IB Biology Sub-topic A2.3 Notes

Viruses

Viruses are inert and acellular pathogens that infect other organisms. Whilst there are many types of viruses with variable shapes and appearances, they are similar in structure and share the following features:

1. Small fixed size
2. Protein capsids
3. No cytoplasm
4. Genetic material as either DNA or RNA
5. Few or no enzymes

Further to this, some viruses are enveloped in host cell membrane. This is advantageous because it can:

1. Facilitate viral entry into the host cell
2. Evade immune system recognition
3. Protect the genetic material

You are not expected to know examples of enveloped and unenveloped viruses, but examples of viruses you need to be aware of include:

1. Bacteriophage λ
2. Coronavirus
3. HIV

Viral life cycles

Since viruses are non-living and do not have a metabolism, they are incapable of growing or reproducing independently. They thus rely on cells for their energy supply, nutrition, protein synthesis and other life functions. They in which they acquire this can occur via one of two cycles: the lytic cycle or the lysogenic cycle. Both will be explored in the context of bacteriophage λ.

In the lytic cycle:

1. The virus attaches to the host cell and injects its viral genome.
2. This degrades the host genome, leaving only the viral genome.
3. The host's cellular machinery replicates the viral genome and synthesizes viral proteins.
4. These are assembled into new viruses and released from the cell.
5. The release process kills the cell, and the new viruses restart the cycle.

In the lysogenic cycle:

1. The virus attaches to the host cell and injects its DNA.
2. The DNA integrates with the host cell's DNA and becomes part of its genome.
3. The host cell replicates into two daughter cells, replicating the virus's DNA.
4. As part of their gene expression, these cells now produce viruses.