Am I a Monkey?: Six Big Questions about Evolution

3 What Is DNA? [Includes Image Plates]

What Is DNA? DNA is short for deoxyribonucleic acid. The chemical structure of DNA is a double helix made up of two complementary strands, which are long chains of four different kinds of nucleotides: adenine (A), cytosine (C), guanine (G), and thymine (T). The genetic information embodied in DNA accounts for three fundamental aspects of life: (1) all living processes in organisms, (2) the precision of biological heredity, and (3) biological evolution. First, DNA holds the genetic information that directs all life processes. The information is encased in long sequences of the four nucleotides in a manner analogous to the way semantic information is conveyed by sequences of letters of the English alphabet. The amount of genetic information in the DNA of organisms is enormous because the total length of the DNA molecules of an organism is huge. For example, the human genome—that is, the DNA that each human inherits from each parent—is 3 billion letters long. Printing one human genome would require 1,000 books, each 1,000 pages long, with 3,000 letters (equivalent to about 500 words) per page. Scientists do not print full genomes of humans or other 30 AB Am I a Monkey? organisms; rather, the DNA information is stored electronically in computers. A second attribute of DNA is that it accounts for the precision of biological heredity. The two strands in the DNA double helix are complementary, both carry the same genetic information , and either one of the two strands can serve as a template for the synthesis of a complementary strand identical to the original complementary strand. Each of the four nucleotides pairs in the complementary strand with only a particular one of the other three. A pairs only with T, and C pairs only with G. For example, if a short segment of one strand consists of the sequence ATTCAGCA, the complementary strand will be TAAGTCGT. This complementarity accounts for the fidelity of biological heredity. In the process of reproduction, the two helically coiled strands unwind and each serves as a template for the synthesis of a complementary strand so that the two daughter double helices are identical to each other and to the mother molecule. Thus the sequence ATTCAGCA would direct the synthesis of a complementary strand, TAAGTCGT, identical to its partner in the original DNA molecule. Similarly , the original strand TAAGTCGT will direct the synthesis of a complementary strand, ATTCAGCA, resulting in a double helix identical in sequence to the sister and to the mother double helix. ThethirdfundamentalpropertyofDNAismutation,which makes possible the evolution of organisms. The information encoded in the nucleotide sequence of DNA is, as a rule, faithfully reproduced during replication, so that each replication results in two DNA molecules that are identical to each other and to the maternal molecule, as just explained. The fidel- What Is DNA? AB 31 ity of the process is enormous but not perfect. Occasionally mutations occur in the DNA molecule during replication so that daughter cells differ from the parental cells (and from each other) in the nucleotide sequence or in the length of the DNA. Mutations often involve a single letter (nucleotide), but occasionally mutations may encompass several or many letters . A mutation first appears in the DNA in a single cell of an organism, and the new, changed DNA is passed on to all cells descended from the first. A notorious example of a mutation with important consequences in recent European history accounts for hemophilia, typically a fatal disease, which is determined by a mutation in the X chromosome. Gender in humans is determined by X chromosomes. Women have two X chromosomes; men have one X and one Y chromosome. Women with the hemophilia mutation in one X chromosome do not suffer from the disease but transmit it to those sons (half on average) who happen to inherit the X chromosome with the hemophilia mutation. A mutation for hemophilia occurred in one of Queen Victoria’s X chromosomes. The mutation was transmitted through her daughters and granddaughters to the Russian, Spanish, and other royal families of Europe. Czarevitch Alexis, the only son of Czar Nicholas II of Russia, inherited hemophilia from his mother, Alexandra, Queen Victoria’s granddaughter. Prince Alfonso, heir to the Spanish crown, inherited it from another granddaughter, Queen Ena, the wife of King Alfonso XIII. Political historians believe that the hemophilia of the throne’s heirs contributed to the fall of the two...