生物学英语教程10 18 19 20 23 24原文及翻译.docx

1、生物学英语教程10 18 19 20 23 24原文及翻译10: The nature of viruses has been apparent only within the last half century, and the first step on this path of discovery was taken by the Russian botanist Dmitri Lvanovsky in 1892 when he studying the tobacco mosaic disease.Viruses are very small entities, ranging in

2、size from 0.02 to 0.3 microns. Unlike the organisms making up the five taxonomic kingdoms of the living world, viruses are acellular, they dont consist of cells and conduct energy metabolism-they dont produce ATP and incapable of fermentation , cellular respiration or photosynthesis. As for these, t

3、he questions of the viruses origin arise. Do they represent a primitive nearly living stage in the evolution of life? Or are they organisms which have lost all cellular components except the nucleus? Could viruses simply be fragments of genetic material derived from cellular organisms? No one really

4、 knows the answers to these questions, but we do know that viruses have been around for a long time, and that almost every form of life is susceptible to viral attack.The basic units of a virus consist of nucleic acid surrounded by a capsid or coat, composed of one or at most a few kinds of proteins

5、. These proteins are so assembled as to give the virion a characteristic shape. As they bud through host cell membranes, many animal viruses also acquire a membrane consisting of lipids and proteins, and many bacterial viruses have specialized tails made of protein. The viral nucleic acid is usually

6、 a single molecule and may be composed of either DNA or RNA, but not both. DNA or RNA can be double-stranded or single-stranded.Viruses are obligate intracellular parasite, that is why they must depend upon specific hosts for their reproduction and development. The cells of animals, plants and bacte

7、ria can all serve as hosts to viruses. Animal viruses attach to special sites on the plasma membrane of the host cell and are then taken up by endocytosis. A given virus can infect only those cells that have a receptor site for that virus. After the membrane breaks down, the viral protein capsid is

8、broken down by cellular enzymes before the viral nucleic acid, in addition, the viral nucleic acid serves to direct the synthesis of new capsid protein by the protein-synthesizing system of the host, and the capsid combine with new viral nucleic acid spontaneously; and in due course, the new virions

9、 are released by the host cell.Plant viruses and bacteriophages must get through a cell wall as well as the host plasma membrane. Infection of a plant usually results from attack by a virion-laden insect vector. The insect uses its proboscis to penetrate the cell wall, and the virions the escape fro

10、m the insect into the plant. Bacterial viruses are often equipped with tail assemblies that inject the nucleic acid into the host bacterium while the protein coat remains outside. Once inside the host cell, the virus genes takes over the metabolic machinery of the cell and generate their own. Someti

11、mes, viral DNA does not immediately take control of the host metabolism, but insert itself into the host chromosome and present as silent provirus until the host cell is exposed to some environmental insult, such as ultraviolet light or radiation.If the viral nucleic acid is RNA, replication needs s

12、pecial enzymes to make the process of RNA-to-RNA synthesis occurs. Some RNA viruses called retroviruses do not carry out RNA-to-RNA transcription. Instead, their RNA is transcribed into DNA is immediately, this reaction is catalyzed by reverse transcriptase, then newly formed DNA is inserted into ho

13、st DNA and then transcribed into RNA and at last new viruses are produced.After replication and combination, most viruses are released by lysis of the host cell. But in other cases, like that of the retroviruses, viruses are released by extrusion, a process similar to budding thereby the virus becom

14、es enveloped in a small piece of cell membrane as it moves out of the cell. Lysis result in the destruction of the cell, but extrusion allows the cell to remain alive and continue to produce new viruses for a long period of time.A common way to classify viruses is to separates them first on the natu

15、re of the nucleic acid component (DNA or RNA) and then on whether the nucleic acid in the virion is single-or double-stranded. Further levels of classification depend on such factors as the overall shape of the virus and the symmetry of the capsid. Most capsid may be categorized as helical, icosahed

16、ral and so on. Another level of categorization is based on the presence or absence of membranous envelope around the virion; still further subdivision relies on capsid size and other criteria. 18: Within a cell, energy is needed at every stage to drive the reactions that keep life in normal states.

