病毒的基本特征
What are the essential characteristics of viruses ?
病毒是最原始的细胞和非细胞质感染因子。俄罗斯植物学家D.J. Iwanowski（1892）首先在受感染的烟草植物中发现了病毒。然而，M.W.Beijerink（1898）创造了病毒 （Virus)一词。然后是美国化学家W.M. Stanley（1935）分离出烟草花叶病毒（TMV）的纯晶体，并得出病毒是由核蛋白制成的。
病毒的一般特征：
（i）病毒是没有细胞质的细胞感染因子。
（ii）它们比细菌小，并且可以通过细菌过滤器。
（iii）病毒可从疾病传播到健康的生物体。
（iv）所有病毒都是专性寄生虫，只能在活宿主细胞内繁殖。
（v）病毒仅含有单一类型的核酸DNA或RNA。
（vi）病毒是宿主特异性的，它们仅感染宿主生物的单个物种和确定的细胞。
（vii）病毒在非常小的剂量下有效。它们对杀菌剂和极端的物理条件具有高度抗性。
病毒的广义结构：
（i）形状和大小：
形状差异很大。它们可以是球形或高尔夫球状，棒状，蝌蚪状，螺旋状或多面体。植物病毒比细菌小。
（ii）化学结构和功能：
病毒的结构非常简单。病毒的核心是由核酸构成的，核酸被称为衣壳的蛋白质外壳包围。核酸总是仅含有一种核酸，即DNA或RNA。病毒的感染性是由于其核酸。 衣壳或蛋白质外套： 它由许多相同的蛋白质亚单位组成，称为壳粒。壳粒由一种或几种蛋白质组成。 Capsomeres以非常对称的方式排列，并为特定病毒提供特定形状。病毒的宿主特异性归因于衣壳的蛋质。 病毒的生物学位置：病毒缺乏细胞质膜，它们没有细胞的基本成分。它们只能在宿主细胞内复制。在宿主细胞外，它们是无生命的。因此，病毒显示出生命和非生命的特征。
（一）病毒的非生命特征：以下病毒特征将其指定为非生命的物质： （a）它们可以结晶。 （b）在细胞外，它们表现得像惰性化学物质。（c）它们不显示生长，发育，营养，繁殖等。 （d）它们可以沉淀。（二）病毒的生命特征： （a）它们在宿主细胞内繁殖。 （b）它们拥有遗传物质，DNA或RNA。 （c）有明确的种族或菌株。（d）它们表现出突变。
由于上述原因，病毒在生物和非生物之间形成了独特的桥梁。
What are the essential characteristics of viruses ?
NIRMALA AGARWAL
Viruses are the most primitive cellular and non-cytoplasmic infectious agents. Russian botanist D.J. Iwanowski (1892) first discovered virus in an infected tobacco plant. However, M.W. Beijerink (1898) coined the term virus. Then American chemist W.M. Stanley (1935) isolated pure crystal of Tobacco Mosaic Viruses (TMV) and concluded that viruses are made of nucleoproteins.
General Characteristics of Viruses:
(i) Viruses are a cellular, non-cytoplasmic infectious agents.
(ii) They are smaller than bacteria, and this can pass through bacteriological filter.
(iii) Viruses are transmissible from disease to healthy organisms.
(iv) All viruses are obligate parasites and can multiply only within the living host cells.
(v) Viruses contain only a single type of nucleic acid either DNA or RNA.
(vi) Viruses are host specific that they infect only a single species and definite cells of the host organisms.
(vii) Viruses are effective in very small doses. They are highly resistant to germicides and extremes of physical conditions.
Generalised Structure of Viruses:
(i) Shape and size:
The shape varies considerable. They may be spherical or golf ball-like, rod-shaped, tadpole-like, helical or polyhedral. Plant viruses are smaller than bacteria.
(ii) Chemical structure and function:
Viruses have a very simple structure. The core of the viruses is made upon of nucleic acid, which is surrounded by a protein coat called capsid. The nucleic acid always contains only a single kind of nucleic acid i.e. either DNA or RNA. The infectious property of a virus is due to its nucleic acid.
Capsid or the protein coats:
It is made up of many identical protein sub-units called capsomeres. The capsomeres are composed of either one or several type of proteins. Capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus. The host specificity of virus is due to proteins of the capsid.
Biological position of viruses:
Viruses lack a cytoplasmic membrane and they do not have the basic component of a cell. They can only replicate inside the host cell. Outside the host cell, they are non-living. Thus, viruses show characters of both living and non-living.
(I) Non-living Characters of Viruses:
Following characters of viruses assign them as non-living:
(a) They can be crystallized.
(b) Outside the cell, they behave like inert chemicals.
(c) They do not show growth, development, nutrition, reproduction, etc.
(d) They can be precipitated.
(II) Living characters of viruses:
(a) They multiply within host cells.
(b) They possess genetic material, either DNA or RNA.
(c) There are definite races or strains.
(d) They exhibit mutations.
Because of the above reasons, viruses form unique bridge between living and non-living things.
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