the main themes - in program 1: (1) How genes work in program 2: (2) Family patterns: inheritance (3) Genetic disorders SUMMARY Program 1: HOW GENES WORK Introduction: Our body is made up of billions of cells. How do our cells know which cell to be? Skin, hair, or muscle? And how is it that we are physically similar yet different to the parents from whom we came? 2

3 Starting from the most simple places - the base of a single hair - Program 1 unravels the amazing story of the structure and functions of chromosomes, DNA, genes, and the protein production directed by the genes. At the end of the journey we find the answer to how a cell knows which cell to be. The proteins produced by DNA and the genes are the vital factors in coding the physical make up of us all. Oliver has a special additional story. He has Achondroplasia, a condition which affects the long bone growth in humans. By exploring the mechanisms of DNA and its incredible coding process, we can gain insight as to how, by chance, a single genetic spelling error has left Oliver with a different stature. We have long known that some human conditions run in families. Like eye colour, blood group and many other characteristics, they are inherited - controlled by factors called genes. In recent years, geneticists - scientists who research inheritance - have deciphered the code system of genes and how it controls the production of all the different proteins essential for our bodies to grow and function. In the beginning... the Nucleus The program starts the journey with a single cell at the base of the hair. Through graphic animation, we take a look inside where we find all the basic components needed for the cell to function and reproduce itself - the nucleus, chromosomes, genes, DNA, and ribosomes First, the chromosomes: inside every animal and plant cell nucleus, chromosomes are found. In a human body cell there are 46 chromosomes, with 2 of each type making 23 pairs. Second, the DNA: every chromosome consists of long chains of atoms which form the DNA molecule. The DNA carries the instructions which build the proteins for all parts of our bodies... and the characteristics we inherit from our family. The Genes: So, the program asks, how does the DNA carry the instructions? The answer is in the genes - the section of the DNA which carries the base code triplets which code for the construction of particular proteins from various combinations of the 20 available amino acids. If we could describe that the chromosomes as the Book of Plans from which to build a human being, then the DNA would be the pages and the genes would be the designs on those pages. The program goes on to explain that using these sections of code, the body or plant and animal cells can make all of the proteins it needs for the cell to function. Proteins are vital 3

4 building blocks of all living things. And that leads to the next question, how do the Genes control the making of proteins? Making Proteins: Once again graphic animation tells this part of the story. Here we see how new proteins are made outside the nucleus on tiny workbenches within the cell called ribosomes. The DNA molecule splits in two down the middle to make an messenger RNA copy (mrna). This mrna contains a mirror image of the DNA information and when it hits the ribosome work bench its coded information triggers the construction of an identical amino acid chain and a new protein molecule is created. Each protein has a specialised task and forms the muscle, nerve, hair, haemoglobin or any other of the hundreds of specialised functions in the body. But then, the program asks, how do the proteins in the hair, for example, know how to keep producing exactly the same type of proteins to give the hair its characteristic colours and shape? The answer is in mitosis cell division. Mitosis and the Cell cycle New cells are constantly being produced which is basically how our body grows and maintains itself ; for example, red blood cells and skin cells last only a few days. Mitosis is the name given to the process of a cell dividing to form two cells which then continue to grow and divide further. This part of the story goes like this: Before cell division begins, each double stranded DNA molecule unzips from one end. Both sides of the old strand act as a template for two new strands to be built. This allows two double stranded molecules identical to the original to be built. During Mitosis the DNA copies itself, coils up tightly and now consists of two identical molecules - called chromatids joined at one point called the centromere. The membrane around the nucleus dissolves and threads form across the cell. The threads become attached to the centromere of each chromosome which contract and pull to the opposite side of the cell. The copied chromosomes separate forming two identical bunches. All the other complex parts of the cell machinery, such as the ribosomes need to be duplicated as well. Eventually the cell divides and a membrane is built around each new nucleus. The result of this extraordinary process is two new cells with exactly the same set of DNA molecules, genes and chromosomes, as the cell they were formed from. What an event! Now, in a further refinement, while each body cell has a full set of genes, only certain ones are switched on in each type of cell so it is given the message of the type of cell it will 4

