Obligatory intracellular parasite
Require living host cells to multiply
Describe the basic characteristics of viruses
Obligatory intracellular parasite, Contain DNA or RNA, contain protein coat, No ribosomes, no ATP generating mechanism
What is meant by host range?
Host range is the spectrum of host cells a virus can infect, most viruses infect only specific types of cells in one host (determined by specific host attachment sites and cellular factors)
What is a bacteriophage?
Virus that infects bacteria, range from 20nm to 1000 nm in length
Define and describe the components of a virion
A virion is a complete, fully developed viral particle, nucleic acid (DNA or RNA), can be single or double stranded; linear or circular, Capsid (protein coat made of capsomeres/subunits), Envelope (lipid, protein, and carbohydrate coating on some viruses), spikes (projections from outer surface for attachment or gaining entry into a host)
Helical viruses
Hollow, cyclindrical capsid
Polyhedral viruses
Many-sided, geometric in shape
Enveloped viruses
Has outer coating lipid, protein, or carbohydrate
Complex viruses
Complicated structures (bacteriphages)
Nonenveloped viruses
naked viruses
Suffix -virus
Genus names end in -virus
Suffix -viridae
Family name ends in -viridae
Suffix -ales
Order name ends in -ales
Viral species
A group of viruses sharing the same genetic information and ecological niche (host)
- Descriptive common names are used for species
- Subspecies are designated by a number
What do viruses need to be grown in labs?
Viruses must be grown in living cells
How are bacteriophages grown in a lab?
Bacteriophages are grown in bacteria, bacteriophages form plaques, which are clearings on a lawn of bacteria on the surface of agar-each plaque corresponds to a single virus; can be expressed as plaque-forming units (PFU)
Growing animal viruses in the lab
- In living animals
- In embryonated eggs (virus injected into the egg/ viral growth is signaled by changes or death of the embryo)
- In cell cultures (Tissues derived from living organisms)- Tissues are treated with enzymes to separate cells, virally infected cells detected via their deterioration, known as cytopathic effect (CPE), continuous cell lines are used-Henrietta Lacks
Describe the one-step growth curve for viral multiplication
- Inoculum
- Eclipse period- virus invaded host cell
- Burst- virions released from host cell *acute infection
- Viral titer-measure of how many viruses in filtrate
Describe the lytic cycle for phages
- Attachment – Phage attaches by the tail fibers to the host cell
- Penetration – the phage injects its DNA into the host cell by penetrating through the cell membrane
- Biosynthesis – the phage DNA replicates inside the cell, synthesizing new phage DNA and proteins
- Maturation – the replicated material assembles into fully formed viral phages (each made up of a head, a tail and tail fibers)
- Lysis – the newly formed phages are released from the infected cell
Describe the lysogenic cycle for phages
In contrast to the lytic cycle, phages infecting bacteria via the lysogenic cycle do not immediately kill their host cell. Instead, they combine their genome with the host genome, allowing the bacteria to replicate the phage DNA along with the bacterial genome. The incorporated copy of the phage genome is called the prophage.
Generalized transduction
Random bacterial genes incorporated into capsid; can be transferred to another bacteria when infected; lytic or lysogenic phages
Specialized trasnduction
Specific bacterial genes transferred to another bacterium via a phage; When prophage excises, extra host DNA is taken; changes genetic properties of the bacteria
Multiplication of animal viruses
- Attachment- viruses attach to cell membrane
- Entry- by receptor mediated endocytosis of fusion
- Uncoating- by viral or host enzymes
- Biosynthesis- production of nucleic acids and proteins
- Maturation- nucleic acid and capsid proteins assemble
- Release- by budding (enveloped viruses) or rupture
The biosynthesis of DNA viruses
DNA viruses replicate their DNA in the nucleus of the host using viral enzymes; Synthesize capsid in the cytoplasm using host cell enzymes.
