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Note on mutation/ gene mutation notes

A mutation is defined as a sudden change in the sequence of DNA. The corresponding phenotype is called mutant. Mutations are caused by chance or by mutagenic agents, which may be chemicals, biologicals, physicals, and radiation. Mutations are of two types genetic mutation and chromosomal mutation. 

When the change involves a small number of bases, then the mutation is genetic. When it is associated with changes in a more significant part of the chromosome, it is called a chromosomal abnormality. Gene mutations come in many forms. In base replacement, only one DNA base changes. Bases are added or removed to the addition or deletion respectively. This change can modify the gene product with adverse effects on the body.

Chromosomal abnormalities change the number or structure of chromosomes. Trisomies (an extra chromosome) occur more often than monosomes (lack of one chromosome) and cause less damage. The numerical chromosomal abnormalities of the sex chromosomes are less severe than those of the autosomes. Numerical chromosomal abnormalities are created by the non-separation of chromosomes during reduction.

Deficiency is the loss of genetic material from a chromosome. During reciprocal displacement two non-homologous chromosomes exchange segments. Inversion is the incorrect placement of a chromosome segment that has been cut and positioned after a 180-degree turn. Chromosomal abnormalities are studied by creating the karyotype, which represents the chromosomes of the individual being studied.

The mutations contribute to the creation of genetic diversity in the population and are responsible for many inherited diseases, as well as for many cases of cancer. Mutations can occur in any germ or body cell of an organism. Only germ cell mutations, however, can be passed from one generation to the next. But that does not mean that bodily mutations are less important for health. In fact, they are the majority of mutations, since an adult organism consists of about 1013 somatic cells.

One type of mutation recently discovered is trinucleotide extension, in which the number of replications of a particular triplet of nucleotides increases from generation to generation. Trinucleotide extension is the cause of myotonic dystrophy, an autosomal dominant form of muscular dystrophy that presents with more severe symptoms from one generation to the next.