*Part 1
The study of viruses is called virology. This branch of microbiology is related to the structure of viruses, their classification (taxonomy), artificial methods of cultivation, isolation processes, various assays, genetics, infections caused by viruses in animals, humans, and plants, modes of infection, evolution, vaccines, and therapy.
*Virus*
Viruses are unique in nature; they are the smallest self-replicating organisms, so small that they cannot be seen with a regular compound microscope. At a very basic level, a virus consists of a small segment of nucleic acid surrounded by a protein shell. They do not have the capability of metabolism but attack cells and parasitize the cell's biosynthetic machinery to meet their needs.
Nucleic acid is either DNA or RNA, known as the virus genome. Based on this, viruses are classified into seven families. This classification was introduced by American biologist David Baltimore and is known as the Baltimore classification. Here is a brief overview:
1. Viruses with double-stranded DNA (dsDNA), where two strands are complementary to each other.
2. Viruses with single-stranded DNA (ssDNA).
3. Viruses with double-stranded RNA (dsRNA), where two strands are complementary.
4. Viruses with a single strand of positive-sense RNA (+ssRNA), which can be directly translated into proteins.
5. Viruses with a single strand of negative-sense RNA (-ssRNA), which first needs to form a complementary strand for translation.
6. Viruses with single-stranded RNA that undergo reverse transcription to form DNA (ssRNA-RT).
7. Viruses with double-stranded DNA, where one strand has a gap, leading to partial dsDNA. This gap is filled during replication to form complete strands, which then produce RNA strands and ultimately the DNA genome (dsDNA-RT).
Step by step, we will discuss the structure of viruses, replication, invasion of cells, entry/penetration into cells, how our immune system protects us from viruses, and a detailed look at the mentioned families.
*For The Students Of Allied Health Sciences (FSc Group) 👇👇👇*
*Third Part (3)*
*Study of Viruses (Virology)*
In the last part, we talked about the experiments and observations that led to the discovery of viruses. Today, we will talk about the structure of viruses and what components they have.
*Structure*
All viruses have nucleic acid (which can be either DNA or RNA, but not both in the same virus). The nucleic acid carries genetic information unique to each virus. This means each virus has different information. The form of a virus that infects a host cell from outside (extracellular) is called a virion. The genetic information produces different types of proteins; some build the structure of the virus and others are functional. Structural proteins form a shell called a capsid that surrounds the nucleic acid. Some viruses also have proteins inside the capsid that act as enzymes, helping in the replication of the *virus.*
*Here, I think it's important to discuss some terms to differentiate viruses from similar entities.*
*Viroids* are disease-causing organisms that have only nucleic acid without any structural proteins.
Some virus-like particles are called prions, which are primarily proteins with a small nucleic acid molecule entangled with the protein molecule.
The real reason viruses can cause infection is because of their nucleic acid. In many viruses (though not all), if the capsid is separated from the nucleic acid, the virus can still infect the host cell but not as effectively as the whole virion.
The capsid serves three functions:
1. Protects the virus's nucleic acid from enzymes (nucleases) that can break it down.
2. Provides a surface that recognizes and attaches to specific molecules (receptors) on the host cell.
3. Provides a place for proteins that facilitate the virus's entry into the host cell or, in some cases, inject the virus's nucleic acid into the host cell.
The combination of nucleic acid and various proteins forms the nucleoprotein or nucleocapsid. Some viruses have multiple layers of proteins surrounding the nucleic acid. Some also have a membrane made of lipids and proteins (lipoprotein), taken from the host cell's membrane, known as an envelope.
The amount and arrangement of proteins and nucleic acid determine the size and shape of the virus. Viruses mainly have two shapes:
1. Rods or filaments, where proteins and nucleic acid are arranged linearly.
2. Spherical, which are primarily icosahedral in shape.
Some large and complex viruses, like bacteriophages, have both filamentous and icosahedral features, forming a combination of shapes.
The genome of double-stranded DNA viruses consists of one large molecule, while in double-stranded RNA viruses, it consists of small segments, each representing a gene (discussed in detail in the first part).
In the next part, tomorrow we will discuss how viruses attack host cells.
*Note-:* Below are the first two images related to the structure of viruses, and the third image shows the shapes of viruses.
*Source Of diagrams -: Encyclopedia Britannica.*
