Chapter 19 - Viruses Flashcards
what is a virus?
an infectious particle consisting of little more than genes packed in a protein coat
what are the three classes of single stranded RNA viruses ?
1) ssRNA: serves as mRNA
- Can be translated into viral protein immediately after
infection
2) ssRNA: serves as template for mRNA synthesis
- RNA genome is transcribed into complementary RNA strands which function both as mRNA and as templates for the synthesis of additional copies of genomic RNA – use viral enzymes for RNA → RNA synthesis
3) ssRNA: serves as template for DNA synthesis
which are the RNA animal viruses with the most complicated replication cycles
retroviruses
equipped with an enzyme called reverse transcriptase which transcribes an RNA template into DNA
whats an example of a retrovirus?
HIV is the retrovirus that causes AIDS – they are enveloped viruses that contain two identical molecules of single stranded RNA and two molecules of reverse transcriptase
newly made viral DNA enters cell nucleus and integrates into DNA of a chromosome – the integrated viral DNA, called a provirus, never leaves the host’s genome
prophages leave host genome at start of a lytic cycle
what’s a prophage?
when DNA is integrated into the bacterial chromosome like this, the viral DNA is a prophage
what are the candidates for original sources of viral genomes?
plasmids and transposons
what are plasmids?
small circular eukaryotes called yeasts – can replicate independently of bacterial chromosome and are occasionally transferred between cells
what are transposons?
DNA segments that can move from one location to another within a cell’s genome
what type of viruses affect plants? people?
Viroids infect plants
Prions infect animals
what three processes contribute to emergence of viral diseases?
1) The mutation of existing viruses
2) dissemination of a viral disease from a small, isolated human population
3) spread of existing viruses from other animals
what are viroids?
circular RNA molecules that infect plants – they don’t encode proteins but can replicate in host plant cells using host cell enzymes
Cause errors in regulatory systems that control plant growth = stunted plant growth
(nucleic acids)
what are prions?
infectious proteins which cause a number of degenerative brain diseases in various animal species - a prion is a misfolded form of a protein normally present in brains cells
can be transmitted by food (Mad Cow disease)
They act very slowly with an incubation period of at least 10 years which prevents sources of infection from being identified which allows even more infection to occur
virtually indestructible….
