Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 9
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The back of the book alsoincludes a large set of Supplementary Problems, without answers, for still more practice. There isnothing better than solving problems not only to test your knowledge but to make it part of yourlong-term memory.Page xxivA stylized version of a bacteriophage that very much resembles the phage T2 used in theHershey-Chase experiments.[Courtesy of Paul Dowrick, Phage et al Ltd.]Page 1Chapter 1—The Molecular Basis of Heredity and VariationCHAPTER OUTLINE1-1 DNA: The Genetic MaterialExperimental Proof of the Genetic Function of DNAGenetic Role of DNA in Bacteriophage1-2 DNA Structure: The Double Helix1-3 An Overview of DNA Replication1-4 Genes and ProteinsTranscription of DNA Makes RNATranslation of RNA Makes Protein1-5 Mutation1-6 How Genes Determine TraitsPleiotropy: One Gene Can Affect More Than One TraitEpistasis: One Trait Can Be Affected by More Than One GeneEffects of the Environment1-7 EvolutionThe Molecular Continuity of LifeAdaptation and DiversityThe Role of Chance in EvolutionChapter SummaryKey TermsReview the BasicsGuide to Problem SolvingAnalysis and ApplicationsFurther ReadingGeNETics on the webPRINCIPLES• Genes control biologically inherited traits; a trait that is genetically determined can also beinfluenced by environmental factors.• Genes are composed of the chemical deoxyribonucleic acid (DNA).• DNA replicates to form (usually identical) copies of itself.• DNA contains a code specifying what types of enzymes and other proteins are made in cells.• DNA occasionally mutates, and the mutant forms specify altered proteins.• Genes interact with one another in sometimes complex ways.• Organisms change genetically through generations in the process of biological evolution.CONNECTIONSCONNECTION: It's the DNA!Oswald T.
Avery, Colin M. MacLeod, and Maclyn McCarty 1944Studies on the chemical nature of the substance inducing transformation of pneumococcal typesCONNECTION: Shear MadnessAlfred D. Hershey and Martha Chase 1952Independent functions of viral protein and nucleic acid in growth of bacteriophagePage 2The members of any biological species are similar in some characteristics but different in others.For example, all human beings share a set of observable characteristics, or traits, that define us as aspecies. We have a backbone and a spinal cord; these traits are among those that define us as a typeof vertebrate.
We are warm blooded and feed our young with milk from mammary glands; thesetraits are among those that define us as a type of mammal. We are, in finer detail, a type of primatethat habitually stands upright and has long legs, relatively little body hair, a large brain, a flat facewith a prominent nose, jutting chin, distinct lips, and small teeth. These traits set us apart from otherprimates, such as chimpanzees and gorillas. The biological characteristics that define us as a speciesare inherited, but they do not differ from one person to the next.Within the human species, however, there is also much variation. Traits such as hair color, eyecolor, skin color, height, weight, and personality characteristics are tremendously variable from oneperson to the next.
There is also variation in health-related traits, such as predisposition to highblood pressure, diabetes, chemical dependence, mental depression, and the Alzheimer disease. Someof these traits are inherited biologically, others are inherited culturally. Eye color results frombiological inheritance; the native language we speak results from cultural inheritance. Many traitsare influenced jointly by biological inheritance and environmental factors. For example, weight isdetermined in part by inheritance but also in part by eating habits and level of physical activity.The study of biologically inherited traits is genetics.
Among the traits studied in genetics are thosethat are influenced in part by the environment. The fundamental concept of genetics isInherited traits are determined by elements of heredity, called genes, that are transmittedfrom parents to offspring in reproduction.The elements of heredity and some basic rules governing their transmission from generation togeneration were discovered by Gregor Mendel in experiments with garden peas. His results werepublished in 1866. Mendel's experiments are among the most beautifully designed, carefullyexecuted, and elegantly interpreted in the history of experimental science.
Mendel interpreted hisdata in terms of a few abstract rules by which hereditary elements are transmitted from parents tooffspring. Three years later, in 1869, Friedrich Miescher discovered a new type of weakly acidsubstance, abundant in the nuclei of salmon sperm and white blood cells. At the time he had no wayof knowing that it would turn out to be the chemical substance of which genes are made. Miescher'sweak acid, the chemical substance of the gene, is now called deoxyribonucleic acid (DNA).However, the connection between DNA and heredity was not demonstrated until about the middleof the twentieth century.
How was this connection established?1.1—DNA: The Genetic MaterialThe importance of the cell nucleus in inheritance became apparent in the 1870s with the observationthat the nuclei of male and female reproductive cells fuse in the process of fertilization. Thisobservation suggested that there was something inside the sperm and egg nucleus that wasresponsible for inherited characteristics.
The next major advance was the discovery of thread-likeobjects inside the nucleus that become visible in the light microscope when stained with certaindyes; these threads were called chromosomes. As we shall see in Chapter 3, chromosomes have acharacteristic "splitting" behavior in cell division, which ensures that each daughter cell receives anidentical complement of chromosomes. By 1900 it had become clear that the number ofchromosomes is constant within each species but differs among species.
The characteristics ofchromosomes made it seem likely that they were the carriers of the genes.By the 1920s, more and more evidence suggested a close relationship between DNA and the geneticmaterial. Studies using special stains showed that DNA, in addition to certain proteins, is present inchromosomes. Furthermore, investigationsPage 3disclosed that almost all cells of a given species contain a constant amount of DNA, whereas theamount and kinds of proteins and other molecules differ greatly in different cell types. The indirectevidence that genes are DNA was rejected because crude chemical analyses of DNA had suggested(incorrectly) that it lacks the chemical diversity needed for a genetic substance.
In contrast, proteinswere known to be an exceedingly diverse collection of molecules. And so, on the basis of incorrectdata, it became widely accepted that proteins were the genetic material and that DNA merelyprovided the structural framework of chromosomes. Against the prevailing opinion that genes areproteins, experiments purporting to demonstrate that DNA is the genetic material had also todemonstrate that proteins are not the genetic material. Two of the experiments regarded as decisiveare described in this section.Experimental Proof of the Genetic Function of DNAThe first evidence that genes are DNA came from studies of bacteria that cause pneumonia.Bacterial pneumonia in mammals is caused by strains of Streptococcus pneumoniae that are able tosynthesize a slimy "capsule" around each cell.
Strains that lack a capsule do not cause pneumonia.The capsule is composed of a complex carbohydrate (polysaccharide) that protects the bacteriumfrom the immune response of the infected animal and enables the bacterium to cause the disease.When a bacterial cell is grown on solid medium, it undergoes repeated cell divisions to form avisible clump of cells called a colony.
The enveloping capsule gives the colony a glistening orsmooth (S) appearance. Some strains of S. pneumoniae are not able to synthesize a capsule. As aresult, they form colonies that have a rough (R) surface (Figure 1.1). The R strains do not causepneumonia, because without their capsules, the bacteria are attacked by the immune system of thehost. Both types of bacteria "breed true" in the sense that the progeny formed by cell division havethe capsular type of the parent, either S or R.When mice are injected either with living R cells or with dead S cells killed by heat, they remainhealthy.
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