1 Using information stored in DNA One important property of DNA is that it carries genetic information in the simple coding language of just four bases. These bases, which can be arranged in a huge variety of sequences, represent a vast potential store of information. In this unit, we consider how this information is used by the cell. The key structural feature of complementary base pairs, which plays an important role in both stability and replication, is also the basis for how DNA functions as genetic material. How do
Learning outcomes After studying this unit you should understand: how the linear sequence of DNA within a gene is related to the linear sequence of amino acids of a protein how the information in DNA is carried via RNA to make a protein how RNA is synthesised from DNA by the process of transcription where the processes of transcription and translation occur within the cell
Introduction This unit explores how information contained in DNA is used, explaining the flow of information from DNA to RNA to protein. Also introduced are the concepts of transcription (as occurs between DNA and RNA) and translation. This unit is an adapted extract from the course Human genetics and health issues
(SK195)
Acknowledgements The content acknowledged below is Proprietary (see terms and conditions) and is used under licence. Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:
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1.1.6 Late-onset multifactorial disorders It is becoming clear that many, if not most, of the common diseases that affect the Western world are multifactorial disorders with some inherited genetic component. Some of the genes that render individuals susceptible to diabetes, coronary heart disease, hypertension and many cancers, including breast cancer, have been identified and can be tested now for the presence of mutations. Multifactorial disorders present a real challenge for genetic medicine. For example, while it may be true that
1.1.5 Late-onset single-gene disorders An individual might know that a late-onset disease such as Huntington's disease (HD) is present in their immediate family and that they might have inherited the disease gene(s). The problems of genetic testing for HD revolve around the fact that it is pre-symptomatic. One dilemma is the long delay between testing positive and developing the clinical symptoms of the disorder in middle age. Is it better not to know and live in hope, or as one victim cried ‘get it over, I'm so tir
1.1.4 Genetic testing of adults Huntington's disease is a good example of a late-onset disorder because it is fatal, non-treatable, relatively frequent and has a strong genetic element that can be tested for. There are others that fall into a similar category, i.e. mainly relate to a single gene, such as adult polycystic kidney disease. The issues surrounding late-onset multifactorial diseases, such as diabetes and breast cancer, will be dealt with separately. To date, relatively few diseases that fall into both these categ
1.1.3 Genetic testing of children Within clinical genetic services, a difference has grown up between the testing of children and the testing of adults. Sometimes the genetic testing of children is relatively uncontroversial. For example, the genetic test may simply be to confirm a medical diagnosis that has been made on clinical grounds. So a three-year-old with low weight, blocked lungs and poor digestion may be given a genetic test to see whether they have CF or not. There are other cases where a test is used predict
1.1.2 Pre-natal diagnosis The type of genetic testing that the majority of us are most likely to come across is still pre-natal diagnosis (PND). This involves testing a fetus during pregnancy, to see whether it is likely to suffer from a number of different disorders — some genetic, some not. While recent developments allow tests for certain multifactorial genetic diseases (such as spina bifida), pre-natal diagnosis has been available since the 1960s to test for Down's syndrome. Most cases of Down's syn
1.1.1 Genetic counselling In the UK and many other countries, genetic testing is provided by the National Health Service (NHS) or its equivalent, only after patients have undergone genetic counselling. This is defined as the provision of information and advice about inherited disorders, and includes helping people to: Understand medical facts; Appreciate the way in which inheritance contributes to the disease in question; Understand the
Learning outcomes After studying this unit you should understand: something of the role of a genetic counsellor and its non-directiveness the difference between pre-natal diagnosis, childhood testing and adult testing and give some examples of diseases that may be tested for the ethical and moral difficulties involved in making decisions on whether or not to carry out such tests
Introduction This unit looks at three different uses of genetic testing: pre-natal diagnosis, childhood testing and adult testing. Such tests provide genetic information in the form of a predictive diagnosis, and as such are described as predictive tests. Pre-natal diagnosis uses techniques such as amniocentesis to test fetuses in the womb. For example, it is commonly offered to women over 35 to test for Down's syndrome. Childhood testing involves testing children for genetic diseases that may not
Acknowledgements The content acknowledged below is Proprietary (see terms and conditions) and is used under licence. This content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:
2.5 Summary When we read about genetics and the future of medicine, we should also think about genetics and the future of health services that have to deliver medical care. The advent of predictive medicine, based on more detailed DNA profiling of individual genotypes using technologies like gene chips, rather than screening for one gene at a time, may shift the relationship between doctor and patient. People will be seeking advice on how to manage their susceptibilities or genetic risks, rather than loo
2.4 Current UK provision One way of describing the organizational shift that the advent of predictive medicine would demand is to suggest that genetics would become a general, rather than a specialist service. But it is much easier to say that than to explain how it will happen. For all the publicity about genes, genomes and genetic information, medical genetics is a very small part of current health services. In the UK, an indication that a patient or a family has a genetic problem will lead to a referral to a
2.3 Scaling up They may look at dozens of alleles, and involve thousands of people, but existing screening programmes have been concerned with individual genes. But the technologies now being developed will soon permit the recording of hundreds of genes at a time. So-called gene chips combine the skills of microchip designers with DNA sequence information to offer rapid, easy-to-read results for an individual covering hundreds of genetic variants. A gene chip is a thin slice of glass about the
2.2.4 Longer-term considerations Something else to ponder is the effect that screening might have on the longer-term incidence of disease and (not the same thing) on the incidence of gene variants linked to disease. Sometimes, the impact on a disease can be dramatic. Take thalassaemia, a haemoglobin disorder similar to sickle cell disease, in which premature destruction of haemoglobin-containing red blood cells leads to anaemia. It is relatively common in some Mediterranean countries. Like sickle cell disease, it is understo
2.2.3 Cystic fibrosis A different model for the genetic tests of the future is screening for cystic fibrosis (CF). This is a DNA-based test, which became possible after the gene involved in CF was identified in 1989. CF is a recessive disease, and it should be easy to test to see if prospective parents carry a mutated allele. A simple mouthwash yields enough cells for DNA extraction. If both partners are carriers, they can consider further counselling before conception, and/or pre-natal testing of any potentially
2.2.1 Phenylketonuria The classic example of population screening is testing new-born babies for phenylketonuria (PKU). Individuals with PKU fail to make a protein, a certain enzyme, and develop mental retardation. The absence of the enzyme results in both an accumulation of phenylalanine, which causes the mental retardation, and a deficiency of tyrosine in the body, as shown in Author(s):
2.2 Population screening for genetic disease: the precedents Knowing about particular genes, or their effects, also permits screening – the search in a population for persons with certain genotypes that are associated with a particular disease. Thus the test may be offered to one and all. Until now, screening programmes have focused on one gene at a time, or one disease at a time, in cases where a mutated gene poses serious health problems and something can be done for those who are found to carry the mutation. What that something is varies with the
