Cola, tea, coffee


Food additive


Pesticides, herbicides, cigarette smoke


Antiseptic in veterinary medicine

Sodium nitrite

Smoked meats

Tris (2,3-dibromopropyl phosphate)

Flame retardant in children's sleepwear

cell cannot be repaired or does not die, it often turns cancerous. This is why many years of sunburns can cause certain types of skin cancer.

Because a gene consists of a sequence of hundreds of building blocks, mutation can alter a gene in many ways—just like a typographical error can occur on this page in many ways. Different mutations in the same gene can produce different severities of symptoms. The most common mutation in the gene that causes cystic fibrosis, for example, causes severe lung infection and obstruction and digestive difficulties and affected individuals often die young. Other mutations are associated with less severe effects, such as frequent bronchitis or sinus infections. This second group generally lives longer than people with the more common mutation.

A type of disorder called an "inborn error of metabolism" results from inheriting a mutation that alters an enzyme. Such an enzyme block in a biochemical pathway has two general effects: the substance that the enzyme normally acts on builds up, and the substance resulting from the enzyme's normal action becomes scarce. It is similar to blocking a garden hose: water pressure builds up behind the block, but no water comes out after it.

The biochemical excesses and deficiencies that an inborn error of metabolism triggers can drastically affect health. The specific symptoms depend upon which pathways and substances are affected. Figure 4.26 shows how blocks of different enzymes in one biochemical pathway lead to different sets of symptoms. Clinical Application 4.4 describes phenylketonuria (PKU), one of these inborn errors about which we know a great deal.

99 How are DNA molecules replicated?

^9 What is a mutation?

^9 How do mutations occur?

□ What kinds of mutations are of greatest concern?

Q How does the nature of the genetic code protect against mutation, to an extent?





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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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