Definition of a Virus

Virus comes from the Latin venom which means poisonous liquid. Based on their nature, viruses are classified into separate kingdoms. Viruses are also not included in cells, because they do not have cytoplasm and nucleus.

Based on its location, the virus can exist outside or inside the cell.

  1. Outside the cell, the virus cannot carry out its life activities, so it is only a submicroscopic particle containing nucleic acid and is wrapped in proteins and other macromolecules.
  2. In the cell, the virus can carry out living activities and multiply, then it can infect living cells so that these cells can experience disturbances and even death in infected living things.

Once inside the cell, the virus has the ability to change its protein structure in a relatively fast time.

After knowing the malignancy carried out by the virus, surely some of you are wondering what the characteristics of the virus have? How to live and reproduce? This will be discussed in the subtopic below.

Virus Characteristics

Viruses have characteristics that distinguish them from other microorganisms, including:

  1. The size is very small, which is 20-30 millimicrons.
  2. It is a subrenic organism, because of its small size so that it can only be seen with an electron microscope.
  3. Its body contains either nucleic acids, DNA, or RNA only.
  4. Does not have metabolic enzymes, ribosomes, or other cell organelles.
  5. Only requires nucleic acid for the reproduction process.
  6. Cannot live outside the cell, so its life depends on other living cells to reproduce itself.
  7. Because their life depends on other cells, the virus is a parasitic microorganism.
  8. Each type of virus can only infect a certain type or type of host, the type of host that can be infected by a virus is called the host range. This determination is based on the ‘lock and key’ suitability theory, which is between the external protein that the virus has and the specific receptor molecule on the surface of the host cell.
  9. Can be crystallized (as an inanimate object) and reclaimed (as a living thing)

Virus Body Structure and Shape

Viruses vary in size, shape and chemical composition. Virus forms, namely:

  1. Ball
  2. Oval
  3. Astigmatism
  4. stem
  5. Letter T

The main structure of viruses is the nucleic acid which can be either RNA or DNA or neither. These nucleic acids are surrounded by protein subunits called capsomeres. The arrangement of the capsomeres then forms a mantle called the capsid. The capsid and nucleic acids in viruses are called nucleocapsids.

In some viruses, there are those that have a wrapping structure, namely a membrane. The membrane is composed of lipid bilayers and proteins (generally glycoproteins). In addition, there are also viruses that have a tail such as the Bacteriophage virus. The tail is composed of more than 20 kinds of protein.

the shape and structure of the virus

a) The form of the nucleocapsid virus, b) the form of the membrane virus, c) the form of the bacteriophage virus
Image source:

Classification of Viruses

Viruses can be classified based on:

Classification of Viruses Based on Place of Life

Viruses cannot carry out their own life activities, but instead live to become parasites for their host or their place of life, namely bacteria, plants, animals, and humans.

There is not a single bacteria that does not contain viruses. Viruses that infect bacteria are bacteriophages. Bacteriophage can reproduce quickly and in a short time can destroy a number of bacteria. Examples of bacteriophages: E. coli.

Most plant diseases are caused by viruses. The genetic material of plant viruses is RNA. In plants, viruses can infect directly or through vectors such as insects. Viruses can multiply in the digestive tract of insects and can be transmitted to plants after an incubation period in insects. Examples of plant viruses: Tobacco Mozaic Virus (TMV) and Beet Yellow Virus (BYV).

The genetic material of animal viruses is DNA double helix or single polynucleotide RNA. As in plants, animal viruses can infect directly or through vectors. Examples of animal viruses: influenza virus, Vaccina virus, and Poliomylitis virus.

As in plants and animals, human viruses can infect directly or through vectors. One of the human viruses that can be transmitted through direct contact or vectors, namely the Corona virus or COVID-19, which is currently a hot topic of conversation. The virus is known to be transmitted between humans and from animals such as bats. These virus-carrying vectors can infect after an incubation period in their body and make direct contact with humans through droplets or body fluids. Examples of other viruses found in humans: chickenpox virus, measles, hepatitis, dengue fever, diarrhea.

Classification of Viruses Based on the Molecules That Make Up Nucleic Acid

The constituent molecules are divided into:

  1. Single strand DNA (DNA ss)
  2. DNA double strand (DNA ds)
  3. Single band RNA (RNA ss)
  4. Double band RNA (RNA ds)

Classification of Viruses Based on the Existence of the Sheath

  • Viruses that have a sheath
    Has a nucleoplasm wrapped by a membrane.
  • Uncoated virus (naked virus)
    It only has capsid (protein) and nucleic acids.

Virus Reproduction (Virus Replication)

There are two types of virus reproduction, namely the lytic cycle and the lysogenic cycle.

1. Lytic Cycle

reproduction of the lytic cycle virus

Lysolytic Cycle in Viruses
Image source: P., FF, & Ariewibowo, M. (2009). Practical Learning Biology 1: for Class X SMA. Jakarta: PT. Balai Pustaka.

The phases carried out by the virus in the lytic cycle are as follows:

  1. The virus attaches to the bacterial cell wall
  2. Viral DNA is inserted into the bacterial cell wall which has been perforated and dissolved by viral enzymes.
  3. Viral DNA does transcription in bacterial cells and use bacterial metabolics to produce viral components or structures such as capsid, tail, tail fibers, and head. New viruses that are formed in one lytic cycle can reach 200-1000 viruses.
  4. The new virus that is formed will secrete lysozyme enzymes to lyse and destroy the bacterial cell walls so that they can infect and repeat the lytic cycle in other bacterial cells. To produce a new virus in one lytic cycle, it takes only 20 minutes.

2. Lysogenic cycle

lysogenic cycle of viral replication

Lysogenic cycle in viruses

Not all viruses that enter the cell walls of living things can immediately destroy and lyse the walls. The viral DNA that enters the bacteria will become part of the host DNA through recombination. However, even though it is part of the host’s DNA, the virus does not directly take over the metabolism of the host cell. Such a cycle is called the lysogenic cycle. The following phases are carried out by viruses in the lysogenic cycle:

  1. The virus lives in a specific place on the surface of the bacterial cell to be able to lyse the cell wall and penetrate the genetic material into the bacterial cell body.
  2. The viral DNA then inserts into the bacterial DNA and forms a profile.
  3. If the bacteria divide, the profage in these cells will also divide. So that later the number of bacteria that contain profage will also increase. If the environment is favorable, the virus will undergo maturation and will enter a lytic state.
  4. A new virus is formed and ready to attack other cells.


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