VIRUS, a very simple, non-cellular
parasite that can reproduce only inside living cells. The simple
structure of viruses is their most distinctive characteristic. Most
of them consist only of a genetic material—either DNA
(deoxyribonucleic acid) or RNA (ribonucleic acid)--and a protein
coat. Some also have membranous envelopes. But all lack the
structures normally found in cells that are necessary for
metabolism, growth, and reproduction.
病毒，一种非常简单的，只能在活细胞内繁殖的非细胞寄生物。病毒的简单结构是它们最独特的特色。它们大多都只是由遗传物质组成—或者是DNA（脱氧核糖核酸），要么是RNA（核糖核酸）并有一个蛋白质外壳。有些还有膜状的包膜。但所有的病毒都缺乏通常在细胞中发现的新陈代谢，生长和繁殖所必需的结构。
Viruses are “alive” in that they can reproduce
themselves—although only by taking over a cell’s synthetic
machinery—but they have none of the other characteristics of living
organisms. While scientists once thought that viruses might be an
ev
olutionary link between nonliving chemicals and living cells, they
now think that viruses probably evolved from cells. This could
happen if a small piece of cellular DNA were excised from a
chromosome. If the excised DNA could replicate, it could then
evolve independently of the parental chromosome. Such independently
replicating pieces of DNA frequently occur in bacteria, where they
are called plasmids. Those pieces of replicating DNA that evolved a
mechanism for infecting additional cells would have an obvious
survival advantage.
病毒是“活的”，因此他们能自我繁殖---尽管只控制了细胞的合成机制---但它们却没有生物体的其它特征。虽然科学家曾经认为病毒可能是在非生物化学物质与活细胞之间的一种进化链接，但他们现在认为病毒可能由细胞进化而来。如果从一个染色体上切除一小块DNA细胞，这种情况就可能发生。如果剥离的DNA能够复制，那么它就能不依赖亲代染色体而进化。这样独立复制的DNA片段通常在细菌中发生，它们被称为质粒。进化出感染额外细胞机制的那些复制DNA
的片段会具有明显的生存优势。
Viruses cause a large variety of significant diseases of
plants and animals, including humans. In addition, they have
provided a simple tool with which scientists can study DNA
replication and protein synthesis, and thus have contributed to the
solution of some of the most fundamental problems of genetics and
molecular biology.
病毒会引起植物和动物，包括人类各种各样重大的疾病。另外，它们提供了一种简单工具，科学家可以用它研究DNA复制和蛋白质合成，从而有助于解决一些遗传学和分子生物学最基本的问题。
Size and Structure.
Viruses are small—much smaller than the cells they infect. Roughly
speaking, they range in size from 20 to 400 nanometers (nm) in
diameter. (One nanometer is one billionth of a meter. About 25
million nanometers make up an inch.) For comparison, a small
bacterium such as Escherichia coli has an average diameter
of about 1,500 nm, and a typical plant or animal cell has a
diameter of about 10,000nm. With few exceptions, viruses are too
small to be seen under the light microscope. Scientists knew they
existed because extracts of infected tissues, which had been
filtered to remove bacteria and cell debris, could infect other
susceptible hosts, viruses could not be seen, however, until after
the invention in 1933 of the electron microscope, with it great
magnifying power.
大小与结构
病毒都很小---比它们感染的细胞小得多。概略地说，它们的直径大小从20纳米到400纳米不等。（一纳米是一米的十亿分之一。大约2500万个纳米组成一英寸）。作为比较，像大肠杆菌这样小的细菌平均直径约为1500纳米，而典型的植物或动物细胞直径大约为10000纳米。除了极少例外，病毒都太小，在光学显微镜下无法看到。科学家们了解它们的存在是因为被感染组织的提取物经过过滤而去除细菌和细胞碎片，依然会感染其它的易感宿主，然而，直到1933年发明了电子显微镜，随着它巨大的放大率才看到了病毒。
The two most common shapes of viruses are the icosahedron and
the rod. An icosahedron is a regular polyhedron having 20
triangular faces, 12 corners, and 30 edges. In icosahedral viruses,
the nucleic acid with its associated proteins forms an inner core.
