American Medical Association Family Medical Guide - American Medical Association [22]
DNA: The blueprint of life
Each of us begins life as a single cell containing genetic information from each of our parents. This original cell divides to about 100 trillion specialized cells, each containing a complete set of genes identical to that in the original cell. Your unique set of about 30,000 genes is called your genome. Your genome orchestrates the activity of all the cells in your body, enabling them to work together in harmony to keep your body healthy and functioning normally.
Cells
You have many different types of cells in your body that perform different functions depending on their location—for example, in muscles, in the brain, in the stomach, or in fat. Although each of your cells contains exactly the same genes, cells differ from each other because different combinations of genes are switched on and off in each cell. These different gene combinations determine what proteins each cell makes and enable the cell to perform a specialized task.
Chromosomes
Inside the nucleus of every cell are structures called chromosomes. Chromosomes are long threads of genetic material made up of strands of deoxyribonucleic acid (DNA). You have 23 pairs of chromosomes; each of your parents contributed one chromosome to every pair. Each chromosome contains from several hundred to several thousand genes. Just before cells divide, the chromosomes make duplicate copies of themselves; the two copies of each chromosome are held together at a narrowed region called a centromere, which gives the chromosome an X shape.
Genes
Genes are the basic physical and functional units of heredity, which provide instructions for making proteins, the building blocks of the body. Scientists believe that genes make up only about 2 percent of the total genome; the rest consists of regions whose functions include keeping the chromosomes intact and regulating where, when, and in what quantity proteins are made.
The genetic code
The information carried by a gene is determined by the structure of its DNA. DNA is arranged in the form of a double helix, or ladder, with rungs supported by a twisting ribbon of sugar and phosphates. The two intertwined DNA strands are linked together by varying patterns of substances called nucleotide bases. There are four nucleotide bases—cytosine (C), guanine (G), adenine (A), and thymine (T).
Along each helix, or side of the ladder, the bases are arranged in groups of three, called base triplets, to form words of genetic text. For example, base triplets, such as TAC, CGG, and TCA, carry specific amino acids (the building blocks of proteins) to assembly points in a cell where they are strung together to make functioning proteins. To form the rungs of the DNA ladder, the bases can join in only two ways: C joins only with G, and A joins only with T.
If you think of the nucleotide bases as the letters of the genetic alphabet, then base triplets are the words, genes are the sentences, chromosomes are the chapters, and the genome is each organism’s book of life. The human genome contains a total of about 3 billion nucleotide base pairs.
Proteins: Building Blocks of the Body
Proteins are essential for life. Some proteins make up structures such as muscles and skin; other proteins—including enzymes, antibodies, and many hormones—dissolve in the body’s fluids and are carried to wherever they are needed. Genes control the production of proteins inside cells and provide the instructions for making specific proteins in precisely the right number and at precisely the right time. Different proteins have different functions in the body:
• Enzymes regulate the rate of chemical reactions in cells.
• Antibodies, produced by the immune system’s white blood cells, destroy invading microorganisms and provide protection against infections.
• Proteins in muscle cells provide mobility, pump blood through the body, and help move food through the digestive