The following points highlight the four main Components of Viruses. The components are: 1. Nucleoid 2. Capsid 3. Envelope 4. Enzymes.
Component # 1. Nucleoid:
It represents the viral chromosome. Nucleoid or viral chromosome is made of a single molecule of nucleic acid. It may be linear or circular with various degrees of coiling. Nucleoid is the infective part of virus.
The nucleic acid is either DNA or RNA but never both. DNA containing viruses are called de-oxy-viruses while RNA-containing viruses are termed as riboviruses. Each of them has two subtypes, double stranded and single stranded.
(i) Double Stranded or dsDNA:
It occurs in T2, T4 bacteriophages, coli-phage Lambda, Cauliflower Mosaic, Pox Virus, Adenovirus, Herpes Virus (linear), Polyoma Virus, Simian Virus-40 (SM40), Hepatitis В (circular).
(ii) Single Stranded or ssDNA:
Coli-phage MS 2, Coli-phage fd (linear), Coli-phage ф x 174 (circular). The single strand of DNA is called plus strand. A complementary or negative strand DNA is synthesized to produce DNA duplex for replication during multiplication of virus.
(iii) Double Stranded or dsRNA:
It is found in Reovirus and Tumour Virus (both linear)
(iv) Single Stranded or ssRNA:
The condition is more common in riboviruses The single strand RNA is generally linear, e.g., Poliomyelitis Virus, Foot and Mouth disease Virus, Influenza Virus, Tobacco Mosaic Virus (TMV), Tobacco Necrosis Virus Potato Mosaic Virus, Bean Mosaic Virus, Retroviruses.
Retroviruses have two copies of single stranded RNA (hence diploid), e.g., HIV (Human Immunodeficiency Virus, HTLV-III, AIDS Virus) HTLV-1 HTLV-11 (Human T-lymph trophic Viruses), Rous Sarcoma Virus (RS V of M).
In some riboviruses, the RNA can directly function as template and take: part in replication (e.g. TMV, Influenza Virus, Paramyxo Virus). In other riboviruses, the RNA of the nucleoid is first employed in synthesizing complementary DNA through reverse transcription (e.g., Oncogenic Viruses, HIV).
Because of the latter, these viruses are called retroviruses. The viral chromosome or nucleoid does not contain many genes. T4 bacteriophage contains about 100 genes. Viral chromosome or nucleic acid is coiled with the help of some polyamines or internal proteins.
Component # 2. Capsid (Sheath, Coat):
It is the proteinaceous covering around the virus which protects the nucleoid from damage from physical and chemical agents. The capsid consists of a number of subunits called capsomeres or capsomers. The capsid of TMV has 2130 capsomeres. In binal bacteriophages the capsid sheath of tail is contractile.
Component # 3. Envelope:
It is a loose membranous covering that occurs in some animal viruses, rarely plant and bacterial viruses. In contrast to enveloped viruses, the viruses without an envelope are called naked.
Envelope consists of proteins from (virus), lipids and carbohydrates (from host). It has subunits called peplomeres or peplomers. Surface of envelope can be smooth or have outgrowths called spikes. Common enveloped viruses are HIV, Herpes Virus, Vaccinia Virus, etc.
Component # 4. Enzymes:
They are occasional. Enzyme lysozyme is present in the region that comes in contact with host cell in bacteriophages. Other enzymes are neuraminidase in Influenza Virus, RNA polymerase, RNA transcriptase, reverse transcriptase.