Viruses Flashcards
Do viruses have ribosomes?
- no
If viruses don’t have ribosomes, how do they form viral proteins?
- they use ribosomes of their host cells to translate viral messenger RNA into viral proteins
Where do viruses get energy?
- they derive their energy and all other metabolic functions from host cells
Do viruses contain DNA?
- all of them contain nucleic acid: either DNA or RNA but not both
- they also have a protein coat which encases a nucleic acid
What are some viruses enclosed by?
- an envelope of fat and protein molecules
What is a virion?
- a virus particle in its infective form outside the cell
- each virion contains at least one unique protein synthesized by specific genes
What are virioids?
- disease-causing organisms that contain only nucleic acid and have no structural proteins
- virus-like
What are prions?
- virus-like particle
- composed primarily of a protein tightly integrated with a small nucleic acid molecule
How are plant viruses transmitted?
- they can’t penetrate plant cell walls
- transmitted by insects or other organisms that feed on plants
How are bacterial viruses like T4 bacteriophage transmitted?
- they have a tail which attaches to bacterium surface by means of proteinaceous pins
- tail contracts and tail plug penetrates cell wall: inject viral nucleic acids
How are viruses classified into families and genera?
- the type and size of their nucleic acid
- size and shape of the capsid
- whether they have a lipid envelope surrounding the nucleocapsid (the capsid enclosing nucleic acid)
Two main types of virus shape
- rods or filaments:: linear array of nucleic acid and protein subunits making up capsid
- sphere: 20-sided polygon
What is the capsid?
- protein shell enclosing nucleic acid
Protein shell enclosing nucleic acid
- capsid
- nucleocapsid
What is capsid composed of?
- proteins organized in subunits known as capsomers
3 functions of capsid
- protect nucleic acid from digestion by enzymes
- contain sites on surface that allow virion to attach to host cell
- provide proteins that enable the virion to penetrate host cell membrane // to inject infectious nucleic acid into cell cytoplasm
What is envelope?
- glycoprotein envelope surrounding the nucleocapsid
What is the envelope composed of?
- 2 lipid layers interspersed with protein molecules (lipoprotein bilayer)
- material from the membrane of the host cell
How does virus obtain lipid molecule for envelope?
- from cell membrane during viral budding process
- replaces the proteins in cell membrane with its own proteins creating a hybrid structure of cell-derived lipids and virus-derived proteins
2 types of RNA based viruses
- with + strand: acts as messenger RNA for direct synthesis of viral protein
- with - strand: virion has an enzyme called RNA-dependent RNA polymerase (transcriptase)
Virus with - RNA strand
- has an enzyme called RNA-dependent RNA polymerase (transcriptase)
- have to catalyze production of complementary messenger RNA from virion genomic RNA before viral protein synthesis occurs
6 steps in viral replication cycle
- attachment
- penetration
- uncoating
- replication
- modification/assembly
- release
What is a lysogenic cycle?
- viral genome is incorporated by genetic combination into a specific place in host’s chromosome
- viral genome here is known as provirus or prophage
What is viral replication?
- formation of biological viruses during infection process in target host cells
Where do DNA viruses develop?
- on the nucleus of host cell
Where do RNA viruses develop?
- in the cytoplasm
Attachment stage in viral life cycle
- binding between viral capsid proteins (or via glycoproteins in viral envelope) and specific receptors on the host cellular surface
