tsikhngl13 Flashcards
Viruses
Acellular particles capable of infecting host cells and causing disease
Require a host cell in which to multiply
Features
Acellular (doesn’t have plasma membrane)
Obligate intracellular parasites disrupting host functions
Contains single nucleic acid type (DNA or RNA) and protein coat
Host Range and Viral Size
Can infect animals, plants, fungi, protozoa, and bacteria
the virus must recognize the features on the host cell surface to infect a cell
Sizes range 20-1000 nm; requires electron microscope to see
Viral Architecture
Nucleic Acid: DNA or RNA, single/double stranded, linear/circular
Capsid: Protein coat formed by capsomeres.
Envelope: Lipid bilayer membrane; contains viral proteins.
Viral Morphology
Polyhedral: 20 triangular faces.
Helical: Long rods, rigid or flexible.
Enveloped: Spherical with lipid bilayer.
Complex: Polyhedral head with helical tail (bacteriophages)
Classification and Naming
Based on nucleic acid type, capsid structure, and presence of envelope
Family = ends with suffix; viridae
Ex. Herpesviridae
Genus = ends with suffix; virus
Ex. Simplexvirus
Species = virus species given a descriptive name
Ex. Human herpesvirus 2
Multiplication of Animal Viruses
Absorption: Attachment to host cell;
viruses have attachment sites where they recognize protein or glycoprotein of host membrane
Penetration: Entry into host cell, naked virus enters the cell through endocytosis, enveloped viruses enter by fusion; lipids of envelope fuse with the host’s cytoplasmic membrane
Uncoating: Viral nucleic acid is freed from the capsid
Biosynthesis: Viral nucleic acids replicated, DNA replication occur in nucleus, RNA replication occurs in cytoplasm, viral proteins synthesized in cytoplasm, relies on the host metabolic machinery
Maturation and Assembly: Virions are assembled, capsomeres form the capsid, nucleic acid enters the capsid and forms the nucleocapsid
Release: Naked viruses burst out and ruptures host cell causing host cell to die, enveloped viruses bud out and pushes through the cytoplasmic membrane, has a steady release of mature viruses, host cell stays alive for a long time
Interactions with Hosts
Host defense crucial; most humans carry viruses and antibodies.
Acute Infections
Disease symptoms result from tissue damage caused by viral replication and host immune responses.
Release and spread of virus particles lead to lysis of host cells.
Host defense systems gradually eliminate the virus, leading to recovery.
Examples:
Influenza
Mumps
Polio
Persistent Viral Infections:
Viruses remain continuously present in the body, often without causing noticeable symptoms
Virus may or may not cause disease, and symptoms may be intermittent or absent.
Infected individuals can serve as reservoirs and transmit the virus to others.
Host immune responses may control viral replication but fail to completely clear the infection.
Example:
Human cytomegalovirus (HCMV)
Chronic Viral Infections
Following an acute phase, infectious virus persists in the body and may cause chronic symptoms
After an initial acute phase, infectious virus remains present at all times.
Virus replication continues at a low level, leading to persistent viremia.
Chronic symptoms may develop, reflecting ongoing viral activity and immune responses.
Example:
Hepatitis B virus (HBV)
Latent Viral Infections
Infection begins with an acute phase followed by a period of symptomless latency
After an acute infection, the virus enters a dormant state within host cells.
The virus may integrate its genome into the host cell chromosome, becoming a provirus.
Periodic reactivation of the virus can occur, leading to recurrent symptoms or disease
Example:
Varicella-zoster virus (VZV): Causes chickenpox (varicella) followed by latency and reactivation, leading to shingles (herpes zoster).
Herpes simplex virus (HSV): Can cause oral or genital lesions during primary infection and recurrent episodes due to viral reactivation
Viruses and Human Tumors
Proto-oncogenes and tumor suppressor genes control cell growth.
Cancer-causing viruses integrate into host genome (e.g., Hepatitis B, HPV)
(read more)
Virus-like Infectious Particles
Viroids: Naked RNA causing plant diseases
Prions: Infectious proteins linked to fatal diseases (spongiform encephalopathies), has no genetic material
Mode of Infection
Prions transmitted through food; resistant to high temperatures
Causes transmissible spongiform encephalopathies; fatal with no cure
Eaten by cows = Mad Cow Disease
Eaten by humans = variant Creutzfeldt-Jakob disease
