From DNA to gene to chromosome. (a) Chromosomes consist of a continuous DNA double helix and associated proteins and are located in the cell's nucleus. (b) A transmission electron micrograph of a chromosome in its replicated form, just prior to cell division. Each half of the chromosome is a chromatid. Note the constriction, where the centromeres are attached (25,000x).

A gene's nucleotide sequence tells a cell how to link a certain sequence of amino acids together to construct a specific protein molecule. Recall from chapter 4 (p. 123) that the information in a DNA sequence is transcribed into a molecule of mRNA, which, in turn, is translated into a protein. The protein ultimately determines the trait associated with the gene, as figure 24.2 illustrates for cystic fibrosis (CF).

All of the DNA in a human cell constitutes our genome (je-nome). The genome includes about 35,000 protein-encoding genes interspersed with many highly repeated sequences whose function is not known. In all cells except for the eggs and sperm, the DNA is distributed among 23 pairs of chromosomes, for a total of 46 chromosomes. These nonsex, or somatic cells, are said to be diploid because they have two sets of chromosomes. Recall from chapter 22 that sperm and eggs, which contain 23 individual chromosomes, are haploid. They have half the amount of genetic material of other cell types.

Genetic information functions at several levels. It is encoded in chemicals, affects cells and tissues, and is expressed in the individual, yet is also passed to the next generation. At the population level, genetic change drives evolution. Until recently, the field of genetics dealt mostly with single genes and the rare disorders that can be traced to the malfunction or absence of single genes. However, knowing the human genome sequence has made it possible to view physiology at the microscopic level, as a complex interplay of gene function. Looking at the human body in terms of multiple, interacting genes is a new field termed genomics. Figure 24.3 views genomics at the whole body, cellular, and molecular levels.

The science of genetics has traditionally focused on disorders caused by single genes. However, the field now increasingly recognizes that genes provide our variability as well as illnesses, including eye, skin and hair color; height and body form; special talents; and hard-to-define characteristics such as personality traits. Clinical Application 24.1 highlights a few interesting nonmedical traits rooted in the genes.

As important as genes are, they do not act alone. Often the environment influences how genes are expressed. The environment includes the chemical, physical, social, and biological factors surrounding an individual that influence his or her characteristics. For example, a person who inherits genes that confer susceptibility to smoking-induced lung cancer will probably not develop the illness if he or she never smokes and breathes clean air. Intelligence is a good example of a characteristic that has many genetic and environmental influences.

Organs affected in cystic fibrosis

Chromosome 7

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