Viral Structure and Replication Quiz

Viruses are minuscule infectious particles, smaller than the resolution of light microscopes, requiring electron microscopes for visualization.

All viruses contain either DNA or RNA as their genetic material, enclosed within a protein coat called a capsid. They may also harbor a few enzymes, but lack cytoplasm and a plasma membrane.

Viruses that use RNA as their genetic material directly replicate their RNA within the host cell.

Retroviruses are a special type of RNA virus that convert their RNA into DNA using the enzyme reverse transcriptase. This DNA is then integrated into the host cell's genome. This allows the virus to control the infected cell's functions and to replicate itself.

HIV is a retrovirus that attacks and weakens the human immune system.

Non-retroviral RNA viruses directly utilize their RNA to create proteins and replicate.

SARS-CoV-2, the virus responsible for COVID-19 disease, is an example of a positive-sense RNA virus. Its RNA acts as mRNA and is directly translated into proteins.

Influenza virus is a negative-sense RNA virus. It needs to be converted into positive-sense RNA by its RNA-dependent RNA polymerase enzyme before it can be translated into proteins.

The capsid is a protein shell that surrounds the viral genome. It provides protection and defines the virus's shape, ranging from simple spherical to complex, multilayered structures.

Viruses can contain either single-stranded or double-stranded DNA or RNA.

An envelope is a membrane that surrounds the capsid of some viruses. It is derived from the host cell's membrane and helps the virus evade the immune system and attach to new host cells.

Bacteriophage lambda is a virus that infects bacteria, mainly Escherichia coli.

Bacteriophage lambda can exist in two life cycles: the lytic cycle, where it destroys the host cell, and the lysogenic cycle, where it integrates its genome into the host's DNA.

Viruses are obligate intracellular parasites, meaning they depend entirely on host cells for their reproduction. They lack the necessary organelles and structures to replicate on their own.

Viruses can enter host cells through various mechanisms, including endocytosis (engulfment by the host cell membrane) and receptor-mediated fusion (binding to receptors on the host cell membrane).

The lytic cycle is a rapid reproduction method where the virus takes over the host cell's machinery to produce new viral particles. Eventually, the host cell bursts, releasing new viruses.

The lysogenic cycle integrates the viral genome into the host cell's DNA. The virus remains dormant as a prophage and replicates passively with the host cell. This allows the virus to spread without harming the host immediately.

The origins of viruses are debated. Some theories suggest that viruses evolved from ancient RNA or DNA molecules, others propose that they were derived from viroids (infectious RNA agents) or transposons (mobile genetic elements). Another theory proposes that viruses originated from cells that lost their independence and became dependent on other cells for reproduction.

The study of convergent evolution in unrelated organisms has led to the observation that viruses infecting different cell types or host organisms can independently evolve similar mechanisms to adapt to their environments and hosts. These similarities include mechanisms for entry, replication, immune evasion, and even physical characteristics.

Viral evolution is accelerated due to high mutation rates, particularly in RNA viruses that use error prone enzymes like reverse transcriptase and RNA polymerase, as well as genetic exchanges between viruses through recombination and horizontal gene transfer.

RNA viruses, especially retroviruses, can accumulate genetic errors faster due to the inherent inaccuracy of their replication machinery, facilitated by the enzymes reverse transcriptase and RNA polymerase. These enzymes are more prone to mistakes than DNA polymerase, leading to a higher mutation rate in RNA viruses.

Viruses can exchange genetic material between themselves through various mechanisms, such as recombination, where their genetic material mixes during replication, and horizontal gene transfer, where they swap genetic snippets directly. This gene exchange contributes to their rapid evolution by introducing new genetic variations.

Viruses have short generation times and high reproductive rates, allowing them to proliferate quickly within their host cells. This rapid replication enhances the rate of mutation accumulation and increases their overall rate of evolution.