DNA is the basis of inheritance and of evolution. DNA, or deoxyribonucleic acid, is important to evolutionary science in four ways:
• Because of DNA, traits are heritable. • Because of DNA, traits are mutable.
• The study of DNA allows comparisons of evolutionary divergence to be made among individuals (see DNA [evi- dence for evolution]).
• The study of DNA allows the genetic variability of popula- tions to be assessed (see population genetics).
In order for natural selection to work on traits within a population, those traits must be heritable (see natural selection). Characteristics that are induced by environ- mental conditions, though sometimes called adaptations, are not heritable (see adaptation). Scientists, and most other people, have long known that traits are passed on from one generation to another, that organisms reproduce not only “after their own kind” but that the offspring resemble their parents more than they resemble the other members of the population. Some scientists in the 17th century believed that tiny versions of organisms were contained within the repro- ductive cells, such as homonculi within sperm; that is, entire structures were passed on from one generation to another, a theory called preformation. Other scientists believed that structures formed spontaneously from formless material, a theory called epigenesis. They were both partly right and partly wrong. In the 20th century scientists discovered that the instructions for making the structure, rather than the structure itself, were passed from one generation to another through the reproductive cells.
In the 19th century, many scientists believed that charac- teristics that an organism acquired during its lifetime could be passed on to later generations. The scientist most remembered for this theory is Jean Baptiste de Lamarck (see Lamarck- ism). The inheritance of acquired characteristics was not his special theory, but rather the common assumption of scien- tists in his day. Until Gregor Mendel (see Mendel, Gregor; Mendelian genetics), nobody had performed experiments that would adequately test these assumptions. Charles Dar- win (see Darwin, Charles) was so perplexed and frustrated by the general lack of scientific understanding of inheritance that he invented his own theory: pangenesis, in which “gem- mules” from the body’s cells worked their way to the repro- ductive organs and lodged there, carrying acquired genetic information with them. His theory was essentially the same as Lamarck’s, and equally wrong. Darwin either had never heard of Mendel’s work or overlooked its significance.
How DNA Stores Genetic Information
By the early 20th century, data had accumulated that a chemi- cal transmitted genetic information from one generation to the next, and that the chemical was DNA. In 1928 microbiologist Frederick Griffith performed an experiment in which a harm- less strain of bacteria was transformed into a deadly strain of bacteria by exposing the harmless bacteria to dead bacteria of the deadly strain. Some chemical from the dead bacteria had transformed the live harmless bacteria permanently into gener- ation after generation of deadly bacteria. Research in 1944 by geneticist Oswald Avery and associates established that it was
the DNA, not proteins, that caused the transformation that Griffith had observed. Research in 1953 by geneticists Alfred Hershey and Martha Chase established that it was the DNA, not the proteins, of viruses that allowed them to reproduce: The DNA from inside the old viruses produced new viruses, while the protein coats were merely shed and lost.
Many scientists doubted that DNA could be the basis of inheritance and suspected that proteins might carry the genetic information. DNA was a minor component of cells, compared to the abundance of protein. Furthermore, DNA was a structurally simple molecule, compared to proteins, and was thus considered unlikely to carry enough genetic information. It was not until the structure of DNA was explained by chemists James Watson and Sir Francis Crick in 1953, based upon their data and data from colleagues such as chemist Rosalind Franklin, that DNA became a truly believ- able molecule for the transmission of genetic information.
DNA is an enormously long molecule made up of smaller nucleotides (see figure on page 133). Each nucleotide consists of a sugar, a phosphate, and a nitrogenous base. In DNA, the sugar is always deoxyribose. The nitrogenous base in a DNA nucleotide is always one of the following: adenine, guanine, cytosine, or thymine. The general public has been well exposed to the abbreviations of these bases (A, G, C, and T). The nucleotides are arranged in two parallel strands, like a rope ladder. The parallel sides of the ladder consist of alternating molecules of phosphate and sugar. The rungs consist of the nitrogenous bases meeting together in the cen- ter. A large base is always opposite a small base, and the cor- rect bonds must form; therefore, A is always opposite T, and C is always opposite G. Because of this, both strands of DNA contain mirror-images of the same information: if one strand is ACCTGAGGT, the other strand must be TGGACTCCA. DNA not only stores information but stores it in a stable fashion: All of the information in one strand is mirrored in the other strand. If mutations occur in one strand, the base sequence in the other strand can be used to correct them. Mutations in DNA are usually but not always corrected. All cells use DNA to store genetic information. Therefore the mutations that occur in one strand are frequently corrected by an enzyme that consults the other strand.
