Exam 3- Viruses Flashcards
What is a virus?
a non-cellular particle that must infect a host cell to reproduce
What are the general characteristics of viruses?
- most viruses are very small
- contain nucleic acid (DNA or RNA)
- nucleic acid is packaged in a protein coat
- some are enclosed in an envelope
- some have spikes
- each species infects a particular group of host species called the host range
What is the size of a typical virus?
50-100 nm
What is a capsid?
made of repeating protein subunits called capsomeres, packages & protects the viral genome
What is an envelope?
a lipid bilayer that surrounds the capsid & is usually derived from the host cell membrane
What are the spikes on a virus?
glycoproteins in the envelope
What are spikes involved in?
attachment to a host cell & escape from a host cell
Explain tissue tropism.
tissue &/or cell type that virus can infect
All viral replication cycles must achieve the following:
- host recognition & attachment
- genome entry
- biosynthesis
- assembly of virions
- exit & transmission
What do all viruses require for reproduction?
a host cell
Explain bacteriophage.
viruses that infect bacteria, most mesophyilic & mildly thermophilic
Lytic cycle of a bacteriophage
- attachment- phage attaches to host cell
- penetration- phage inserts DNA into host cell
- biosynthesis- production of phage DNA & phage proteins by the infected cell
- maturation- self-assembly of phage particles; virus genome is packaged into capsid
- release- phage degrades infected host cell wall; host cell lyses (bursts); new phage particles are released
Describe a virulent phage.
reproduce only through the lytic cycle, resulting in lysis of the host bacterial cell
Describe a temperate phage.
can reproduce through the lytic cycle; can also undergo lysogeny, where the phage genome inserts into the host genome
Lysogenic cycle of a bacteriophage.
- attachment- phage attaches to host cell
- penetration- phage inserts DNA into host cell
- integration- phage DNA integrates into bacterial chromosome by recombination; the integrated phage DNA is called a prophage
- excision- prophage excises from chromosome (phage induction); excision can be stimulated by UV or chemicals
- lytic cycle- excised phage enters lytic cycle
General life cycle of animal viruses
- attachment- virus attaches to host cell membrane
- penetration- virus enters host cell by endocytosis or fusion
- uncoating- capsid removed by viral or host enzymes
- biosynthesis- infected host cell produces viral nucleic acid & viral proteins –> may require viral enzymes
- maturation- viral nucleic acid & viral capsid proteins assemble into virus particle
- release- virus escapes host cell by budding (enveloped viruses), exocytosis, or rupture
What is the primary factor to determine the life cycle of an animal virus?
the form of its genome
What do DNA viruses use during the life cycle?
the host replication machinery
What do RNS viruses use during the life cycle?
an RNA-dependent RNA-polymerase to copy their RNA
What do retroviruses use during the life cycle?
a reverse transcriptase to copy the RNA genome into DNA & an integrase to insert the DNA into the host chromosome
Describe Human Papillomavirus.
-HPV- small icosahedral virus; no envelope
- 8 kb genome
- most infections go away without treatment
-infects epithelial cells that cause genital warts
-also integrate into genome of host basal cells that can cause cancer
- direct contact
-prevention is Gardasil HPV vaccine
-80% adults in US are infected
What DNA viruses can cause cancer?
-Epstein-Barr virus
-HPV
-Hepatitis B virus
-Human herpes virus-8
What RNA viruses can cause cancer?
-human T-cell leukemia virus
-hepatitis C virus
Describe Influenza A virus.
-(-) stranded RNA virus, segmented genome, enveloped with spikes
-upper respiratory infection- fever, sore throat, headache
-high risk- very old, very young, immunocompromised
-transmission through droplets
-seasonal flu vaccine
-Tamiflu (Oseltamivir) & Zanamivir