5 develop into eg. The switch says form skin pigment, or form nerve, or form haemoglobin etc, etc, etc. Finally, the question about Genetic conditions: At this point the program returns to Oliver and the young girl with Down Syndrome and asks, why? Here look closer at alleles, the different forms of a particular gene and the work done in the recent year by genetic researchers in locating alleles on chromosomes and identifying various forms of genetic misinformation. When a cell divides the DNA is copied exactly. Usually the copy is perfect. However occasionally a mistake is made : this is called a mutation. Some mutations involve whole chromosomes or parts of chromosomes (whole lengths of DNA) or, as in the case of Oliver and his condition called Achondroplasia, just one base. The difference in one base changes the code for just one of the amino acids that make up the protein concerned; that is enough to change the way that it works in the body. An incredibly small change in the genetic code has a dramatic effect. Once the duplication process has occurred it is replicated in every subsequent copy through mitosis. Mutations sometimes occur spontaneously, however other mutations can be induced by chemical substances or radiation. A key question we are left with is why can a child acquire a genetic condition like Down Syndrome when neither of her parents have the disorder? Is there part of the story we haven t been told? Obviously there is and that leads to Program two about inheritance : FAMILY PATTERNS. AFTER THE PROGRAM ADDITIONAL NOTE: More on... DNA - Deoxyribonucleic acid The chemical structure of DNA is a double helix - imagine it being similar to a spiral stair case with the hand rails formed by alternating sugar and phosphate molecules, and each steps being a pair of bases of either adenine, thymine, guanine, or cytosine: A with T and C with G. The deoxyribonucleic acid or DNA that makes up genes is able to carry complex inherited information because of certain features of its structure. All living things have the same DNA. Just the order of the bases in the immense DNA molecules is different - different order, different code, different proteins and enzymes and different organisms. 5

6 The different bases are: Adenine (A) Guanine (G) Cytosine. (C) Thymine (T) Geneticists discovered that the bases were strung together along each chain in an irregular pattern. They realised this was a coded message - a sort of code spelt out by the sequence of bases. The really clever bit about the code is the fact that: DNA is a two sided molecule. each side contains a sequence of As, Ts, Cs, and Gs. But A will only pair with T and C will only pair with G - we say that T is complementary to A and C is complementary to G. This allows each side of the DNA molecule to make a sort of mirror image of itself using the rule about the complementary pairs. STUDENT ACTIVITIES 1. Jumbled up genetics. Match these genetic terms to their correct meanings: Chromosomes Genes DNA Nucleus Inheritance Ribosome Geneticists Mitosis Mutation Scientists who study genetics Unit of chromosome that determines inheritance The process of characteristics being passed on. Part of DNA that codes for a specific protein Control centre of the cell: contains DNA Deoxyribonucleic acid An incorrect gene structure creating variation A long strand of DNA containing genes The process of cell division and reproduction 2. Seeing how it works: 6

7 a) Draw or make your own model of DNA, labelling all the components. Make sure it can unzip and carry the code in the same way as suggested by the program. b) Role play cell division in mitosis. Everyone can participate by forming nucleus and cell membranes as well as representing the dividing chromosomes. 3. Research. a) Describe some characteristics that have been inherited in your family (eg., hair colour). Draw a pedigree to show how the genetic characteristic has been passed on from one generation to the next. b) Choose you favourite pet (dog, bird, cat, mouse or horse etc) note the characteristic features of its breed. With the help of your phone book, local pet shop owner, vet or other specialist, locate a breeder and interview them about how they go about breeding the animal for particular characteristics. (The local KCC or Horse Racing Clubs may be of particular assistance and offer speakers to the class) c) Make a list of the genetic conditions mentioned in program one and note (1) how the condition occurs and (2) what effect the condition has on the person s development. Research at least three other genetic conditions NOT mentioned in the program and gather the same type of information. Present your findings to the class. d) In a group write up questions that occurred to you as you watched the video. Collate the questions on a large sheet of paper and group them into similar topic areas. Have you group present your Questions for further Explanation to the class. e) Watch the film Jurassic Park. Make some notes on the genetic science that underpins the story. After the film, discuss the accuracy and the possibility of this science in class. Perhaps, write a letter to Mr Speilberg, the director, telling him what you think of the scientific basis of his film, outlining where the film is accurate and where the story becomes pure fantasy. OTHER RELATED PROGRAMS AVAILABLE ON 14 DAY PREVIEW FROM VEA GENE GENIUS GENE CUISINE GENETICS IN THE 90S And more! Please call in Australia or in New Zealand for details 7

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