Mutations occur relatively infrequently, and evolution proceeds slowly, in all species of organisms. Some viruses (which are not true organisms) use a related molecule, RNA (ribonucleic acid), to store genetic information. RNA is sin- gle-stranded, and its mutations cannot be corrected. Because of this, RNA viruses evolve much more rapidly than DNA viruses. RNA viruses such as colds and influenza evolve so rapidly that the human immune system cannot keep up with them. This is why a new flu vaccine is needed every year. Last year’s flu vaccine is effective only against last year’s viruses. In contrast, DNA viruses such as the ones that cause poliomyelitis (polio) evolve slowly enough that old forms of the vaccine are still effective. The human immunodeficiency virus is an RNA virus and evolves rapidly (see AIDS, evolu- tion of).
DNA is capable of replication. If the two strands sepa- rate, new nucleotides can line up and form new strands that exactly mirror the exposed strands. In this way, one DNA DNA (raw material of evolution)
molecule can become two, two can become four, and so on. This occurs only when the appropriate enzymes and raw materials are present to control the process. Cells use DNA information in three ways: • A single cell, such as a fertilized egg cell, can develop into a multicellular organism. Most of the 70 trillion cells in a human body contain nearly an exact copy of the DNA that was in the fertilized egg from which the person developed. • During the daily operation of each cell, the genetic instruc- tions in the DNA determine which proteins are made, and what the cell does.
• DNA can be passed on from one generation to the next. Usually this involves separating the information into two sets and then recombining them in sexual reproduction (see meiosis; sex, evolution of). DNA is a potentially immortal molecule: Each person’s DNA molecules are the descendants of an unbroken line of replication going back to the last universal common ancestor of all life on Earth (see origin of life).
In eukaryotic cells, DNA is organized into chromosomes (see eukaryotes, evolution of). The evolutionary origin of chromosomes has not been explained. One hypothesis, pro- posed by John Maynard Smith (see Maynard Smith, John), is that within a chromosome all of the DNA must replicate at the same time. This system prevents some segments of DNA from replicating more often than others, and prevents any of them from getting lost. If each segment of DNA replicated on its own, some of them might replicate faster than others and lead to the death of the cell (see selfish genetic elements). A nucleus can simultaneously replicate all of its chromo- somes, of which most nuclei have fewer than 50, more eas- ily than it could simultaneously replicate many separate DNA segments.
Two important facts result from the fact that genetic information is passed on by DNA:
• Acquired characteristics cannot be inherited. Lamarck, and Darwin, were wrong about this. Although various regula- tory molecules can pass from one generation to another through egg cells, the only characteristics that are transmit- ted from one generation to the next are the ones coded by DNA. One 20th-century fundamentalist preacher said, “If evolution is true, why are Hebrew babies, after thousands of years of circumcision, still born uncircumcised?” It was not only evolution but DNA that this preacher misunder- stood. His question, though ridiculous to modern scientists, might have bothered Lamarck.
• Traits that seem to have disappeared can reappear in a later generation. Traits do not blend together like different col- ors of paint, as scientists once believed; instead, the traits are discrete units. A trait may be hidden by other traits, but its DNA is still there and can reappear under the right conditions. This was also a problem with which Darwin wrestled, and which has been solved by an understanding of DNA and of genetics.
