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1.1.1 The chemical structure of DNA

This unit explores the chemical nature of the genome. Genomes are composed of DNA, and a knowledge of the structure of DNA is essential to understand how it can function as hereditary material. DNA is remarkable, breathtakingly simple in its structure yet capable of directing all the living processes in a cell, the production of new cells and the development of a fertilized egg to an individual adult.

DNA illustrates beautifully the precise relationship between molecular structure and b
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Learning outcomes

After studying this unit you should understand:

  • the basic composition and structure of DNA;

  • what is meant by complementary DNA base pairing;

  • how base pairing allows a mechanism for DNA replication;

  • the number of DNA molecules within a chromosome.


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4 From DNA to RNA: transcription

In the process of transcription, the information in a gene, i.e. the DNA base sequence, is copied, or transcribed, to form an RNA molecule. RNA is therefore an intermediary in the flow of information from DNA to protein. Before we consider the details of transcription, we will first look at the structure of RNA.

The name ribonucleic acid suggests that RNA is chemically related to DNA. Like DNA, RNA is a chain of nucleotides.

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3 The flow of information from DNA to RNA to protein

The information flow from DNA to protein is more complex than shown in Figure 1. The genetic information encoded within the DNA of a gene is carried via an intermediary molecule, RNA (ribonucleic acid). Information within a cell can therefore be seen as passing from DNA, via RNA, to a protein. This flow of information can b
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2 One gene–one protein

A gene is a short section of a long DNA double helix molecule, which comprises a linear sequence of base pairs.

SAQ 1

What is the basic (primary) structure of a protein?

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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
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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


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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)


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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
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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
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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
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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
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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
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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
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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


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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
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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:


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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
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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
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