Surrounding the core is the capsid, or coat, which is composed of
protein subunits called capsomeres. Together the core plus the
capsid form the nucleocapsid. The nucleocapsid is the entire virus,
except in the case of viruses that have envelopes. Icosahedral
viruses appear to be roughly spherical in electron micrographs. The
diameters of the capsids range from about 20 to 100 nm.
Adenoviruses are typical icosahedral viruses.
病毒最普通的两个形态是二十面体和杆状病毒。二十面体是一种拥有20个三角形面，十二个角和30条边的正多面体。在二十面体的病毒中，核酸与其相关的蛋白质形成一个内核。围绕内核的是病毒的衣壳，或外壳，它构成了称为壳粒的蛋白亚基。内核加上衣壳一起形成了核蛋白壳。除了拥有包膜的病毒外，核蛋白壳就是整个病毒。二十面体病毒在显微镜中大体上呈球形。病毒衣壳的直径大约为20到100纳米不等。腺病毒是典型的二十面体病毒。
The coat-protein subunits of the rod-shaped viruses are
arranged in a helical pattern around the axis of the rod, with the
nucleic acid sandwiched between them in adjacent turns of the
helix. The capsomeres completely cover the nucleic acid. Tobacco
mosaic virus is an example of a helical virus. The rod is 300 nm
long and has a diameter of 18 nm.
杆状病毒的外壳蛋白基围绕杆的轴心呈螺旋状排列，核酸夹在它们之间形成相邻的螺旋圈。壳粒完全覆盖了核酸。烟草花叶病病毒就是螺旋病毒的一个例证。杆长为300纳米，直径18纳米。
Both icosahedral and helical viruses may have envelopes. The
envelope contains lipids and poly-saccharides in addition to
proteins. It has a structure similar to that of the outer membrane
of cells. In order to be enveloped, helical nucleocapsids must be
flexible enough to fold into an irregular core within the envelope.
Herpes-viruses are enveloped icosahedral viruses, and myxoviruses
are enveloped helical viruses.
二十面体的和螺旋的病毒都可能有包膜。除了蛋白质外，包膜包含了脂类和多糖。它与细胞的外膜结构拥有相似的结构。为了包膜，螺旋形核衣壳必须灵活，足以在包膜内折叠成不规则的核心。疱疹病毒都是包膜的二十面体病毒，而黏液病毒是包膜的螺旋形病毒。
Certain bacteriophages, (viruses that infect bacteria) have a
relatively complex structure. These are the “T-even” phages. They
consist of a polyhedral head containing the DNA and of a tubular
tail. A thin disk called the collar is located between the head and
the tail. The tail has a hexagonal plate at its base, and a fiber
is attached to each corner of the plate. The T-even phages are
about 210 nm from head to tail.
某些噬菌体（感染细菌的病毒）拥有相对复杂的结构。这些都是“T-偶”噬菌体。它们由包含DNA的多面体的头和管状尾巴组成。一个称为项圈的薄片位于头与尾之间。尾的基底有一个六角形的片状体，而且一根纤维连接至片状体的每个角。那个“T-偶”噬菌体从头到尾大约为210纳米。
Classification.
Viruses are classified according to the type of organism that they
infect: animals, plants, or bacteria.
分类。病毒是根据它们感染有机体的类型：动物、植物，或细菌来分类的。
Bacterial viruses, or phages, may contain either DNA or RNA.
They were discovered, probably independently, by British
bacteriologist F. W. Twort in 1915 and by the French
micro-biologist Felix d’Herellel in 1917. Because multiplication of
phage within a bacterium results in the lysis, or rupture,
of the host cell, d’Herelle thought that phages could be used to
control bacterial diseases. He was wrong, but investigations of
phages have contributed much of the fundamental knowledge of gene
function and protein synthesis.
细菌病毒，或者噬菌体可能含有DNA和RNA。它们很可能是由英国细菌学家F.
W.