DNA encodes genetic information, allowing it to be passed on from one generation to the next in almost perfect form, but with just enough imperfection to allow mutations,
DNA is an extremely long molecule that consists of two strands that form a double helix (spiral). Each strand contains sugars, phosphates, and nitrogenous bases. Within each strand, sugars () and phosphates () form a backbone. Nitrogenous bases A, C, T, and G meet in the middle of the molecule between the two strands and hold them together by weak bonds. A is always opposite T, and C is always opposite G. This illustration does not show the arrangements of atoms.
the raw material of the genetic variability of populations, and of natural selection.
How Genes Determine Proteins
DNA stores information as a four-letter alphabet (A, C, T, G) forming three-letter words called codons. Each codon of three nucleotides specifies one amino acid. Proteins are large molecules made up of smaller amino acids; therefore 3,000 DNA nucleotide pairs specify the order of amino acids in a protein that is made of 1,000 amino acids. The DNA that specifies the structure of one protein or group of related pro- teins is called a gene. The proteins do all of the work of the cell. A human is different from a snail largely because many of their proteins differ. An organism’s characteristics result from the work of its proteins; and its proteins are specified by its DNA.
DNA remains in the nucleus of the cell. Proteins are manufactured out in the cytoplasm of the cell. Enzymes copy or transcribe genetic information from the DNA into messenger RNA; it is the messenger RNA that travels from the nucleus out to the cytoplasm. Each gene has a group of nucleotides (see promoter) which identifies it and indicates where the gene begins. Different groups of genes have differ- ent kinds of promoters. A cell transcribes only the genes that have promoters that are appropriate for that cell’s functions. Once the messenger RNA molecule arrives in the cyto- plasm, structures called ribosomes produce proteins whose
amino acid sequence matches the RNA nucleotide sequence. Small molecules called transfer RNA attach to amino acids and bring them to the ribosomes. Each transfer RNA mol- ecule recognizes and attaches only to its particular kind of amino acid. Each transfer RNA molecule recognizes only particular codons on the messenger RNA molecule. This is how the transfer RNA molecule brings the appropriate amino acid to the right position in the growing protein mol- ecule. The correspondence between the nucleic acid codons and the amino acids is called the genetic code. Nearly all cells use exactly the same genetic code (see table at left). Mitochondria, and some ciliates, have a slightly different genetic code. Evolutionary scientists take this as evidence that the genetic code was established in the common ances- tor of all life-forms now on the Earth. The genetic code seems not to be an arbitrary coupling of codons and amino acids. A computer simulation was used to randomly link up codons and amino acids and produced over a million alter- nate genetic codes. The simulation indicated that the genetic code actually used by cells was one of the most efficient pos- sible codes. This suggests the possibility that the common ancestor of all cells was itself the product of a long period of evolution, during which less efficient genetic codes were tried and eliminated by natural selection.
DNA stores information digitally, just like a computer. A computer uses the bits 0 and 1, while DNA uses the four bases A, C, T, and G. A computer has bits organized into bytes, which specify letters, just as bases are organized into codons that specify amino acids. Bytes make up words, just as codons make up genes.
The processes of transcription and translation are more complex than here described. In particular, cell components can transcribe and translate different portions of the DNA, then modify the resulting protein, so that one gene can encode several different proteins.
How Genes Determine Characteristics of Organisms
The transcription and translation of genes produces proteins, which form many structures and do nearly all the work in the cell. No cell transcribes or translates all of its genes. It tran- scribes and translates only the genes for which the promoter site is open, and which have not been chemically altered: • In some cases, inhibitor molecules can block a promoter
site. The inhibitor molecule may consist partly of the end product of the series of reactions that the gene begins. When the end product is abundant, the end product itself helps to block the promoter. When the end product is scarce, the promoter is open. This process helps to keep the amount of the gene product more or less constant in the cell. Usually, the interactions of control molecules, most of them proteins, is very complex, especially in eukaryotic cells.
• In some cases, the genes can be altered by a process called methylation. The nucleic acid sequence of the methyl- ated gene is intact, but the gene cannot be transcribed. In some cases an entire chromosome can be inactivated, as with one of the two X chromosomes in female mammals.