17、On the earth, almost all the energy that fuels life today comes from the sun and is captured in the process of photosynthesis by plants. Most nonphotosynthetic organisms obtain energy by ingesting photosynthetic organisms or others that have themselves ingested photosynthetic organisms, and the ener

18、gy stored by photosynthesis is usually released through a process known as respiration. In this chapter, we shall discuss these two processes.Photosynthesis is a logical starting point for our discussion of the basic energy transformation of life. In simple terms photosynthesis consists of the reduc

19、tion of atmospheric CO2 to carbonhydrate by use of light energy, with an associated release of oxygen from water. This reaction can be summarized by the following generalized equation.Like many other physiological processes, photosynthesis consists of a number of sequential steps: trapping of light

20、energy by chloroplasts; pigments other than chlorophyll(e.g carotenoids)play an accessory role in photosynthesis by transferring energy to chlorophyll a. splitting of water and release of high-energy electrons and o2.electron transfer leading to generation of chemical energy in the form of ATP and t

21、he reducing power as NADPH2.terminal steps involving expenditure of energy of ATP and the reducing power of NADPH2 to fix CO2 molecules, and finally convert this compounds into more complex carbohydrates, such as sucrose, starch, cellulose and so on.Carbon dioxide is an exceedingly energy-poor compo

22、und, whereas carbohydrates is energy-rich. Photosynthesis, then, converts light energy into chemical energy. In chemical terms, the energy is said to be stored by the addition of one more electron-stores energy in the substance being reduced.Although, photosynthesis can occur in any chlorophyll-cont

23、aining parts of the plant, leaves that expose the greatest area of green tissue to the light are the principal organs of photosynthesis. Through a microscope it can be seen that the outer surfaces of the leaf have a layer of epidermis, which is covered by waxy layer of cuticle. The region between th

24、e upper and lower epidermis constitutes the mesophyll portion of the leaf. The cells of mesophyll contain many chloroplasts, which is the organelles that photosynthesis takes place. The CO2 required for photosynthesis can enter through some holes called stomata between the spaces of mesophyll cell.

25、Chloroplast is bounded by two concentric membrane and a third set of internal membranes that form a series of flattened, interconnected sacs known as thylakoids, where chlorophyll molecules and most of the electron-transport-chain molecules are located. The light reaction, in which light energy is t

26、rapped and converted into chemical energy, takes place on the thylakoid membrane. The dark reaction, in which CO2 is reduced to carbohydrates, occurs in the more fluid stroma that surrounds the thylakoid sacs.19: It is evident that the phenomenon of inheritance and variation is of universal importan

27、t in the living world. To understand it, we must explore how hereditary material expresses itself in new combinations, and whether principles can be formulated about so complex event. This exploration is the study of the branch of biology known as genetics.Some of the basic concepts of heredity grew

28、 out of experiments performed by Gregor Mendel in the mid-1800s. Mendel spent most of his lifetime as a monk in an Austrian monastery and during this time he cultivated garden peas, and did a series of experiments to study inheritance in plants. We now know that owing to his elaborate design, unique

29、 technique, Mendel formulated two excellent Laws of Inheritance from his breeding experiments on the garden peas: one is the Law of Segregation, the other is the Law of Independent Assortment.The Law of Segregation emphasize on single traits and it can be stated as follows: the inheritance of each i

30、ndividual trait is determined by hereditary factors (genes), each organism possesses two inheritance factors for each character, when gametes are formed, the two factors separate into separated gametes. An offspring formed by the fusion of two gametes therefore receives one factor for each character

31、 from each parent. The Law of Independent Assortment involving two or more traits and states that during gametes formation, when two or more genes are involved in a cross, the alleles of one gene are inherited independently of the alleles of another gene. His great achievement laid a solid foundatio

32、n for the genetics. In order to further understanding of these two laws, some details can be known either:1. Hereditary traits are controlled by discrete units that pass unchanged from generation to generation. For example, the trait white flower seems to disappear in the F1 generation, but reappear

33、s on the F2 progeny, and that there are no intermediate colors, only red or white.2. Each trait is produced by two hereditary factors. This is a necessary assumption to account for the way in which a trait such as flower color appears in successive generations in a predictable ratio.3. When two contrasting hereditary factors are present in