特沃特于1915年，以及法国微生物学家菲力克斯·德赫雷尔于1917年独立发现的。因为，细菌内的噬菌体增殖导致了宿主细胞的溶解或破裂，德赫雷尔认为噬菌体可被用于控制细菌性疾病。他错了，不过他对噬菌体的调查对基因功能和蛋白质合成的基础知识贡献很大。
Virtually all plant viruses contain RNA as their genetic
material. They have either helical or icosahedral structures. In
1899 the Dutch botanist M. W. Beijerinck proposed that the agent
causing tobacco mosaic disease was a self-reproducing, subcellular
form of life. In 1935 the American biochemist Wendell Stanley
proved the unusual nature of the tobacco mosaic virus (TMV) when he
crystallized it. At that time, crystallization was thought to be a
property of inorganic molecules. Now it is known that organic
substances, too, including purified virus, will crystallize if
their molecules or particles can form orderly structural
arrays.
事实上，所有植物病毒都包含了作为它们遗传物质的RNA。它们要么具有螺旋形的，要么具有二十面体的结构。1899年，荷兰植物学家M.
W.
拜耶林克提出，引起烟草花叶病的介质是一种生命的亚细胞形式的自我繁殖。1935年，美国生物化学家温德尔·斯坦利在他将烟草花叶病病毒（TMV）结晶时，证明了它的反常性。当时，结晶被认为是无机分子的属性。现在人们知道了有机物质，包括净化的病毒，如果它们的分子或粒子能够形成有序的结构排列，也会结晶。
No generalizations can be made about the structure of animal
viruses. They may have either DNA or RNA for their genetic
material. Some have envelopes, while others exist as naked
nucleocapsids. They may be as simple in structure as is the polyoma
virus, the DNA of which weighs about a billionth of a billionth of
a gram and contains fewer than 10 genes, or they may be as complex
as the poxviruses which have almost 50 times as much DNA as the
polyoma virus. Poxviruses are big enough to be seen with the light
microscope.
关于动物病毒的结构问题不能一概而论。它们的遗传物质可能具有DNA或RNA。一些可能具有包膜，而另一些会以裸核壳体存在。它们的结构可能像多瘤病毒的结构一样简单，其DNA的重量大约为十亿分之一克，并包含少于10个的基因，或者它们可能像痘病毒一样复杂，拥有几乎像多瘤病毒DNA数量的50倍。痘病毒够大，用光学显微镜可观察到。
REPRODUCTION
In orders to reproduce, a virus must first enter a
susceptible host cell. Once inside, the virus takes over the
synthetic machinery that normally makes all cellular components and
uses it to synthesize viral constituents and reproduce the virus in
large numbers. Synthesis of cell constituents stops as a result of
viral infection. Reproduction of many kinds of viruses results of
viral infection. Reproduction of many kinds of viruses results in
the dissolution and death of the host cell. Such virus-caused
dissolution is called lysis, and the virus is said to
lyse the host cell.
繁殖
为了繁殖，病毒必须首先进入一个易受影响的宿主细胞。一旦进入，病毒便控制了通常制造所有细胞成分的合成机制，并用它来合成病毒成分，并大量繁殖病毒。由于病毒感染的结果，细胞成分的合成停止。许多种类的病毒繁殖都是病毒感染的结果。许多种类的病毒繁殖导致了宿主细胞的分解和死亡。这种造成分解的病毒被称为溶解，而且据说病毒分解了宿主细胞。
The exact mechanism by which a virus reproduces depends on
the type of nucleic acid it contains., double- or single-stranded
DNA and double- or single-stranded RNA , All four possible types
have been found. The life cycles of phages have been most
thoroughly studied and are the best understood. In general, the
reproduction of animal and plant viruses parallels that of phages
with the same type of nucleic acid.
