WEEK 11: 11.5 Viral Life Cycle and Replication Flashcards
(20 cards)
What are the 3 stages of the life cycle of a virus
Viral entry
Translation and Genome Replication
Assembly
Describe stage 1: viral entry
viruses encounter the cell randomly
they attach to cell surface receptor (can have multiple)
enter the cell passively through assistance (as they are too large to diffuse actively)
virus transfers their own genome into the host cell
Describe stage 2: translation and genome replication
viral RNA or DNA is transcribed into mRNA, this is translated into viral proteins
What are the transcription sites for common viruses?
Nucleus: DNA viruses
cytoplasm: RNA viruses
nucleus & cytoplasm: HIV, HBV
Describe stage 3: assembly
non-enveloped viruses are spontaneously assembled into a capsule
enveloped viruses follow a different pathway; viral proteins embed themselves within the plasma membrane to create a lipid bilayer envelope
Why must all viral genomes make mRNA that can be read by host ribosomes
because viruses do not encode translational machinery
What is the Baltimore Classification?
classifying viruses based on the polarity of their genetic material in comparison to the ribosome ready mRNA
Define positive sense RNA
RNA is the same polarity as its mRNA
Define negative sense RNA
RNA is an opposing polarity to its mRNA
What are the 2 rules for RNA virus replication?
- RNA dependent RNA polymerase is the key enzyme that replicates viral RNA to make progeny genomes
there is a relationship between the enzyme and the sense of genome
2, the RNA genome must be copied from end to end with no loss of sequence; RNA is synthesised by template directed stepwise incorporation of nucleotides
Describe the steps in replication of + strand viruses
- RNA infect host cell, express viral genome, RdRp produced and accumulates
- once RdRp levels become sufficient, replication complex activates; RdRp creates -RNA
- -RNA acts as a template for the synthesis of +RNA, this mRNA is utilised by the host’s translational machinery
Describe the steps in replication of - strand viruses
- -RNA infects the host cell, it carries its own RNA polymerase enzyme
- RNA polymerase produces mRNA that codes for viral proteins
- -RNA is transcribed to +RNA, transcribed back into -RNA - viral replication
in essence, the positive strand genome is used to create a - strand full length complement which is used as a template to create a new positive strand genome
What occurs as a result of - strand viruses having no cellular enzyme present to copy RNA into mRNA
they must carry their own enzyme to initiate protein synthesis
Why might there be delays in infection in DNA virus replication?
viral DNA replication always requires the synthesis of at least 1 viral protein
where does replication of DNA virus initiate at
specific sites on the DNA template (origins of replication)
what is DNA virus replication catalysed by?
a DNA dependent DNA polymerase (DdDp) which is initiated by a primer
How can smaller and larger genomes then affect DNA viral replication?
smaller genomes require more host proteins; have a restricted coding capacity
whereas
larger genomes encode most/all of the proteins required for DNA synthesis & DNA viral genomes vary in size and config
what are the steps of DNA virus replication?
- attachment and entry via mediated endocytosis
- Host RNA polymerase transcribed immediate, early and late mRNA
- specific translated proteins replicate the viral DNA genome
- capsids are assembled in the nucleus and release virions
define protein capsid
a protective protein shell that encloses the DNA genome of a virus
define virion
a complete infectious viral particle consisting of DNA/RNA, enclosed in a protein capsid
