A2.3 viruses (HL) unfinished Flashcards
characteristics of viruses
- small (very simple)
- fixed size (cannot grow)
- nucleic acid (single/double stranded RNA or single/double stranded DNA)
- capsid made of proteins (unique based on the presence of specific amino acids and structures produced when bonding)*
- no cytoplasm
- few or no enzymes
*this unique composition determines the ability of a virus to infect a particular host cell, specialized sites on the capsid allow attachment to specific host cells. Some viruses contains specialized proteins that allow the genetic material of the virus to penetrate the host. Some viruses are also enveloped (from host cell’s plasma membrane) this is involved in host cell recognition and attachment. Glycoproteins and other antigens on the surface of the envelope belong to the host organism and therefore
it is difficult to be recognized by the immune system.
viruses are not living
they cannot carry out the functions of life without a host cell
diversity of structures in viruses
viruses are highly diverse in their shape and structure. they can have single or double stranded RNA or DNA and can be enveloped or not enveloped in the host cell’s membrane.
bacteriophage lambda structure
- a capsid head that protects the
1 double-stranded DNA core - tail tube where genetic material is injected through
- tail sheath made of proteins that contract to drive the tail tube through the host cell’s outer membrane
- tail fibres that attach the virus to the host cell
- 1 double stranded DNA that is injected through the tail into the host cell
non-enveloped
host: e-coli bacterium
corona virus structure
- spherical shape
- 1 single-stranded RNA
- spike proteins on the envelope
enveloped
host: epithelium cells in airways and lungs of humans
HIV structure
- 2 single-strands of RNA protected by capsid
- its own reverse transcriptase encoded in the RNA
- phospholipid envelope with spikes made of protein and carbs
enveloped
host: T-helper cells in immune system (WBC)
the lytic cycle
- attachment: phage attaches to host cell
- penetration: phage penetrates the host cell and injects its DNA
- replication: DNA is replicated to make copies
- biosynthesis: synthesis of viral proteins using mRNA transcribed from viral DNA
- assembly: assembly of new viruses with DNA inside a protein coat
- lysis: cell bursts to release the new virons
the lysogenic cycle
- phage attaches to host cell and injects its DNA
- phage DNA circulates and enters the lysogenic cycle
- phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage
- lysogenic bacterium reproduces normally (virus is inactive)
- a spontaneous event occurs that triggers the prophage to separate from the bacterial chromosome, initiating a lytic cycle
the origin of viruses
3 hypotheses:
- virus first hypothesis:
- viruses originated before cells (based on the simplicity of viruses compared to cells)
- ancestors of modern viruses could have provided the materials necessary for the development of the first cells - regressive hypothesis:
- viruses were once very small, simple cells
- gradually became parasites and lost the cell-like morphological features through gene reduction (an evoluationary event where genes
no longer needed by an organism are gradually lost) - escape hypothesis:
- genetic material RNA and DNA escaped from larger cells and became surrounded by an outer boundary
divergent vs convergent evolution
divergent: an ancestral form evolves into distinct forms which need to adapt to different conditions
convergent: the INDEPENDENT
evolutionary development of features in order to to adapt to a specific “environment”
evidence supporting convergence in viruses:
1. genetic material DNA or RNA enclosed in a capsid
2. A protein “coat” the capsid
3. ALL OF THEM ARE OBLIGATE PARASITES
antigenic drift (HIV)
antigenic shift (flu)
