The Beauty of Mutations
Why So Many Errors in our DNA?
Pines, Maya, ed . "Blazing a Genetic Trail." Bethesda, MD: Howard Hughes Medical Institute, 1991.
As scientists learn to read the instructions in our genes, they are discovering that much of our DNA is riddled with errors. Fortunately, most of these errors are harmless. Considering the difficulties involved - the 6 feet of DNA in a human cell consists of 6 billion subunits, or base pairs, coiled and tightly packed into 46 chromosomes, all of which must be duplicated every time a cell divides - our general state of health is something of a miracle.
We each inherit hundreds of genetic mutations from our parents, as they did from their forebears. In addition, the DNA in our own cells undergoes an estimated 30 new mutations during our lifetime, either through mistakes during DNA copying or cell division or, more often, because of damage from the environment. Bits of our DNA may be deleted, inserted, broken, or substituted. But most of these changes affect only the parts of DNA that do not contain a gene's instructions, so we need not worry about them.
Problems arise only when an error in DNA alters a message that tells certain cells to manufacture a particular protein. Such messages are spelled out in varying sequences of the four chemical bases that make up DNA: adenine (A), thymine (T), guanine (G), and cytosine (C).
To stay alive and functioning, the human body requires a daily crop of billions of fresh protein molecules - about 50,000 different kinds of proteins that must be supplied in the right quantities, at the right times, and in the right places. We need hemoglobin to carry oxygen through the bloodstream, antibodies to fight foreign substances, hormones to deal with stress, neurotransmitters to evoke movements, emotions, and thought, and many other proteins to give structure to organs or speed up chemical reactions.
Our cells are kept extremely busy linking together amino acids - the building blocks of proteins - in the right order to produce these diverse proteins. The order is determined by the genes. According to the genetic code, each triplet of bases in the genes' instructions either calls for a particular amino acid or gives a signal to start or stop making a protein.
An error in just one base can bring the wrong amino acid, altering the protein. And should one or two bases be missing, each succeeding triplet will be read in the wrong combination; such "frame shifts" generally prevent cells from making the protein at all.
Actually, the DNA's instructions are not transmitted directly; a copy consisting of ribonucleic acid (RNA) acts as an intermediary. The original DNA remains safely in the nucleus, somewhat like the printing block in a printing press, while the RNA copy is produced by transcribing just one strand of DNA, which carries the instructions for manufacturing protein.
Reading the DNA of humans and other mammals is complicated by the astonishing fact discovered a little over a decade ago that the genes' instructions are split into separate segments of DNA. These instructions must be spliced together before they can be carried out by a cell. Only about 5 percent of the DNA in mammalian genes actually contains the recipe for making a protein. The remaining 95 percent consists of intervening sequences, or "introns," whose function is unknown.
Splicing together the "exons," the protein-coding sequences, is a very delicate, precise operation that involves snipping out the introns to end up with a much shorter strand of potent RNA. At the exon-intron boundaries are splicing signals, which researchers can now identify. Several genetic diseases have been traced to disrupted splicing. Much of the recent progress in reading DNA has come from analyses of genetic errors.
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