病毒繁殖的准确机制取决于它所包含的核酸类型。已被发现的全部四种可能的类型有，双链-或单链DNA和双链-或单链RNA。人们对噬菌体的生命周期已做了最彻底的研究和最全面的了解。总之，动物和植物病毒的繁殖与拥有相同类型核酸的噬菌体的繁殖类似。
Phages with DNA. The
first step in the multiplication of any virus is adsorption—that
is, attachment to the susceptible cell. Viruses usually attach to a
specific, limited range of cell types. The specificity of
attachment depends both on the viral coat (or envelope) and on the
presence of receptors on the cell surface. The T-even phages attach
to the bacterium Escherichia coli by means of the tail
fibers. The virus then injects its double-stranded DNA through the
tail into the bacterial cell. The period following injection is the
eclipse, during which infectious viruses cannot be recovered
from disrupted bacteria. The virus coat remains outside the cell,
and naked viral nucleic acid is only infectious under special
circumstances.
带有DNA的噬菌体。任何病毒增殖的第一步就是吸附---换言之，就是附着到易受影响的细胞上。通常，病毒是附着到特定的，有限范围的细胞类型上。吸附的特征不仅取决于病毒的外膜（或包膜），而且取决于细胞表面存在的受体。T偶噬菌体通过尾部纤维附着在大肠杆菌上。
Synthesis of viral components and enzymes completely replaces
that of bacterial constituents in infected cells. The viral DNA
replicates in the normal manner according to the principles of
base-pairing suggested by James Watson and Francis Crick. Viral
genes direct the synthesis of messenger RNA’s and, ultimately, of
viral coat proteins. For descriptions of the replication of DNA and
RNA and of the synthesis of cell proteins, see GENE.
病毒成分与酶的合成完全替代了在受感染细胞中细菌成分的合成。病毒的DNA根据詹姆斯·沃森和弗朗西斯·克里克提出的碱基配对原则，以正常的方式复制。病毒基因指挥着信使RNA的合成，而且最终指挥着病毒外壳蛋白的合成。关于DNA和RNA复制的描述，以及细胞蛋白合成的描述，可参阅基因词条。
During maturation, the various components assemble
spontaneously to form complete infectious virus particles. A
hundred or more may form in each cell. The final step is the
release of the newly synthesized viruses. This is accomplished by
means of a viral enzyme, called lysozyme, that breaks down the
bacterial cell wall. The viruses then escape from the bacterium
into the surrounding medium, where they can infect additional
cells, and the lysed bacterium dies.
在成熟期间，各种成分自发地聚集起来，形成完全的传染性病毒粒子。在每个细胞中可形成100个或更多的粒子。最后一步是释放新的合成病毒。这是通过一种称为溶菌酶的病毒酶完成的，会破坏细菌的细胞壁。然后，病毒从细菌中逃入周围的介质中，在那里它们可能感染额外的细胞，并且使细胞溶解的细菌死亡。
A few simple phage, such as ФX174,
have a single-stranded DNA molecule for their genetic material.
WhenФX174 infects a bacterium, the first
step, once the DNA is inside the cell, is the synthesis of a DNA
strand (the minus strand) complementary to the original parental
(or plus) strand. The resultant double-stranded DNA replicates to
form additional double-stranded progeny. Later in
theФX174 life cycle, only plus strands
are formed. These associate with capsid proteins to form virus
particles.
一些简单的噬菌体，像ФX174，它们的遗传物质具有一种单链的DNA分子。当ФX174感染细菌时，一旦DNA进入细胞，第一步是DNA链（负链）的合成与原始亲本链（或正链）互补。由此导致的双-链DNA复制形成额外的双-链后代。在ФX174生命周期的后期，只有正链形成。这些与衣壳蛋白一起形成病毒粒子。
A virus does not “grow” in the usual sense. Instead, the
virus is “fully grown” as soon as its component parts have
assembled themselves into the structure that is characteristic of
that particular virus.
