Viruses Overview

Viruses Overview

VIRUSES OVERVIEW

• Small biological agents that hijack hosts to reproduce

Structure

1. Capsid (nucleocapsid) – protects the genome
2. Nucleic Acid – stores the virus' genetic information

• DNA – single-stranded or double-stranded
• RNA – single-stranded (plus-strand or minus-strand) or double-stranded

3. Envelope – some viruses are surrounded by lipid membrane structure derived from host cell membrane

Life cycles

1. Lytic virus – burst out of host cell killing it
2. Lysogenic virus – integrate into host genome and are replicated when it is replicated

T4 BACTERIOPHAGE

Structure

1. Viral head – where genome is located (dsDNA)
2. Collar
3. Sheath
4. Base plate
5. Tail fibers – help stabilize and attach the virus to bacterial cell membrane

Life cycle – Lytic

1. Bacteriophage attaches to bacterial cell
2. Viral DNA is injected into the cytoplasm
3. Viral mRNA transcription occurs (host DNA, RNA and protein synthesis is stopped)
4. Viral protein synthesis and virus assembly
5. Around 20 minutes after infection, the cell is lysed and hundreds of new viruses are released

Full-Length Text

Here we will learn about viruses.

First, start a table so we can list some key concepts about viruses.
Denote that viruses are small biological agents, which hijack hosts so as to reproduce.
Also denote that viruses are useful for studying cell biology because when they hijack or shut off host cellular proteins, we can learn what those proteins do.

Denote that the structure of virions (virus particles) comprise the following:
The capsid (sometimes called the nucleocapsid), which is made of protein and protects the viral genome.
Nucleic acid, which stores the virus' genetic information.
Some viruses are surrounded by an envelope, a lipid membrane structure surrounding the capsid derived from the host's cellular membrane.
Because viruses are often classified by the type of nucleic acid they use for their genome, let's list the possibilities:
Viral genomes can be either DNA or RNA.
DNA genomes can be either double stranded or single stranded.
RNA genomes can also be either double stranded or single stranded.
Of the single-stranded RNA viruses, they can be either described as plus-strand viruses or minus-strand viruses. Plus-strand viral genomes are messenger RNA which can be immediately translated into protein. Minus-strand viral genomes are anti-messenger sense, they need to be transcribed into mRNA before viral proteins can be synthesized.

Now, as an example, let's explore the T4 bacteriophage. Bacteriophages infect bacteria.
First, we'll draw the bacteriophage, step-by-step.
Draw the viral head, which is where the viral genome is found.
Within the head, draw the viral nucleic acid. In the case of the T4 bacteriophage, it is double-stranded DNA.
Now draw the collar, the sheath, and the base plate. These structural components have a channel in them that the DNA travels through during infection.
Finally, draw the tail fibers. These help stabilize and attach the virus to bacterial cell membrane.

The T4 bacteriophage has a lytic life cycle. Before we explore it, we need to understand the different viral life cycles.
Make a table for viral life cycles.
Write that there are two main types of viral life cycles, lytic and lysogenic.
Write that lytic viruses burst out of the host cell to spread and this kills the host.
Write that lysogenic viruses integrate into the host genome which is replicated when the host replicates. Some lysogenic viruses can become lytic if certain conditions are met.

Now back to the T4 bacteriophage life cycle.
Draw the cell wall and cell membrane of a bacterial cell.
Label the cytoplasm.
When the bacteriophage attaches to the bacterial cell, the structural proteins undergo a conformational change that ejects the viral DNA into the cell.
Draw the viral DNA being ejected into the cytoplasm.
Within two minutes of viral injection, viral mRNA transcription begins as host DNA, RNA, and protein synthesis is stopped.
Draw the viral mRNA strands. This mRNA codes for various viral proteins necessary for the construction of new virions and replication of viral DNA.
Draw new virions with viral DNA in the cytoplasm as viral protein synthesis and assembly occurs.
Draw another section of bacterial cell wall and membrane and indicate that about 20 minutes after infection, the host cell is lysed and hundreds of new virions are released to infect nearby cells.

UNIT CITATIONS:

1. McKee, T. & McKee, J.R. Biochemistry: The Molecular Basis of Life, 5th ed. (Oxford University Press, 2012).

2. Strauss, J. H. & Strauss, E. G., Viruses and Human Disease. (Academic Press, 2002).

3. Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).

4. Trun, N. & Trempy, J. Fundamental Bacterial Genetics. (John Wiley & Sons, 2009).

5. Chaudhari, K. Microbial Genetics. (The Energy and Resources Institute (TERI), 2014).

6. Volberding, P. Global HIV/AIDS Medicine. (Elsevier Health Sciences, 2008).

7. Dudek, R. W. High-yield Cell and Molecular Biology, Volume 845. (Lippincott Williams & Wilkins, 2006).

8. Ropka, M. & Williams, A. HIV Nursing and Symptom Management. (Jones & Bartlett Learning, 1998).