Find basics. There’s way more to it

Find out how viruses are classified based on their being double-stranded, single-stranded, positive-sense, negative-sense, or by having something called reverse transcriptase.

Viral Classification

When choosing a movie you’re going to watch online, you’re going to have a plethora of choices. You can, of course, just type a name into the search box to find something specific. However, if you don’t really know what you’re going to watch, you can choose a movie based on the category it is in. There are horror movies, dramas, comedies and so on. In the same sense, viruses can be classified, or categorized, based on certain features they may have.

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Some are dramatic, some are horrible and all are out to get you – sort of.

Double- and Single-Stranded

In a very basic sense, viruses are classified based on their physical and chemical characteristics. Unfortunately, that’s just the basics. There’s way more to it than that, and we’ll only cover the important points of what is known as the ‘Baltimore classification system,’ which classifies viruses based on their type of genome and method of replication. For instance, viruses may have a nucleic acid genome that is either DNA or RNA, either of which can be double-stranded. Double-stranded viruses are those that have a genome that consists of two chains of complementary nucleotides that are joined together.I know that was a mouthful, so let’s break this down into a simple example.

DNA or RNA is known as a nucleic acid, or the genome of a virus. Each strand of a nucleic acid is made up of little subunits called nucleotides. If you’ve ever seen Tibetan prayer flags, you’ll know what I mean. Each individual flag and the section of string it is attached to is known as the nucleotide.

The nucleotide is also called a ‘monomer,’ or more technically a ‘mononucleotide,’ where mono means ‘one.’Since the prefix ‘poly’ means ‘many,’ a prayer flag string that has many flags hanging off of it is called a ‘polymer’ or a ‘polynucleotide.’ When two polynucleotide chains meet, all of their prayer flags, the chain of nucleotides, attach to one another and form a double-stranded nucleic acid.

Some of the viral nucleic acids, however, are single-stranded, or those consisting of one polynucleotide chain.

Positive and Negative Sense

If you think the definition of single-stranded viruses was too easy to be true, you’re right. We’re about to reach the scariest part of our horror movie.

When an RNA virus is single-stranded, it must have what’s known as a ‘sense,’ something many B-rated Hollywood movies lack. Positive-sense RNA is a strand of viral RNA that can immediately serve as a template for protein synthesis during the process of translation.Negative-sense RNA is a strand of viral RNA that must first be converted into complementary positive sense strands before creating proteins. Each positive sense strand is the same sense as mRNA, which is used for protein synthesis during translation, so it shouldn’t be all that dramatic when you learned that a positive-sense strand can be used directly for protein synthesis.In addition, genomes that are both positive and negative sense are called ‘ambisense,’ from the prefix ‘ambi,’ which means ‘both.’ The proteins created by the viruses of any sense are then used for their replication, reproduction and infectivity.

Keep in mind that while DNA viruses can also have a ‘sense’ like their RNA brethren, this term is mainly used for single-stranded RNA viruses because DNA viruses have slightly different replication schemes. I hope all of that made sense and that I didn’t leave you stranded with my explanations.Furthermore, when we talk about a single- or double-stranded viral nucleic acid, we often abbreviate it as ‘ds’ for double-stranded and ‘ss’ for single-stranded. And when we talk about positive and negative sense, we abbreviate positive with the plus sign and negative with the negative sign.

These abbreviations help us ‘dumb down’ the process of writing out important stuff in our script, like the writers of cheap romantic comedies seem to often do.Finally, each of our viruses usually has an important enzyme that makes strands of polynucleotides called a polymerase in order to help the virus produce vital proteins and help replicate its genome for the production of virus babies. The term ‘polymerase’ should make sense since we defined ‘polymer’ as a long strand of many nucleotides, or polynucleotides, and the suffix ‘-ase’ is used to denote an enzyme.

Hence, a polymerase is an enzyme that makes polymers, or polynucleotides in our case, which are the strands of DNA or RNA in our viruses.

The Baltimore Classification System

With all of that in mind, we can begin grouping viruses by their type of specific genome using something known as the Baltimore classification system. This system is the one that puts viruses into groups, like movies are grouped into their different genres.

  • Group I includes dsDNA viruses
  • Group II has ssDNA viruses
  • Group III contains dsRNA viruses
  • Group IV involves +ssRNA viruses, while
  • Group V covers -ssRNA viruses
  • Group VI comprises ssRNA-RT viruses, while
  • Group VII encompasses dsDNA-RT viruses

There are important things to keep in mind. For our purposes, DNA viruses must replicate within the nucleus, while RNA viruses replicate within the cytoplasm of the cell. In addition, the first two groups, the DNA viruses, use a DNA polymerase to help replicate its genome.

The next three groups use an RNA polymerase to help replicate its genome.

Reverse Transcriptase

The final two groups of viruses have something called reverse transcriptase, or ‘RT’ for short. Reverse transcriptase is a polymerase that uses an intermediate nucleic acid, different from that of the original viral nucleic acid, to replicate the viral genome.Basically, the reverse transcriptase helps the virus to replicate through an RNA intermediate in the case of a dsDNA-RT virus or a DNA intermediate in the case of a ssRNA-RT virus. The most famous of these reverse transcriptase viruses is HIV, and this enzyme, the reverse transcriptase, is a frequent target of drugs used to try and stop the HIV.

Lesson Summary

This is by all means just the bare bones basics of the replication strategies and steps viruses use, but I probably fried your brain enough even with that ‘paltry’ amount of information. So let’s review the important points of this lesson instead.Viral nucleic acid genomes can be double-stranded. Double-stranded viruses are those that have a genome that consists of two chains of complementary nucleotides that are joined together. Some of the viral nucleic acids, however, are single-stranded, or those consisting of one polynucleotide chain.

When an RNA virus is single-stranded, it must have what’s known as a ‘sense.’ Positive-sense RNA is a strand of viral RNA that can immediately serve as a template for protein synthesis, whereas negative-sense RNA is a strand of viral RNA that must first be converted into a complementary positive-sense strand before creating proteins.Furthermore, each of our viruses usually has an important enzyme that makes strands of polynucleotides, called a polymerase, in order to help it produce vital proteins and replicate its genome for virus babies. Finally, some viruses have something called reverse transcriptase.

Reverse transcriptase is a polymerase that uses an intermediate nucleic acid different from that of the original viral nucleic acid to replicate the viral genome.

Learning Outcomes

When this lesson is finished, you should be able to:

  • Describe the difference between double-stranded and single-stranded genomes
  • Understand what ‘sense’ means to single-stranded genomes
  • Define the difference in polymerase and reverse transcriptase
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