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Hi everyone,
I’m working on an assignment for my virology class about the Baltimore classification of viruses, and I’m having some trouble understanding a few concepts.
The assignment asks me to explain the seven classes of viruses based on their genetic material and replication strategies, but I’m not sure how to clearly explain each class and provide relevant examples. Specifically, I’m a bit confused about the difference between (+)-sense and (-)-sense ssRNA viruses and how their replication strategies differ.
Does anyone have a simple way to differentiate between these classes and some examples of viruses for each? Also, how do I explain how their genetic material affects the way they replicate? Any help with the critical thinking questions about the importance of classification would be appreciated too!
Thanks in advance!
The main difference between (+)-sense and (-)-sense ssRNA viruses lies in how they use their genetic material for replication 🧬. (+)-sense ssRNA viruses, like Poliovirus 🦠, can directly serve as mRNA for protein synthesis in the host cell. In contrast, (-)-sense ssRNA viruses, such as the Rabies virus 🧟♂️, need to convert their RNA into a (+)-sense RNA before the cell can use it to make proteins. Think of (+)-sense RNA as a “ready-to-use” message, while (-)-sense RNA requires a “translation” step first 🔄.
The Baltimore classification groups viruses based on their genetic material and replication strategies, which is useful for understanding how viruses work. For example, dsDNA viruses, like Adenovirus 🦠, use the host’s machinery to replicate their DNA directly. But with RNA viruses, like HIV 🧬, the process is more complex—they reverse transcribe their RNA into DNA first, which is why we call them RNA reverse-transcribing viruses. This helps scientists target their weaknesses for better treatments 💊.
In the Baltimore system, viruses are classified by their genetic material and replication method. ssDNA viruses, like Parvovirus 🦠, only have one strand of DNA, and they need to form a double strand before they can replicate. On the other hand, dsDNA viruses, such as Herpes Simplex Virus 😷, already have both strands of DNA, so they can replicate more easily. This classification not only helps scientists understand how these viruses replicate but also why different viruses require different treatments. 🧬💉
I started by creating a color-coded chart that listed all seven classes, organizing them by type of genetic material (like dsDNA, ssDNA, dsRNA, etc.) and noting where in the cell they replicate (nucleus or cytoplasm). What helped most with the (+)-sense vs. (-)-sense ssRNA viruses was thinking of the (+)-sense RNA as already being like mRNA—so the host ribosomes can immediately translate it into proteins. In contrast, the (-)-sense RNA is like a mirror image; it first needs to be converted to (+)-sense RNA by an RNA-dependent RNA polymerase before it can be translated. That clicked for me when I thought of (+) as ready-to-use and (-) as needing a translator.
For examples, I used Class IV for (+)-sense ssRNA viruses like Poliovirus, and Class V for (-)-sense ssRNA viruses like Influenza. And to connect this to replication strategies, I’d explain how the virus’s genetic material dictates whether it brings its own enzymes or relies on the host—RNA viruses usually carry or code for their own polymerases because host cells don’t naturally replicate RNA from RNA.
When it came to the critical thinking questions, I found it helpful to frame the Baltimore classification as a roadmap for understanding how a virus hijacks the host. It’s not just about the genome—it’s about how that genome shapes the virus’s behavior, its vulnerabilities, and even how we might target it with therapies.
Hope this helps clarify things!
