Chapter 8 Study Notes: Viruses

Definition: All viruses are obligate intracellular parasites, meaning they cannot reproduce independently and must enter a living cell.
- Reproduction Process:
- A virus must utilize the host cell's machinery to replicate its components.
- Viral particles (virions) cannot perform functions such as energy production or macromolecule synthesis when outside a host cell.
- This raises the question of whether viruses are alive or non-living beings.
Nature of Classification:
- The binary classification (e.g., living vs. non-living) is often inadequate since other intermediate categories exist.
- Example: Terms like "tall" or "short" lack precise definitions when it comes to height.
- Viruses can reproduce in a very specific manner but this does not necessitate classifying them as living.
Viruses as Parasites:
- Viruses can be classified as parasites as they derive resources from the host, offering little to no benefit in return.
Hosts Requirement: Viruses require hosts for reproduction; without a host, new virions cannot be produced.
- Notably, bacteriophages (viruses that infect bacteria) are likely more numerous than bacteria itself.
Numerical Data:
- Estimated number of bacteria on Earth: $10^{30}$ (equivalent to approximately 70 gigatons of organic carbon).
- Estimated number of virions: $10^{31}$ , making them the most abundant type of organism.

Composition:
- Every virus consists of one or more pieces of nucleic acid and a protective shell called a capsid, composed of several proteins.
- Some viruses have an additional membrane derived from the host cell (envelope).
Types of Nucleic Acids:
- DNA: All cells use double-stranded DNA (dsDNA) for genetic information; viruses can use:
- Double-stranded DNA (dsDNA)
- Single-stranded DNA (ssDNA)
- RNA: Viruses may also use RNA, which is generally more error-prone during replication.
- dsRNA: More stable than ssRNA but less common in viruses.
- ssRNA: Highly mutable, associated with rapid evolution and mutations (e.g., HIV, SARS-CoV-2).

Genetic Coding:
- Minimum information required for a virus includes details on copying their nucleic acid and making capsid proteins.
- Some viruses have hundreds of genes encoding various proteins and functions.
Capsid Structure:
- Capsids often form through self-assembly of identical protein subunits, typically leading to a limited number of shapes (e.g., helical, icosahedral).
Envelope Characteristics:
- Enveloped viruses contain lipid-rich membranes that protect genetic material and carry proteins crucial for cell interaction.
- Enveloped viruses are sensitive to detergents; non-enveloped viruses may have variable sensitivity.

Cytoplasmic Membrane:
- Surrounds every cell; semi-permeable, essential for maintaining cellular integrity.
- Contains various proteins facilitating transport and communication between cells.
Nucleus:
- Houses the cell's genetic material (dsDNA arranged in chromosomes).
- Surrounded by a nuclear membrane with large pores for molecule passage, separating genetic activity from cytoplasmic processes.
Cytoplasm:
- Composed of numerous proteins categorized into structural proteins and enzymes:
- Structural proteins: components of muscle, membrane stiffness, and structural frameworks.
- Enzymatic proteins: catalysts for biochemical reactions, categorized based on their function (e.g., proteases, sucrases).

Ribosomes:
- Sites of protein synthesis, translating RNA sequences into polypeptide chains.
- Amino acids are assembled linearly based on mRNA instructions derived from gene sequences in the DNA.
Triplet Code:
- DNA uses combinations of three nucleotides (triplet code) to encode 20 different amino acids (e.g., TTT = Phe).

Infection Process:
- Recognition: Virus binds to a specific receptor on host cells (e.g., SARS-CoV-2 spike protein to ACE2 receptor).
- Entry: Can occur through different mechanisms (e.g., phagocytosis for naked viruses and membrane fusion for enveloped viruses).
Replication and Assembly:
- Viruses utilize the host cell’s machinery to produce mRNA, proteins, and genomic nucleic acids necessary for virus replication.
- Assembly of new virions occurs as proteins and nucleic acids come together.
Exit Strategy:
- Naked viruses often cause cell lysis, while enveloped viruses typically bud from the cell, taking a portion of the cellular membrane with them.

Understanding these processes unveils the intricate dynamics of viral life cycles and their interaction with host cells, from entry to replication and eventual release. The remarkable evolutionary adaptations that allow viruses to thrive as obligate parasites make them unique biological entities.