Lesson 3.6 Viruses

3.6.2 Virus size and general structure

STOP AND THINK:

Since viruses are able to infect bacteria, animal and plant cells, would you predict that they would be as big as these cells, larger than these cells, or smaller than these cells? Explain why.

STOP AND THINK:

In terms of size, from smallest to largest, list the order of prokaryotic cells, viruses, and eukaryotic cells.

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 Figure 3.6.2

Figure 3.6.2 : The electron microscope can achieve a resolution of up to 100 picometers, allowing eukaryotic cells, prokaryotic cells, viruses, ribosomes, and even single atoms to be visualized (note the logarithmic scale).

As seen in Figure 3.6.2, viruses are much smaller than bacteria. In fact, looking at the chart, you can infer that some viruses are a little larger than ribosomes!! You know that bacteria are smaller than plant and animal cells, so a listing of these from smallest to largest would look like: viruses, prokaryotic cells, eukaryotic cells.

Typically, viruses are composed on one type of nucleic acid, either DNA or RNA, enclosed by a protein coat. The nucleic acids may be single-stranded or double-stranded dependent on the type of virus.

The protein coat surrounding the nucleic acid is referred to as a capsid and its function is to protect the nucleic acid. Occasionally, the capsid will also contain an enzyme in addition to the nucleic acid. An example of this can be seen in retroviruses. In the capsid of these viruses, both RNA and RNA reverse transcriptase, an enzyme essential to the replication of the virus, are found.

The size and shape of capsids vary dependent upon the type of virus, as illustrated in Figure 3.6.3. 

Figure 3.6.3

Figure 3.6.3 shows two of the more common shapes of a viral capsid. Diagram (a) illustrates the capsid of the tobacco mosaic virus, while Diagram (b) illustrates the capsid of a typical adenovirus.

Some viruses will also have an envelope that surrounds the capsid. Composed primarily of phospholipids and proteins like those found in a host cell, the viral envelope is thought to allow a virus to attach to and be drawn into host cells. In this way, a virus can initially “hide out” from the host’s immune system. 

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 Figure 3.6.4

Figure 3.6.4 illustrates the location of a viral envelope.