GENETIC PROBLEMS AND INVESTIGATIONS

DNA replicates itself millions of times and with amazing accuracy over a lifetime but, sometimes, things can go wrong.



WHERE GENETIC PROBLEMS ARISE



When there is a change in DNA (due to an internal error in the normal functioning of a cell or an attack from an external environmental cause, known as a mutagen), problems can arise on three main levels. In the first level, a change in a gene affects the protein for which it codes. The next level sees a change in the number of chromosomes. Thirdly, problems can occur when there are alterations in several genes plus environmental triggers. There is a fourth level, affecting mitochondrial DNA, but this is unusual compared with the other three levels.



GENE LEVEL



Faulty genes can be inherited, can spontaneously mutate in an embryo, or can accumulate mutations after long exposure to mutagens such as UV from the Sun, radiation, or tobacco.




MULTIFACTORIAL LEVEL



Some diseases are influenced by mutations in a number of genes plus environmental factors that affect susceptibility. For example, Alzheimer's disease and breast cancer are multifactorial in their origins.




CHROMOSOMAL ' LEVEL



Errors can occur when chromosomes are divided up during mitosis and meiosis (see pp.50-51), such as the inheritance of an incorrect number of chromosomes.



MITOCHONDRIAL LEVEL



J; Mutations can occur in the DNA I contained in the cell's mitochondria* the structures that give cells the energy they need to work. Their DNA codes for proteins that are needed to keep mitochondria working properly.



MUTATIONS



Any permanent alteration in the code of a DNA sequence is known as a mutation. It can be as small as a one-"letter" change in a gene or as large as a chunk of chromosome. The effect of a chromosomal mutation depends on the size and location of the structural change and whether any DNA is lost. It usually occurs either in the sperm or eggs, or early in embryonic development. Gene mutations can be inherited or they can occur spontaneously in an embryo. But, often, they occur in body cells when the intricate system for replicating DNA slips up somewhere. A gene mutation can have a negative effect if it impairs the normal functioning of a gene.



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