viruses
Viruses
Overview
Estimated 10 nonillion (10,000,000,000,000,000,000,000,000,000) individual viruses exist on our planet.
Enough to assign one to each star in the universe.
Debate on Existence of Life: Scientists discuss whether viruses can be classified as alive. Most scientists conclude that viruses are nonliving.
Importance of Viruses
Retroviral Genomes: The human genome contains approximately 500,000 retroviral genomes, which accounts for about 8% of the genome's total length.
These retroviral genomes are termed viral fossils, being genes inserted by retroviruses.
Syncytin: A protein involved in fusing placenta cells to form a wall, encoded by the envelope gene of a human endogenous retrovirus (HERV-W).
Its role in placental development is considered very significant.
Pandemics Caused by Viruses:
The Black Death: Approximately 25 million deaths.
Asian Flu: Around 1.1 million deaths.
AIDS: Estimated 35 million lives lost.
Spanish Flu: 50 million deaths.
COVID-19: Over 7 million deaths.
Structural Features of Viruses
Size: Extremely small and fixed size, ranging from 10 to 400 nm.
Genetic Material: Contain either RNA or DNA as their genetic material.
Capsid: Enclosed by a boundary made of a protein called capsid.
Does not contain cytoplasm inside, is hollow except for RNA/DNA.
Enzymes: Possess few, if any, enzymes.
Capsid Structure: The unique composition and structure of the capsid—based on specific amino acids—determines the virus's ability to infect particular hosts.
Bacteriophages
A bacteriophage is a virus that specifically infects bacteria.
Bacteriophage lambda: Infects E. Coli.
Features:
A capsid head that protects the double-stranded DNA core.
Tail fibers that attach the virus to the cell.
A tail sheath made of proteins that contracts to drive the tail tube through the host cell's outer membrane.
DNA is injected through the tail into the host cell.
Coronaviruses
The COVID-19 pandemic has led to over 5 million deaths globally, with more than 205 million cases.
Coronaviruses: Characterized by:
Spherical shape.
Single stranded RNA.
An envelope surrounding the capsid.
Numerous spike projections on the envelope.
HIV (Human Immunodeficiency Virus)
HIV is an STD transmitted through promiscuity.
Features:
Has an envelope outside the capsid.
Contains two identical strands of RNA protected by the capsid.
Encodes reverse transcriptase, allowing for DNA production from the viral RNA template.
Designated as a retrovirus due to its ability to make a DNA copy of its RNA code.
Envelope spikes of HIV are made of proteins and carbohydrates.
A mother with AIDS must undergo a C-Section to prevent the transmission of AIDS to the child during birth, as the placenta filters maternal blood, keeping the infant safe until natural delivery.
Host Specificity
Certain viruses specifically bond to certain host cells:
HIV: Attaches to white blood cells (CD4 cells).
Poliovirus: Targets spinal nerve cells.
Hepatitis Virus: Infects liver cells.
Norovirus: Infects intestinal system cells.
Life Cycle of Viruses
Lytic Cycle
Attachment: Virus attaches to a specific site on the host cell.
Penetration: Incorporates genetic material into the host cell's cytoplasm.
Biosynthesis: Uses the host's processes to produce new viral components.
Maturation: Assembles viral components into new cell entities.
Release: Bursts the host cell and releases new viruses into the surrounding environment.
Lysogenic Cycle
Does not result in immediate release of the new virus.
The bacteriophage's DNA integrates with bacterial DNA, forming a prophage.
This prophage does not alter the bacterial DNA, allowing the bacteria to continue normal functions, including reproduction.
Subsequent bacterial generations will carry the prophage within their genome.
Origin of Viruses
Virus First Hypothesis: Suggests that viruses originated before cells due to their simplicity and may have contributed materials for developing first cells.
Regressive Hypothesis (Reduction or Degeneracy Theory): Proposes that viruses were once small cells that became parasites, shedding unnecessary structures through gene reduction.
Escape Hypothesis (Vagrancy Hypothesis): Indicates that portions of genetic material from larger organisms escaped and formed viruses after being surrounded by a protective boundary.
Each hypothesis faces challenges for credibility, and it’s possible that one, none, or all hypotheses could be correct.
Convergent Evolution: Suggests that viruses developed similar traits due to environmental conditions, indicating commonalities:
Obligate parasites; cannot replicate or perform life functions alone.
Possess a protein outer boundary (capsid) without cytoplasm.
Share genetic material (DNA or RNA) between viruses and all of Earth's organisms.
Vaccination and Viral Evolution
Annual vaccinations for influenza and HIV are necessary due to the rapid evolution of these viruses through mutation.
Antigenic Drift produces small incremental changes over long periods.
Antigenic Shift occurs when two or more viruses infect the same cell and combine genetic material, creating major changes rapidly.
Antigenic drift leads to slow variation in viral surface proteins.
Antigenic shift results in entirely new viruses that can potentially form pandemics as the immune system does not recognize them.
Notably, HIV can undergo rapid antigenic drift, complicating the immune response.
Influenza can undergo antigenic shift, resulting in new virus strains that are unrecognized by the immune system.
Vaccines
Vaccines are useful in preventing and controlling many viruses.
Influenza vaccines must be updated annually due to antigenic drift, which occurs at a manageable pace for vaccine adjustments.
Antigenic shifts present challenges for annual vaccines because the rapid changes make it unpredictable, significantly complicating successful vaccination efforts.