chapter 5 Flashcards
4 facts about viruses
- viruses infect us and use that to create more viruses
- humans do not have viruses that make proteins
- 8-10% of our DNA is viruses
- viruses are also called obligate intracellular parasites
how to classify viruses
- by the host they invade
- by what you can see
- the structure they have/take
- their genetic makeup
- DNA genome
- RNA genome
- can be both and can be single or double stranded
properties of viruses
- are obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals
- estimated 10^31 virus particles on earth, approximately 10 times the number of bacteria and archaea combined
- are ubiquitous in nature and have had major impact on development of biological life
- are ultramicroscopic in size, ranging from 20nm up to 1,000 nm (diameter)
- are not cells; structure is very compact and economical
- do not independently fulfill the characteristics of life
- basic structure consists of protein shell (capsid) surrounding nucleic acid core
- nucleic acid can either be DNA or RNA, but not both
- Nucleic acid can be double stranded DNA, single-stranded DNA, and sing or double stranded RNA
- molecules on virus surface give them high specificity for attachment to the host cell
- multiply by taking control of host cell’s genetic material and regulating the synthesis and assembly of new viruses
-lack enzymes for most metabolic processes - lack machinery for synthesizing proteins
the viral debate
the two sides of the debate:
- since viruses are unable to multiply independently from the host cell, they are not living things and should be infectious molecules
- even though viruses do not exhibit most of the life processes of cells, they can direct them, and thus are certainly more than inert and lifeless molecules
viruses are better describes as active or inactive rather than alive or dead
viral components
capsid: protein shell that surrounds the nucleic acid:
- Nucleocapsid: the capsid together with the nucleic acid
- naked viruses consist only of a nucleocapsid
Envelope: external coverings of a capsid, usually modified piece of the host’s cell membrane
spikes can be found on naked or enveloped viruses:
- project from the nucleocapsid or the envelops
Virion: a fully formed virus that is able to establish an infection in a host cell
viral capsid
Capsid:
- most prominent feature of viruses
- constructed from identical protein subunits call capsomeres
- capsomeres spontaneously self-assemble into the finished capsid:
Two different types:
- helical
- icosahedral
viral envelope
Enveloped viruses: take a bit of the cell membrane when they are released
- enveloped viruses can bud from:
- cell membranes
- nuclear envelope
- endoplasmic reticulum
more flexible than the capsid so enveloped viruses are pleomorphic
nucleic acid: at the core of a virus
Genome: the sum total of the genetic information carried by an organism
- viruses contain DNA or RNA, but not both
- the number of viral genes is quite small compared with that of a cell:
- four genes in hepatitis B virus
- hundreds of genes in some herpes viruses
- possess only the genes needed to invade host cells and redirect their activity
how viruses multiply
viruses are minute parasites that seize control of the synthetic and genetic machinery of cells
The way this cycle works dictates:
- the way the virus is transmitted
- what it does to its host
- responses of immune defenses
- human measures to control viral infection
multiplication cycles in animal viruses
general phases of the animal viral replication cycle:
- adsorption
- penetration
- uncoating
- synthesis
- assembly
- release
The length of the replication cycle varies from 8 hours in polioviruses to 36 hours in herpesviruses
adsorption
a virus can invade its host cell only through making an exact fit with a specific host molecule
Host range: the limited range of cells that a virus can infect:
- hepatitis B: live cells of humans
- poliovirus: intestinal and nerve cells of primates
- rabies: various cells of all mammals
cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus
tropisms: specificities of viruses for certain tissues
penetration and uncoating
- the flexible membrane of the host is penetrated by the whole virus or its nucleic acid
- penetration through endocytosis happens when an entire virus is engulfed by the cell and enclosed in a vacuole or vesicle
- direct fusion of the viral envelope with the host cell membrane:
envelope merges directly with the cell membrane, liberating the nucleocapsid into the cell’s interior
Lift cycle of dsDNA viruses
Early Phase:
- viral DNA enters the nucleus, where genes are transcribed into a messenger RNA
- RNA transcript moves into the cytoplasm to be translated into viral proteins (enzymes) needed to replicate the viral DNA
- the host cell’s DNA polymerase is involved in this phase
Late Phase:
- parts of the viral genome are transcribed and translated into proteins required to form the capsid and other structures
- new viral genomes and capsids are assembled
- mature viruses are release d by budding or cell disintegration
Assembly and release
Assembly: virus is put together using “parts” manufactured during the synthesis process
Release: the number of viruses released by infected cells is variable, controlled by:
- size of the virus
- health of the host cell
- pox virus - infected cell: 3,000 to 4,000 virions
- poliovirus - infected cell: 100,000 virions
- immense potential for rapid viral proliferation