在通常意义上，病毒不会“生长”。相反，一旦病毒的组成部分将它们自身组合到那种特殊病毒具有的特定结构中，病毒就是“发育完全的”。
Pages with RNA. The
RNA phages, including phages f2 and
Qβ, have only enough
RNA to code for THREE proteins. These are the coat protein, an
attachment protein needed for adsorption of the virus to the host
bacteria, and an enzyme called RNA synthetase or replicase. A
replicase, which transcribes viral RNA to form RNA strands
complementary to it, is necessary for replication of all RNA
viruses. The viral RNA serves both as a messenger for the synthesis
of viral proteins and as a template on which the replicase
synthesizes a complementary minus strand. The minus strand in turn
serves as a template for the synthesis of additional plus RNA
strands. These plus strands have THREE options: they can serve as
templates for the synthesis of minus strands; they can act as
messengers for the synthesis of more viral proteins; or they can be
incorporated into intact virus particles. As many as 10,000 RNA
virus particles may form in a single cell.
带有RNA的噬菌体。RNA噬菌体，包括噬菌体f2
和
Qβ，只有足够的RNA编码三种蛋白质。这些都是外壳蛋白，一种需要将病毒吸附到宿主细菌上的附着蛋白，以及一种称为RNA合成酶或复制酶的酶。转录病毒RNA形成RNA链与其互补的复制酶是所有RNA病毒复制所必要的。病毒RNA不仅作为病毒蛋白合成的信使，而且也是复制酶合成互补负链的模板。反过来，负链又起到了额外的正RNA链合成模板作用。这些正链有三个选择：它们可以作为负链合成的模板；它们可以充当更多病毒蛋白合成的信使；或者它们可以被整合到完整的病毒颗粒中。在一个单细胞中可形成多达10000个RNA病毒颗粒。
Animal Viruses.
Although the reproduction of animal viruses resembles that of the
corresponding types of bacteriophages, some differences exist. The
complete naked virus—not just the nucleic acid—usually enters the
host cell. In the case of enveloped viruses, the envelope first
fuses with the cell membrane, after which the nucleocapsid is
released into the cell. Uncoating of the viral nucleic acid inside
the cell is carried out by cellular enzymes that digest the capsid
proteins.
动物病毒。虽然动物病毒的繁殖类似于相应的噬菌体类型的繁殖，但却存在着某些差异。完全的裸病毒---不只是核酸---通常会进入宿主细胞。在有包膜病毒的情况下，包膜首先熔合细胞膜，溶合之后，核衣壳被释放到细胞中。细胞中的病毒核酸脱壳是由消化衣壳蛋白的细胞酶进行的。
The DNA of most DNA viruses replicates in the nucleus of the
infected cell. (Replication of animal viruses differs in this
respect from replication of phages in bacteria, because bacteria do
not have nuclei.) The messenger RNA of these viruses is also
synthesized in the nucleus but it then migrates to the cytoplasm
where protein synthesis occurs. The proteins must be transported
back into the nucleus since assembly of the nucleocapsids takes
place there. The assembly of the nucleocapsids of animal viruses is
spontaneous and resembles that of the phages. The DNA-containing
poxviruses reproduce entirely in the cytoplasm, as do viruses with
RNA.
大多数DNA病毒的DNA都在受感染的细胞核酸中复制（动物病毒的复制在这方面不同于细菌噬茵体的复制，因为细菌没有细胞核。）这些病毒的信使DNA也是在细胞核中合成的，但它随后迁移至蛋白合成的细胞质。由于核衣壳的构成在那里进行，所以那些蛋白必须被送回到细胞核。动物病毒核衣壳的构成是自发的，而且类似于噬菌体的构成。含有DNA的痘病毒完全在细胞质内复制，带有DNA的病毒也是如此。
Naked viruses may be released by lysis of the cell or by the
formation and subsequent rupture of virus-containing vacuoles at
the cell surface. The release of enveloped viruses is more
complicated. After viral proteins are incorporated into the cell
membrane, the nucleocapsid adheres to its inner surface and causes
the membrane to bud outward. Eventually the membrane completely
surrounds the nucleocapsid and the completed enveloped virus
pinches off from the cell. The lipids and polysaccharides of the
envelope are derived from the host-cell membrane.
