Viruses are fascinating microorganisms that blur the line between living and non-living. They're incredibly diverse, with structures ranging from simple protein shells to complex enveloped particles. Understanding their structure is key to grasping how they infect and replicate.

Viral replication cycles are a crucial part of their life cycle. Whether lytic or lysogenic, these processes showcase how viruses hijack host cells to reproduce. This topic ties into the broader chapter by highlighting the unique characteristics of viruses compared to other microbes.

Viral Structure

Components of a Virus

• Capsid consists of protein subunits called capsomeres that protect the viral genome (DNA or RNA)
• Envelope surrounds the capsid in some viruses, composed of a lipid bilayer with embedded glycoproteins that help the virus attach to and enter host cells (influenza virus)
• Viral genome contains the genetic material (DNA or RNA) that carries the information needed for viral replication

Variations in Viral Structure

• Viruses can be enveloped or non-enveloped, with enveloped viruses having an additional lipid bilayer surrounding the capsid (HIV)
• Capsid shape varies among viruses, with common shapes including icosahedral (poliovirus), helical (tobacco mosaic virus), and complex (bacteriophage T4)
• Some viruses have additional structures, such as the matrix protein layer between the envelope and capsid in influenza viruses or the tail fibers and base plate in bacteriophages (T4 phage)

Viral Replication Cycles

Lytic Cycle

• Lytic cycle is a type of viral replication that results in the destruction of the host cell
• Virus attaches to and enters the host cell, releasing its genetic material into the cytoplasm
• Viral genes are expressed, leading to the synthesis of viral proteins and replication of the viral genome
• New viruses are assembled and released from the host cell through lysis, causing cell death (bacteriophage T4 in E. coli)

Lysogenic Cycle

• Lysogenic cycle is a type of viral replication in which the viral genome integrates into the host cell's genome, becoming a provirus
• Provirus is replicated along with the host cell's DNA during cell division, allowing the virus to persist in a dormant state
• Environmental factors or cellular signals can trigger the provirus to enter the lytic cycle, leading to the production and release of new viruses (lambda phage in E. coli)

Viral Assembly and Release

• Viral assembly involves the packaging of the replicated viral genome into new capsids
• Assembly can occur in the nucleus (herpesviruses) or cytoplasm (picornaviruses) of the host cell, depending on the virus
• Enveloped viruses acquire their envelope by budding through the host cell membrane, incorporating host cell lipids and viral glycoproteins (influenza virus)
• Release of new viruses occurs through lysis (non-enveloped viruses) or budding (enveloped viruses), with budding allowing the host cell to survive

Types of Viruses

Retroviruses

• Retroviruses are RNA viruses that use the enzyme reverse transcriptase to convert their RNA genome into DNA upon entering the host cell (HIV)
• Retroviral DNA integrates into the host cell's genome, becoming a provirus that can be transcribed to produce new viral RNA and proteins
• Retroviruses are known for their ability to establish chronic infections and cause diseases such as AIDS (HIV) and some types of cancer (human T-cell leukemia virus)

Bacteriophages

• Bacteriophages, or phages, are viruses that infect bacteria (T4 phage, lambda phage)
• Phages can undergo both lytic and lysogenic cycles, depending on the specific virus and host conditions
• Lytic phages (T4) quickly replicate and lyse the bacterial cell, while temperate phages (lambda) can integrate their genome into the bacterial chromosome as a prophage
• Phages have been used in research to study gene regulation, DNA replication, and other cellular processes, as well as in phage therapy to treat bacterial infections