What is a virus?
Viruses are among the simplest organisms on Earth. They are invisible to the naked eye as their size ranges from 20-400 nanometers, 10-100 times smaller than a bacterium. They are made up of at least one protein and a single- or double-stranded nucleic acid enclosed by a protective protein covering called capsid which contains enzymes that allow them to enter their host cell. Their genome is contained in their nucleic acid which may either be a deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) but never both. Because of this, the type of their nucleic acid became the basis of their major classification: DNA viruses and RNA viruses.
Viruses can be considered living and non-living. Non-living in the sense that they are acellular (not made up cytoplasm and organelles) and cannot carry out basic biological functions such as growth and replication on their own. In order to reproduce, they need to enter a host cell (a bacterium), thus, viruses are obligatory parasitic in nature. While outside a host cell, they remain in a state of dormancy where biological processes are arrested. But once inside their host cell, they replicate at a very fast rate, hence they become ‘living’ entities.
There are two cycles in viral reproduction, the lytic cycle and the lysogenic cycle. The former is considered the main mode of reproduction which eventually leads to the disruption of the host cell thereby releasing viral progeny ready to infect other cells. The latter on the other hand can occur without causing harm to the host cell.
A virus that uses a bacterium to replicate is called a bacteriophage. Bacteriophages can be further classified into two based on the process they use to reproduce. Those that go through the lytic cycle to replicate are called lytic bacteriophages while phages that replicate by means of the lysogenic cycle are called Temperate phages. Lytic phages are named so because at the end of their replication, they lyse or disrupt their host bacterium. Temperate phages in contrast can replicate by incorporating their DNA into their host’s DNA without causing bacterial lysis.[ad#co-1]
This cycle consists of 5 stages – Adsorption, Penetration, Replication, Maturation and Release. A sixth stage called Reinfection can also be part of this cycle.
When a bacteriophage infects a bacterium, T4 for example, it attaches itself to the bacterium’s cell wall using its tail fibers. The protein on its tail fibers enables the virus to recognize and attach itself to specific receptor sites in the bacterial cell wall which includes lipopolysaccharides, teichoic acids, proteins or even a flagellum. The specificity and compatibility of the bacteriophage’s attachment to its host as determined by the type of receptors limit the type of host cell for the bacteriophage. This just means that there are only certain bacteria that can be infected by a particular bacteriophage.
After attachment, the T4 phage releases an enzyme that weakens the cell wall of its host. The weakened area becomes the spot where the phage injects its DNA into the bacterium. Once inside the host, the phage starts to take over its host as it makes great amount of viral proteins and genetic materials (DNA or RNA). The replication in T4 phages specifically involved three phases of mRNA production followed by protein production. When there are enough viral components synthesized, assembly into complete viruses immediately follows. In the case of T4 phages, the protein coded by the phage DNA acts as enzymes for the assembly of new T4 phages. Finally, an enzyme that weakens the host’s cell wall is then released and later on, lysis or disruption of the host bacterium occurs, releasing the newly formed T4 phages.
As in the lytic cycle, lysogenic cycle begins with the phage’s attachment to the cell wall followed by the injection of the phage’s genome into its host. However, unlike lytic bacteriophages, temperate bacteriophages do not shut down their host cell. After the genome is injected, it becomes integrated into the host’s DNA and is now called a prophage (phage’s genome incorporated into the bacterial DNA). Since the phage’s genetic material is added with that of the host, it is also replicated when the host cell replicates its DNA and divides. Thus, the viral genetic material is transmitted to bacterial daughter cells at each consequent cell division. The prophage can be cut out from the host’s genome by external factors such as ultraviolet radiation. After being removed, it replicates and produce viral components which ultimately leads to lysis of the host cell.
Differences of Lytic and Lysogenic Cycle
Both the lytic and the lysogenic cycle are means in which a virus reproduce. The main difference of these cycles is that in the lytic cycle, bursting or destruction of the host cell inevitably occurs whereas in the lysogenic cycle, the phage can replicate without harming their host. Another noticeable difference between the two is the number of phages produce after every cycle. More are produced in the lytic cycle whereas there are only 2 in the lysogenic cycle as a result of cell division. Also, the resulting products of the lysogenic cycle can further undergo lytic cycle when triggered by external factors such as radiation. But those that undergo lytic cycle follows each stage until the end solely without any diversion or alternative process from the lysogenic cycle.[ad#afterpost]
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