裸病毒可能由细胞的溶解，或由在细胞表面形成和随后破裂含有病毒的液泡所释放。包膜病毒的释放更为复杂。在病毒蛋白成为细胞膜的一部分之后，核衣壳附着到其内表面，并使薄膜向外发芽。最终薄膜完全包围了核衣壳，并且完成的包膜病毒从细胞分离。包膜的脂类和多糖都来源于宿主细胞膜。
Plant Viruses. Much
less is known about the reproduction of plant viruses than about
that of animal and bacterial viruses. This is due mainly to the
previous lack of plant cell cultures in which the virus could be
grown in a synchronized manner—that is, with all the particles in
the same phase of the reproductive cycle. Such synchronization is
required for studying the details of virus reproduction. Suitable
cultures have recently become available, however.
植物病毒。比起对动物和细菌病毒繁殖的了解，人们对植物病毒繁殖的了解要少得多。这主要是由于早先缺乏可以同步方式培养病毒的植物细胞培养---换言之，所有粒子都处于生殖同期的同一阶段。这样的同步化是研究病毒繁殖的细节所必需的。然而，适当的培养在近来成为了可能。
It appears that plant viruses reproduce in manner similar to
that of other RNA viruses. One difference is that some, probably
including TMV, may replicate their RNA in the nucleus. The
cellulose cell walls of plants constitute something of a barrier to
viral infection, which requires wounding of the plant cell. This is
most often done by insect carriers of viruses, but man and his
machinery may also spread viral infections. Transmission between
cells of a plant is through plasmodesmata, links of cytoplasm that
extend through the cell wall and connect cells.
看来，植物病毒的繁殖方式与其它RNA病毒的繁殖方式类似。一种差异是，可能包括TMV，可能在细胞中复制它们的RNA。植物的纤维细胞壁构成了某种对病毒感染的屏障，病毒感染需要对植物细胞造成损伤。这在很多时候是由昆虫携带的病毒造成的，但人与其机构也可能传播病毒感染。在植物细胞之间的传播是通过胞间连丝，贯穿细胞壁连接细胞，链接细胞质。
LYSOGENY
Viruses that always lyse cells are called virulent
viruses. Temperate viruses, however, do not always kill their host
cells. Infection of a susceptible bacterial strain by a temperate
phage produces either of two results. In some cases the virus
multiplies and causes lysis of the cell. In other cases, involving
only DNA viruses, the phage establishes a lysogenic
relationship with the cell. In 1950 the French microbiologist Andre
Lwoff showed that in lysogeny the virus is maintained in the
bacterial cell in a stable, heritable, but noninfectious form
called the prophage, which does not reproduce in the
lysogenized cell.
溶原现象
总是溶解细胞的病毒被称为致命细胞。然而，温和的病毒并不总是杀死它们的宿主细胞。由温和噬菌体感染的易感菌株会产生两种结果中的一种。在某些情况下，病毒会繁殖并导致细胞溶解。在其它情况下，只涉及DNA病毒，噬菌体与细胞建立一种溶原关系。1950年法国微生物学家安德烈·利沃夫证明，在溶原现象中，病毒以稳定、可遗传的，但非传染的，被称为前噬菌体的方式保留在细菌细胞中，不会在溶原化的细胞中繁殖。
One of the best-studied lysogenic relationships is that
between phage lambda and strain K12 of the bacterium E.
coli. The structure of phage lambda somewhat resembles that
of the T-even phages, but it has much less DNA for its genetic
material. Once this phage establishes a lysogenic relationship with
bacterial cell, that bacterium cannot be infected by additional
viruses of the same type. The cell is said to be “immune’ to
infection. (This “immunity” has no relation to the immune system of
higher animals.) Both immunity and the lack of reproduction by the
phage are caused by the presence of a repressor protein that
prevents transcription of the viral genes needed for reproduction
the repressor is specified by a lambda phage gene. The French
molecular biologist Francois Jacob concluded that when a temperate
phage enters a suitable bacterial cell, a race occurs between the
synthesis of the gene products needed for reproduction and the
synthesis of the immunity repressor. The outcome of the race
determine whether the cell will be lysed (destroyed) or will be
lysogenized and survive.
