A2.3 - Viruses

• Viruses: Non-cellular parasite of animals, plants, and bacteria that consists of nucleic acid surrounded by a protein coat (a capsid).
• Parasite: An organism that lives on or in another organism (its host), for most of its life cycle, deriving nutrients from the host.
• Viruses contain nucleic acids, but require living cells to reproduce. This makes viruses obligate parasites, as they can only reproduce within a host cell, since they do not have ribosomes and metabolic enzymes necessary for protein synthesis.
• DNA Viruses Include: Single stranded and double stranded
• RNA Viruses Include: Single stranded, double stranded, human immunodeficiency
• Some infections are caused by microorganisms, such as bacteria. Other infections are cause by viruses.
• Microorganism: A living organism that cannot be seen by the naked eye, such as bacteria, protists, and single celled fungi.
• Capsid: The protein coat of a virus
• All viruses have the following features in common

  • They are extremely small and have a fixed size range
  • They have nucleic acid (DNA or RNA) as genetic material
  • A capsid made of protein surrounds the nucleic acids
  • No cytoplasm
  • Few or even no enzymes
• Viruses are highly diverse in their shape and structure.
• The genetic code is shared between viruses and living organisms, meaning that the host cells transcribe their DNA into mRNA and translate it to protein. RNA viruses include HIV and coronavirus.
• Positive sense RNA: Viral RNA that has the same base sequence as mRNA, which allows it to instantly start protein synthesis during virus replication
• Negative sense RNA: Viral RNA that does not have the same base sequence as mRNA, so it must to replicated to mRNA in order to start protein production
• Viruses have a small genome, which makes them highly adapted for virus replication, in which each infected host cell produces many copies of the viral genome from a single DNA or RNA template.
• The smaller the genome the faster they can eplicate.
• Envelope: A membrane typically from the host cell plasma membrane with viral glycoproteins
• Nucleocapsids: The capsid of a virus with an envelope
• Virion: An isolated but infectious virus particles found outside the host cell
• The stages of viral reproduction are generally:

  • Attachment to host cells
  • Entry (penetration)
  • Replication of viral genome
  • Transcription of virus mRNA and protein synthesis using host cell ribosomes
  • Maturation and assembly of viruses and release from host cell
• Bacteriophages are viruses that infect bacteria.
• Lytic: A phage life cycle where a phage attaches to a host bacteria, injects its DNA which undergoes replication to form new virions, which then lyse the cell
• Lysis: Breakdown typically of cells
• Virulent: The ability of a viruse or bacteria to cause rapid and severe disease
• Antigenic Drift: Small changes caused by mutations in viral genes that can lead to chnages in the surface of a virus
• Antigenic Shift: A major, sudden change in a virus's genes, often when two different viruses mix their genetic material.
• Retrovirus: A virus that has RNA as its nucleic acid and uses the enzyme reverse transcriptase to copy its genome into the DNA of the host cell's chromosomes
• Reverse Transcriptase: An enzyme found in retroviruses that converts RNA into viral DNA
• Provirus: A virus that has integrated into a host's genome and is replicated when the cell replicates its DNA