Nature of Mutations

Mutations can originate in a number of ways. In one common mechanism during DNA replication, a base may pair incorrectly with the newly forming strand, or extra bases may be added. Or, sections of DNA strands may be deleted, moved to other regions of the molecule, or even attached to other chromosomes. In any case, the consequences are similar—genetic information is changed. If a protein is constructed from this information, its molecular structure may be faulty and the function changed or absent. For example, the muscle weakness of Duchenne muscular dystrophy may result from a mutation in the gene encoding the protein dystrophin. The mutation may be a missing or changed nucleotide base or absence of the entire dystrophin gene. In each case, lack of dystrophin, which normally supports muscle cell membranes during contraction, causes the cells to collapse. The muscles weaken and atrophy. Figure 4.25 shows how the change of one base may cause another inherited illness, sickle cell disease.

Fortunately, cells detect damage in their DNA molecules and use repair enzymes to clip out defective nu-cleotides in a single DNA strand and fill the resulting gap with nucleotides complementary to those on the remaining strand of DNA. This restores the original structure of the double-stranded DNA molecule.

If DNA is not repaired, illness may result. A class of disorders affects DNA repair. One such condition is xeroderma pigmentosum (XP).

When other youngsters burst out of their homes on a sunny day to frolic outdoors, a child who has XP must cover up as completely as possible, wearing pants and long sleeves even in midsummer, and must apply sunscreen on every bit of exposed skin. Moderate sun exposure easily leads to skin sores or cancer. Even with all the precautions, the child's skin is a sea of freckles. Special camps and programs for children with XP allow them to play outdoors at night, when they are safe.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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