When arthropod act as a vector it is referred as arbovirus. The size of virus varies from 10nm to 1300x6nm. Their size is not confined and is quite irregular. The shape of virus is not static. It keeps on changing, it may be helical or cuboidal or binal in shape. Viruses can be divided into two types DNA or RNA virus which are also known as Deoxyvira or Ribovira. DNA viruses are mostly animal viruses. Some animal viruses may be RNA type. DNA or RNA virus can be double or single stranded. Double stranded DNA occurs in herpes virus, polyoma virus, pox virus and Hepatitis B. While the viruses with single stranded DNA are few in number like coliphage types. Tumor and Reovirus constitute double stranded RNA viruses. Single stranded RNA viruses are more common. They cause polio, foot and mouth disease, tobacco disease, AIDS. There are other groups of viruses called retroviruses which have two single stranded RNA. Viruses are also divided into other groups such as Plant viruses, animal viruses and Phagineae. This subdivision has been given by Holmes. Plants viruses occur in plants and make them disease affected. For example TMV which stands for Tobacco Mosaic Virus. An animal virus mainly affects the animals including human beings such as Polio, hepatitis or influenza virus. Phagineae viruses are complex type of virus which affects fungi (mycophages), algae (phycophages), bacteria (bacteriophages).
A virus is mainly divided into two parts capsid and nucleoid. Some viruses have envelope covering. Nucleoid is the chromosome of the virus which is made up of single molecule of nucleic acid. Nucleic acid may be either DNA or RNA. Capsid is the protein covering which shields nucleoid. It has number of subunits known as capsomers. Envelope is a membrane covering in few of the animal viruses. Viruses without the envelope covering are referred as naked virus. It has many subunits which are known as peplomers. The envelope surface may be smooth or rough. The rough outgrowths are referred as spikes. The last component of virus is enzymes. The most common enzyme is Lysozyme.
Phagic and Pinocytic are two main types of reproduction in viruses. Phagic is subdivided into lytic and lysogenic type.
What is Lytic Cycle?
Lytic cycle is also known as virulent cycle as occurs in virulent phage. The phage attacks the host cell with its tail fibers. It produces a hole with the enzyme lysozyme. After entering in the host cell viral DNA transcribes early mRNAs to form enzymes. Cytosine bases protect phage DNA and messenger RNA by their methylation. There is a virulent or dormant or eclipse period in which virus enters into host cell and later burst host cells so that more viruses can be liberated.
What is Lysogenic Cycle?
It is also known as Lambda phage. It acts as a parasite over E.coli. It does not possess tail fibers as were present in lytic phase. In this case tail comes in direct contact with host cell and produces a hole.
The phage DNA is pushed into the host cell. After some time it goes in non virulent state. Enzyme integrase plays an important part in integration of viral genome with chromosomal DNA and this form of viral genome is known as Prophage. It divides and it is distributed to the bacteria. A repressor is also formed. Sometimes a repressor is not formed. It is destroyed by external factors such as chemicals, high energy radiations, temperature changes. The co existence of virus with host DNA is referred to Lysogeny.
What are single stranded DNA phages?
These phages have single stranded DNA and this DNA makes an additional strand to become double stranded. These processes occur before multiplication.
