Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 15
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Progress in genetics has often resultedfrom the proper subdivision of a complex trait into distinct types that differ in their genetic or environmentalcausation.1.7—EvolutionOne of the remarkable discoveries of molecular genetics is that organisms that seem very different (for example,plants and animals) share many common features in their genetics and biochemistry. These similarities indicate afundamental "unity of life":All creatures on Earth share many features of the genetic apparatus, including genetic information encodedin the sequence of bases in DNA, transcription into RNA, and translation into protein on ribosomes viatransfer RNAs.
All creatures also share certain characteristics in their biochemistry, including manyenzymes and other proteins that are similar in amino acid sequence.The Molecular Continuity of LifeThe molecular unity of life comes about because all creatures share a common origin through evolution, theprocess by which populations of organisms that are descended from a common ancestor gradually become moreadapted to their environment and sometimes split into separate species. In the evolutionary perspective, the unity offundamental molecular processes is derived by inheritance from a distant common ancestor in which manymechanisms were already in place.Not only the unity of life but also many other features of living organisms become comprehensible from anevolutionary perspective.
The importance of the evolutionary perspective in understanding aspects of biology thatseem pointless or needlessly complex is summed up in a famous aphorism of the evolutionary biologist TheodosiusDobzhansky: "Nothing in biology makes sense except in the light of evolution."One indication of the common ancestry among Earth's creatures is illustrated in Figure 1.16. The tree ofrelationships was inferred from similarities in nucleotide sequence in a type of ribosomal RNA molecule commonto all these organisms. Three major kingdoms of organisms are distinguished:1. Bacteria This group includes most bacteria and cyanobacteria (formerly called blue-green algae).
Cells of theseorganisms lack a membrane-bounded nucleus and mitochondria, are surrounded by a cell wall, and divide bybinary fission.2. Archaea This group was initially discovered among microorganisms that produce methane gas or that live inextreme environments, such as hot springs or high salt concentrations; they are widely distributed in more normalenvironments as well. Like Bacteria, the cells of Archaea lack internal membranes. DNA sequence analysisindicates that the machinery for DNA replication and transcription resembles that of Eukarya whereas metabolismin Archaea strongly resembles that of Bacteria.
About half of their genes are unique to Archaea, however.3. Eukarya This group includes all organisms whose cells contain an elaborate network of internal membranes, amembrane-bounded nucleus, and mito-Page 23Figure 1.16Evolutionary relationships among the major life forms as inferred from similarities innucleotide sequence in an RNA molecule found in the small subunit of the ribosome. Thethree major kingdoms of Bacteria, Archaea, and Eukarya are apparent.
Plants, animals, andfungi are more closely related to each other than to members of either of the otherkingdoms. Note the diverse groups of undifferentiated, relatively simple organisms thatdiverged very early in the eukaryote lineage.[Courtesy of Mitchell L. Sogin.]chondria. Their DNA is organized into true chromosomes, and cell division takes place by means of mitosis(discussed in Chapter 3). The eukaryotes include plants and animals as well as fungi and many single-celledorganisms, such as amoebae and ciliated protozoa.The Bacteria and Archaea are often grouped together into a larger assemblage called prokaryotes, which literallymeans "before [the evolution of] the nucleus." This terminology is convenient for designating prokaryotes as agroup in contrast with eukaryotes, which literally means "good [well-formed] nucleus."Adaptation and DiversityFigure 1.16 illustrates the unity of life, but it also illustrates the diversity.
Frogs are different from fungi, andbeetles are different from bacteria. As a human being, it is sobering to consider that complex, multicellularorganisms came relatively late onto the evolutionary scene of life on Earth. Animals came later still and primatesvery late indeed. What about human evolution? In the time scale of Earth history, human evolution is a matter of afew million years—barely a snap of the fingers.If common ancestry is the source of the unity of life, what is the source of diversity? Because differences amongspecies are inherited, the original source of the differences must be mutation.