研究溶原性关系最好的之一是噬菌体入链与大肠杆菌细菌的K12菌株的关系。噬菌体入链的结构有点类似T-偶数噬菌体的结构，但它的DNA遗传物质要少得多。一旦这种噬菌体与细菌细胞建立起溶原关系，那种细菌就不会受到同类其它病毒的感染。据说，该细胞对感染是“免疫的”。（这种“免疫力”与高级动物的免疫系统无关。）免疫力和缺乏噬菌体繁殖都是由阻抑蛋白的存在引发的，这种蛋白阻止了由入链噬菌体基因指定的，繁殖所需的抑制因子的病毒基因转录。法国分子生物学家弗朗索瓦·雅各布总结道，当温和的噬菌体进入合适的细菌细胞时，在繁殖所需的基因产物综合与免疫力阻抑综合之间会出现竞争。竞争的结果决定是否细胞会被溶解（破坏）或会被溶原化并且存活。
When lysogenization occurs, the viral prophage is inserted
into the bacterial chromosome, usually at a fixed site. Its
replication is controlled by bacterial genes so that it replicates
only when the bacterial chromosome does. Occasionally the
integrated prophage is spontaneously excised, and the virus then
reproduces normally and lyses the cell. A number of treatments,
including irradiation with ultraviolet light, certain chemicals,
and carcinogens, can greatly increase the frequency of
excision.
当溶原化发生时，病毒的前噬菌体就会插入细菌的染色体中，通常在一个固定的地方。它的复制是由细菌基因控制，所以它只有当细菌的染色体变化时才复制。偶尔，整合的前噬菌体被自发地切除，然后病毒正常繁殖并溶解细胞。有一些处理方法会大大增强切除的频率，包括用紫外线照射，特定的化学物品以及致癌物。
TRANSDUCTION
Viruses sometimes carry bacterial DNA from a donor bacterium
to a recipient. The American geneticists Joshua Lederberg and
Norton Zinder who discovered this phenomenon in 1951, called it
transduction. Restricted, or specialized,
transduction is the transfer of a limited number of specific
genes by a lysogenic phage. Specialized transduction requires
excision of the virus from the chromosome of the donor bacterium.
During excision, neighboring bacterial genes are removed together
with viral DNA. Thus only those bacterial genes near the site of
the prophage in the donor’s chromosome are transferred to the
recipient bacterium.
转导
病毒有时会将细菌的DNA从供体携带到受体上。美国遗传学家约书亚·莱德伯格和诺顿·津德尔于1951年发现了这种称其为传导的现象。受限的，或专门的传导是一种由溶原噬菌体转移有限数量的特定基因。专门的传导需要从供体细菌的染色体中切除病毒。在切除过程中，相邻细菌基因随病毒的DNA一起被移除。因此，只有在供体染色体中靠近前噬菌体位置的那些细菌基因才被转移到受体细菌中。
In general transduction, any portion of the bacterial
chromosome may be transferred to a recipient. It is caused by
encapsulation of bacterial DNA into the phage coat. The transducing
phage preparation can be obtained either from lysogenic bacteria or
from a lysing viral infection. Many such transductants carry no
phage genes. They are still infectious, because infectivity is
property of the coat. Only a small fraction of the bacterial
chromosome can be transferred, however, because the bacterial
chromosome is much larger than the normal viral chromosome. Thus
only genes close together on the donor bacterial chromosome can be
transduced to a recipient bacterium.
在一般的转导中，细菌染色体的任何一部分都可能被转移到受体中。这是将细菌DNA包装在噬菌体外壳引起的。传导的噬菌体既可从溶原性细菌中获得，也可从溶解病毒引起的感染中获得。许多这样的转导体并不携带噬菌体基因。它们依然具有传染性，因为传染性是外壳的特性。然而，只有小部分细菌的染色体可以被转移，因为细菌的染色体比正常的病毒染色体要大得多。因此，只有在供体细菌染色体上紧密相连的基因才能传导至受体细菌。
This property of general transduction gave investigators a
tool for constructing detailed maps of bacterial chromosomes. For
example, the American microbiologists Charles Yanofsky and E.
