生物学英语教程10 18 19 20 23 24原文及翻译.docx

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 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, the 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 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. 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 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 bacteria 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 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 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 from 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.Sometimes, 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 special 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 host 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 becomes 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 nature 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, icosahedral 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. 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 energy 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 reduction 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 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 the 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 compound, 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-containing 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 the 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. 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 trapped 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 important 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 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 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 individual 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 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 foundation 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 reappears 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 an organism, such as red-flower color and white-flower color, only one will be expressed. one will be dominant and the other recessive. In the case of the peas, red-flower color is dominant to white-flower color since only the hereditary character of red shows on the F1 generation.4. Each parent contributes only one of the hereditary factors to each gamete. Moreover, as a consequence of segregation, equal numbers of gametes each kind are formed.5. when gametes unite at the fertilization, the two hereditary factors are brought together. Fertilization is a random union in the sense that equal numbers of the different kinds of gametes are produced and it is a matter of chance how they will pair. This being true it should be possible, on a probability basis, to predict the ratio of various characteristics in the offspring. In the flower color experiment, for example, the ratio of approximately three red-flowered plants to each white-flowered plant conform the expected or predicted ratio of 3 to 1.Today, Mendels theories seem rather obvious in the light of the discovery of DNA, genes, chromosomes and meiosis. But it should be remembered that Mendel did his work before all of these theories. He arrived at his great conclusions purely by reasoning from the patterns of inheritance he detected in his experiments. Of course, we know that a variety of influence, including environment, interaction of one gene with another, can cause the appearance changes, and a single gene may have multiple effects on explaining the phenomenon of the testimony to the strength of the Mendels conclusions.20: we know that when a cell divides it must first duplicate the genetic information contained in its chromosomes, so that each daughter cell receive its own copy from the parental cell, but how the genetic information copied on a molecular level? Since 1928, Frederick Griffith published his experiments on pneumococci, a series of experiments led to the realization that DNA is the genetic material. Soon after then, scientists proposed that the linear order of nucleotides in DNA determines the order of amino acids in polypeptides, and therefore determines the structure and function of proteins. This work is now so well established that it is called the central dogma of molecular biology. The dogma states that the information contained in DNA molecules is transferred to RNA molecules, and then from the RNA molecules the information is expressed in the structure of proteins. A summary of this process is:DNA molecule is known to be composed of two nucleotide chains, each nucleotide are joined by covalent bonds between the sugar of one nucleotide and the phosphate group of the next nucleotide in the sequence. The nitrogenous bases are side groups of the chains, the two chains are held together by hydrogen bonds between nitrogenous bases (adenine=thymine and guanine= cytosine) in opposite directions, then, the ladder like double-chained molecule is coiled into a double helix. During replication, the two chains of the DNA separated, and each chain acts as a template for the synthesis of its new partner according to base pairing principle. This process is much like one might unzip a zipper and then produce two complete double-chained molecules, each identical in base sequence to the mother DNA molecule.The transcription of DNA into RNA begins when the enzyme of RNA polymerase binds to a sequence of nucleotides on the DNA called the promoter. Next, the two strands of DNA are separated, and one strand serves as the template for the formation of a complementary strand of RNA. RNA polymerase moves along the DNA and joins the complementary ribonucleotides to the growing RNA strand, one by one. The enzyme works only in the 3 to 5direction on its DNA template, assembling RNA in the 5 to 3 direction. This process is much like the replication of DNA, with one important difference. When DNA replication, once begin, usually copies all DNA in the cell, whereas RNA synthesis transcribes only selected proteins of the DNA. When the polymerase reaches a termination signal on the DNA, it leaves the DNA chain, and the RNA also detached.Sine DNA molecules are double-stranded, any section of DNA could, in principle, be transcribed into two different RNA molecules, one complementary to each strand. In practice, only one of the strands is transcribed in any one segment of DNA. The orientation of the promoter indicates which strand is to be transcribed because the promoter reads correctly only on that strand. The codes for some genes lie on one DNA strand, and codes for other genes lie on its partner.There are four major stages in protein synthesis:1. activation: each amino acid is first activated by reacting with a molecule of ATP. The activated amino acid is then attached to its own particular tRNA molecule with the aid of an enzyme(a synthetase) that is specific for that particular amino acid and that particular tRNA molecule.2. initiation: this stage consists of three steps: mRNA, which carries the information necessary to synthesize one protein molecule, attached to the 40s ribosome. the anticodon of the first tRNA binds to the codon of the mRNA that represents the initiation signal. the 60s ribosome now combines with 40s body. The 60s body has two binding site: one is P site where the growing peptide chain will bind, the other is A site where the incoming tRNA will bring the next amino acid in.3. elongation: at this point the A site is vacant, and each of the 20 tRNA can come in and try to fit itself in. but only one can be the right anticodon that corresponds to the next codon on the mRNA. In the next elongation, the whole ribosome moves one codon along the mRNA. Simultaneously with this move, the dipeptide is translocated from the A site to the P site, while the empties tRNA dissociates and goes back to the tRNA pool to pick up another amino acid. After the translocation, the A site is associated with the next codon on the mRNA. These elongation steps are repeated until the last amino acid is attached.4. termination: after the last translocation, the next codon reads stop (UAA,UAG or UGA). No more amino acid can be added. Releasing factors then cleave the polypeptide chain from the last tRNA in a mechanism not yet fully understood. The tRNA itself is released from the P site, and the whole mRNA is released from the ribosome.23：within multiple-cellular organisms, cells of different tissues divide at very different rates, this process is strictly controlled so that cells divide only when necessary. Occasionally, however, the genetic control fail, a cell begins to grow and divide without restrain, until its offspring begin to crowed surrounding cells and interfere with tissue functions. The alteration has spawned a tumor. Tumor cells divide not only at a horrendous rate, but not respond to the controls telling them when to stop. They will not stop as long as conditions for growth remain favorable. If the tumor cells remain localized, it is said to be benign. Sometimes, however, a tumor cell can metastasize and then grow and divide in other organs of the body, such tumor cells are said to be malignant, or cancerous by definition. A cancer cell has the following characteristics:1. Almost all cancer cells have an abnormal number of chromosomes.2. Cancer cells tend to have a rather spherical shape, making them more mobile than normal cells.3. Profound abnormalities in the plasma membrane. Membrane transport and permeability are amplified. Some proteins at the surface are lost or altered, and new one appears.4. Profound changes in the cytoplasm. The cytoskeleton shrinks, becomes disorganized, or both. Enzyme activity shifts.5. Abnormal growth and division.6. Diminished capacity for adhesion to substrates. Secretions needed for adhesion dwindle; cells cannot become properly anchored in the parent tissue.Our current understanding of cancer suggests that most cancer arise