Viral Particle Replication Flashcards
Viruses are
non-cellular infectious particles that straddle the boundary between ‘living’ and ‘non-living’
They are much smaller than prokaryotic cells
They are typically between 20 and 300 nm in diameter
Viruses are relatively simple in structure
Unlike prokaryotic cells, they do not have a
plasma membrane, cytoplasm or ribosomes
Structurally they have
A nucleic acid core (their genomes are either DNA or RNA, and can be single or double-stranded)
A protein coat known as a ‘capsid’
Some viruses have an outer layer known as an ‘envelope’ formed usually from the membrane-phospholipids of a cell they were made in
Being non-living, viruses do not undergo
cell division
All viruses are parasitic, meaning they can only reproduce by
infecting living cells (known as host cells)
Viruses replicate by injecting their
nucleic acid into a host cell:
First step First, a virus uses attachment proteins on its surface to bind to
complementary receptor proteins on the surface of a host cell
second step The virus then injects its
Nucleic acid into the host cell
third step The host cell then uses its nucleic acid and protein-building machinery (ribosomes) to
produce new viral particles are made forming new capsids
Eventually, the new viral particles are released. This may occur when:
The host cell bursts open(lyses, releasing all the new viral particles at once
Viral particles leave individually through the host cell membrane via a process known as ‘budding’, often taking a section of the membrane with them (this is the ‘envelope’ that surrounds some viruses)
The exiting of viruses damages their host cells, causing disease
For example, HIV infects cells of the immune system (such as helper T cells and macrophages)
When the newly replicated HIV particles exit, these host cells are destroyed
Over time, the immune system becomes severely compromised and this eventually leads to Acquired Immunodeficiency Syndrome (AIDS)
Different viruses have different attachment proteins and therefore can only bind to specific host cell types that have the correct (complementary) receptor proteins. For example, some viruses use prokaryotic host cells, whilst others use eukaryotic animal or plant host cells. In addition, some viruses can only infect one specific cell type, whereas others can infect many different cell types.