Lennox used general transduction to map the region of the E.
coli chromosome containing the genes required for synthesis of
the amino acid tryptophan. Yanofsky later prepared an even more
detailed map of one of these genes. This map served as the basis
for Yanofsky’s proof in 1966 of a tenet of molecular biology that
had long been assumed—namely, that the amino acids that make up a
protein molecule are joined together in exactly the same sequence
as are the corresponding codons (a codon is a sequence of THREE
nucleotides that specifies or codes for a particular amino
acid).
一般转导的这种特性给研究者构建详尽的细菌染色体颁布图提供了工具。例如，美国微生物学家查尔斯·亚诺夫斯基和E.
伦诺克斯利用一般转导绘制了包含氨基酸色氨酸合成所需基因的大肠杆菌染色体区域。后来，亚诺夫斯基甚至为这些基因中的一个基因准备了一张更加详细的分布图。这张图成为了亚诺夫斯基于1966年证明长期以来假定分子生物学原则的基础---也就是说，构成蛋白质分子的氨基酸与相对应的密码子是以完全相同的顺序连接在一起的（密码子是指定或编码特定氨基酸的三个核甘酸序列）。
CULTIVATION
Because viruses are obligate intracellular parasites, they
must be grown in living cells. Virus preparations are needed in
large quantities for the manufacture of vaccines against viral
diseases and in smaller quantities for scientific investigations.
Finding suitable cells for growing a virus is sometimes a major
problem. For example, research on polio was greatly hindered until
1939 when the American microbiologists John Enders, Thomas Weller,
and Frederick Robbins discovered that poliovirus could reproduce in
cultures of certain human or monkey tissues.
培养
因为病毒是专性细胞内的寄生物，所以它们必须在活细胞内生长。为生产预防病毒疾病的疫苗就需要大量的病毒制剂，以及少量用于科学研究的制剂。为培育病毒找出合适的细胞有时是个大问题。例如，有关脊髓灰质炎的研究就受到了很大阻碍，直到1939年美国微生物学家约翰·恩德斯，托马斯·韦勒和弗雷德里克·罗宾斯发现脊髓灰质炎病毒可在某些人类和猴子的组织中培养才得以解决。
Cells that tend to become cancerous are considered unsuitable
for growing viruses for vaccines that will be injected into humans.
Cultures of normal cells may be used for this purpose. Another
alternative is the use of fertile chicken or duck eggs. The rabies
virus, for example, is now grown in duck eggs. Until recently, this
virus had to be cultivated in nerve tissue—the normal site of viral
multiplication—and the vaccines contained some of the nerve tissue,
which often caused dangerous allergic reactions. With the newer
vaccines, immunization against rabies has become less
hazardous.
易癌变的细胞被认为不适合培养用于注射入人体疫苗的病毒。正常细胞的培育可用于这一目的。类似的替代物是利用多产的鸡蛋或鸭蛋。例如，狂犬病病毒目前是在鸭蛋中培育的。直到最近，这种病毒已在神经组织中培育---病毒繁殖的正常部位---而该疫苗包含了一些神经组织，通常会引起危险的过敏反应。随着更新的疫苗出现，抗狂犬病的疫苗接种已很安全。
JEAN L.
MARX
Research News Staff of
“Science”
简·L．马克斯
《科学》的研究消息人员
2022年12月30日译
待续部分：VIRUSES AND HUMAN
DISEASES病毒与人类疾病；Adenoviruses腺病毒；Poxviruses痘病毒；Picornaviruses
小核糖核酸病毒；Myxoviruses
黏液病毒；Paramyxoviruses
副黏液病毒；Rhabdoviruses
杆状病毒；Togaviruses
披膜病毒；Viruses and
Cancer 病毒与癌症；The
Slow Virus Diseases 慢性病毒疾病；DEFENSES
AGAINST VIRAL DISEASE
对病毒性疾病的防御；Interferon
干扰素；Chemotherapeutic
Agents 化疗药剂。
（译者注：该部分词条位列《大美百科全书》1985年版，第28卷，第172页至175页）
