2.2

STRUCTURE AND GENOME

2.2.1

VIRUS STRUCTURE AND FUNCTION

2.2.1.1

Capsid

A capsid is a protein shell required to protect the viral genome from host nucleases.

For some viruses, during infection, the capsid is responsible for attachment to the

specific receptors exposed on the host cellular surface. A capsid can be either

single- or double-protein shells containing few structural proteins. Hence, multiple

copies of the capsid must self-assemble to form the 3D capsid structure, allowing

different viruses to have wide ranges of shape and structure.

2.2.1.2

Envelope

Some viral families have an additional protective coat, called the envelope. The

envelope is a lipid bilayer which is partly obtained from the host cell membrane.

The lipid composition of this envelop closely reflects that of the specific host cell

plasma membrane. The exterior of this bilayer exhibits protruding structures

known as “spikes,” containing virus-coded glycosylated trans-membrane pro-

teins. In enveloped viruses, spikes also assist in the attachment of the virus to the

host cell surface.

The viral envelope serves the function of protecting the viral genetic material.

It also helps in facilitating the entry of the virus while infecting a host cell along

with evading the host immune response. However, due to the envelope’s fragile

nature, non-enveloped or also known as naked viruses, could be more resistant to

parameters such as temperature, pH, and few common chemical disinfectants

(Figure 2.2).

2.2.2

VIRAL GENOME

A virus could have either an RNA or DNA genome. This RNA or DNA genome

could be again categorized into being either positive or negative sense and single or

FIGURE 2.1 Baltimore Classification: This classification is based on the genome of the

virus. +ve RNA: similar to mRNA and can be readily translated by host cell; −ve RNA:

complementary to mRNA and requires conversion to +ve RNA by RNA polymerase prior to

translation; +ve DNA: also called sense/coding strand. Sequence corresponds to the mRNA

transcript ready to be translated; −ve DNA: also called anti-sense/template strand. Reverse

complementary to the sense strand as well as to the mRNA transcript.

Introduction to basic virology

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