Viruses and Prokaryotes: Structure, Replication, and Diversity

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Viruses

General Characteristics of Viruses

Viruses are unique biological entities that exist at the boundary between living and non-living matter. They are much smaller than cells and require a host to reproduce.

Definition: A virus is an infectious particle consisting of genetic material (DNA or RNA) enclosed in a protein coat called a capsid.
Viruses cannot reproduce or carry out metabolism outside of a host cell.
They are considered obligate intracellular parasites.
Viruses infect a wide range of organisms, including animals, plants, fungi, and bacteria.
Host range: Each virus can infect only a limited range of host species or cell types.
Example: The influenza virus infects human respiratory tract cells, while bacteriophage T4 infects Escherichia coli bacteria.

Viral Genomes

The genetic material of viruses is highly variable and forms the basis for their classification.

Viral genomes may consist of either double- or single-stranded DNA or double- or single-stranded RNA.
Viruses are classified as DNA viruses or RNA viruses based on their genetic material.
The viral genome is either a single linear or circular molecule of nucleic acid.
Viral genomes can be very small, containing between three and 2,000 genes.
Example: Bacteriophage T4 has a double-stranded DNA genome; HIV is an RNA virus.

Capsids and Envelopes

The structure of a virus is determined by its protein coat and, in some cases, an outer envelope.

The capsid is the protein shell that encloses the viral genome.
Capsids can have a variety of shapes, including helical, icosahedral, or more complex forms.
Some viruses have accessory structures that help them infect hosts.
Viral envelopes (derived from host cell membranes) surround the capsids of many animal viruses, such as influenza.
Viral envelopes contain a combination of viral and host cell molecules.
Example: The influenza virus has a membranous envelope with glycoprotein spikes.

Bacteriophages

Structure and Function

Bacteriophages, or phages, are viruses that specifically infect bacteria.

Bacteriophages have an elongated capsid head that encloses their DNA.
A protein tail piece attaches the phage to the host and injects the phage DNA inside.
Phages are important tools in molecular biology and genetics.
Example: Bacteriophage T4 infects E. coli and has a complex structure with a head, tail sheath, and tail fibers.

Viral Replication in Host Cells

Viruses can only replicate within host cells, using the host's machinery to produce new viral particles.

Viruses are obligate intracellular parasites; they cannot reproduce independently.
Each virus has a specific host range—the spectrum of host cells it can infect.
After entering a host cell, the viral genome directs the synthesis of viral proteins and the assembly of new viruses.
Host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules are used for viral replication.
Newly synthesized viral components self-assemble into new virus particles.

Key Steps in the Viral Replicative Cycle

Entry and uncoating of the viral genome into the host cell.
Replication of viral nucleic acid and synthesis of viral proteins.
Self-assembly of new virus particles.
Exit of new viruses from the host cell, often destroying the cell.

*Additional info: The lytic and lysogenic cycles, as well as bacterial defenses against phages, are important subtopics but are not fully detailed in the provided images. See expanded notes for these topics in the full content.*