DNA
...led “nucleotides.” There are four different nucleotides, which are labeled adenine (A), thymine (T), guanine (G), and cytosine (C). The human genome is made of a sequence of roughly three billion of these nucleotides, and it is about the same size as the genome of a chimpanzee or a mouse. In contrast, a fruit fly has 180 million, a yeast has 12 million, and the flowering weed “thale cress” has 100 million nucleotides of DNA in its genome. The DNA molecule is composed of two very long strands of A’s, T’s, G’s and C’s, which are tightly paired with each other. An A on one strand is always paired with a T on the other strand, and a G is always paired with a C. This means that if the sequence of nucleotides on one strand is known, the sequence of the other strand will be automatically known as well. One strand of DNA is like a photographic negative of the other strand. A negative can be used to make many copies of a photograph because it contains all of the information that is part of that photograph. Similarly, to read the sequence of A’s, T’s, G’s, and C’s in a genome, it is only necessary to read one strand of DNA to be able to deduce the sequence of the other strand. The sequence of nucleotides in a gene gives it meaning by storing the instructions for building the other molecules necessary for life. These instructions are read as a string of A’s, T’s, G’s, and C’s, such as ACGGTAACT. In the sense that there are 26 letters in the English alphabet, there are four letters in the alphabet of DNA. The letters of the genetic alphabet – A, T, G, and C – are meaningless on their own, but they are combined into useful instructions in genes. Some genes carry enough information for one complete characteristic of an organism, but most characteristics result from combinations of genes. Genes are like chapters in the books that fill the library of the genome. The sequence of letters within a gene is like the letters in a book of instructions. Deciphering the enormously long sequence of A’s, T’s, G’s, and C’s in an organism’s genome reveals useful information. For example, finding a difference in a gene sequence that governs muscle structure raises questions. Could the difference affect health? Just as changing one letter in a word can change its meaning – for example, mice to rice to nice – so changing one DNA letter can sometimes cause illness. Not all of the sequences in the genes of two humans are identical. For example, because your face is unique, the precise set of sequences in the large group of genes that control the shape of your face are presumably unique too. Some special parts of the DNA sequence vary from person to person with unusually high frequency. As you will see, finding sequences in DNA samples can be used to identify individuals and help solve crimes, even when there are no eyewitnesses. Every cell in the body contains all of the DNA sequence, but the composition of each cell depends on which sections of the DNA are used. We know that each cell reads only those chapters from the library of instructions that it needs. The selective reading process creates many different kinds of cells, such as skin, muscle, neural, and bone cells, all of which develop from the many cells of the embryo produced by the growth and division of one cell: the fertilized egg. Studies of the fruit fly, Drosophila melanogaster, have been useful in revealing how organisms develop these cell types, with each cell knowing what chapters to read according to its position in the developing embryo. The new ability to pinpoint the cause of human genetic disease, and to detect those individuals who are predisposed to such diseases, does not mean that modern medicine can prevent them. A child inherits two copies of each gene: one from the mother and one from the father. In most cases, genetic diseases are said to be “recessive,” which means that if just one of the two copies is defective, the other copy will keep the child healthy. A person who inherits one defective copy and one normal copy of a gene is a “carrier” of the mutation. Even though they are unlikely to get sick, they have a fifty-fifty chance of passing the mutation along to each of their children. This is one reason why some diseases seem to disappear in one generation, only to reappear in later generations. Because A, T, G, and C are letters in the genetic alphabet, changing one letter for another can change the meaning of a gene. Just as “time” changes to “tame” when one letter changes, a single nucleotide change in a gene may sometimes cause disease. Since many genes govern the fundamental structure and chemical processes of life, defects in their instructions can potentially interfere with the chemical interactions that control an organism’s growth and health. However, most mutations that are passed on from generation to generation occur in the long stretches of DNA in between the parts of genes that carry instructions, where they do not harm the person. Hemochromatosis results from inheriting two copies of a defective gene, which causes the intestines to absorb too much iron. As a result, a person with these defective genes is likely to develop liver disease in middle age. Hemochromatosis is often hard to identify because the symptoms are shared with other diseases. If a fairly close relative, such as an aunt, uncle, or first cousin has hemochromatosis, it may be prudent to test your children’s DNA for the defect. If a child is known to have inherited the genetic mutations for hemochromatosis, the impact of the disease can often be reduced. Symptoms of the disease usually do not appear until middle age. Even in such cases, health and life expectancy can be improved through a treatment known as “phlebotomy,” in which iron-rich blood is removed from the patient every week and replenished with normal blood by the body. The Centers for Disease Control (CDC) recommends avoiding vitamins that contain iron and restricting vitamin C, which increases iron absorption. The CDC also recommends avoiding behavior that could damage the liver, such as more than mild alcohol consumption. Although patients may eat iron-containing foods, they should avoid eating raw seafood and shellfish, because iron-overload patients are susceptible to infections that these foods may carry. The science of identifying individuals using DNA sequences is very clear, and the probability of scientific error is extremely small. As a result, DNA evidence has been used to help identify perpetrators of crimes and to exonerate innocent people before they become suspects. Most people share very similar gene sequences, but some regions of DNA sequence have been found to vary from person to person with high frequency. Comparing variation in these regions allows us to answer the question of whether two different DNA samples come from the same person. The FBI’s forensic DNA identification system probes thirteen such regions in the genome. Sequences in these special regions involve multiple repetitions of short combinations of letters, such as GATA. Easily detectable differences between people lie in the number of repeats that occur in both copies of their DNA in these regions. For example, at one of these regions a person might have inheri...