However, mutations alone are notsufficient to explain why organisms are adapted to living in their environments—why ocean mammals have specialadaptations that make swimming and diving possible, or why desert mammals have special adaptations that enablethem to survive on minimal amounts of water. Mutations are chance events not directed toward any particularPage 24adaptive goal, like longer fur among mammals living in the Arctic. The process that accounts for adaptation wasoutlined by Charles Darwin in his 1859 book On the Origin of Species. Darwin proposed that adaptation is theresult of natural selection, the process in which individual organisms that carry particular mutations orcombinations of mutations that equip them to survive or reproduce more effectively in the prevailing environmentwill leave more offspring than other organisms and so contribute their favorable genes disproportionately to futuregenerations.
If this process is repeated throughout the course of many generations, the entire species becomesgenetically transformed because a gradually increasing proportion of the population inherits the favorablemutations. The genetic basis of natural selection is discussed in Chapter 15.The Role of Chance in EvolutionNatural selection is undoubtedly the key process in bringing about the genetic adaptation of organisms to theirenvironments.
Hence there is a great deal of appeal in being able to explain why particular traits are adaptive.Unfortunately, the ingenuity of the human imagination makes it all too easy to make up an adaptive story for anytrait whatsoever. One example is the adaptive argument that the reason why blood is red is that seeing it scaresone's enemies when one is injured. This explanation sounds almost plausible, but the truth is that blood is red forthe same reason as rust; it contains oxidized iron. Each hemoglobin chain carries an atom of iron, and oxidized ironis red as a matter of physics, not biological evolution.Made-up adaptive stories not supported by hard evidence are called just-so stories after the title (Just So Stories)of a 1902 book by Rudyard Kipling.
The stories tell how animal traits came to be: how the elephant got its trunk (acrocodile caught a baby elephant by his nose and "pulled and pulled and pulled it out into a really truly trunk sameas all elephants have today"); how the whale got his throat (because he swallowed a sailor who wedged a grate atthe back of his throat that prevented him from eating anything except "very, very small fish—and this is the reasonwhy whales nowadays never eat men or boys or little girls"); how the camel got his hump (a wizard cursed it onhim for not doing his work); and so forth.The rationale for inventing evolutionary just-so stories is the assumption that all traits are adaptive by necessity andone needs only to find a reason why.
But this is not necessarily so. For example, some traits exist not because theyare selectively advantageous in themselves but because they are pleiotropic effects of genes selected for otherreasons. Chance may also play a large role in some major events in the history of life. Many evolutionary biologistsnow believe that a mass extinction was precipitated 65.3 million years ago when an asteroid smashed into thePacific Ocean off the Yucatan Peninsula and spewed so much debris into the air that Earth went dark for years. Themass extinction triggered by this event was not by any means the largest in Earth history, but it led to the extinctionof all dinosaur species and about 90 percent of other species. Until then, dinosaurs were a wonderfully diverse andwell-adapted group of organisms. The demise of the dinosaurs made way for the evolutionary diversification andsuccess of mammals, so one could argue that a chance asteroid impact explains in part why human beings are here.Chapter SummaryOrganisms of the same species have some traits (characteristics) in common, but they may differ from one anotherin innumerable other traits.
Many of the differences between individual organisms result from genetic differences,the effects of the environment, or both. Genetics is the study of inherited traits, including those influenced in partby the environment. The elements of heredity consist of genes, which are transmitted from parents to offspring inreproduction. Although the sorting of genes in successive generations was first put into numerical form by Mendel,the chemical basis of genes was discovered by Miescher in the form of a weak acid now called deoxyribonucleicacid (DNA).
However, experimental proof that DNA is the genetic material did not come until about the middle ofthe twentieth century.Page 25The first convincing evidence of the role of DNA in heredity came from experiments of Avery, MacLeod, andMcCarty, who showed that genetic characteristics in bacteria could be altered from one type to another bytreatment with purified DNA. In studies of Streptococcus pneumoniae, they transformed mutant cells unable tocause pneumonia into cells that could by treatment with pure DNA from diseasecausing forms. A second importantline of evidence was the Hershey-Chase experiment, which showed that the T2 bacterial virus injects primarilyDNA into the host bacterium (Escherichia coli) and that a much higher proportion of parental DNA, as comparedwith parental protein, is found among the progeny phage.The three-dimensional structure of DNA, proposed in 1953 by Watson and Crick, gave many clues about themanner in which DNA functions as the genetic material.
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