Class 1 viruses are double-stranded DNA viruses that replicate their genomes and transcribe their RNA. So they replicate the genomes and transcribe their RNA in the same way or very similar to that that we described for prokaryotic cells. They use slightly different strategies to make sure that the viral messenger RNA takes preference to the host messenger RNA production, but they use many of the host-derived enzymes. They are the most numerous that we have identified, found in bacteria, archaea, and eukarya. Most of them are bacteriophages, which are extensively used in molecular biology as model systems, and that will include this one right here which is T7 the T7 bacteriophage which will be our example in the following slide so each of these Baltimore classifications will look at one example that's in the textbook and just explore that one example although there are others in the book they also include herpes virus that we see here and also the pox viruses so mpox smallpox etc which are very large viruses and packaged with replication enzymes, allowing them to replicate in the cytoplasm.
Pretty much all the other eukaryotic double-stranded DNA viruses have to move to the nucleus to replicate, but not the pox viruses. They are among the largest known and they carry those enzymes with them to allow that replication in the cytoplasm to occur. Bacteriophage T7 infects E. coli and some other related enteric or gut bacteria.
It has an icosahedral head as we've described before and a helical tail and its double-stranded DNA is injected into the host cell. As that DNA is injected in remember the protein coat, the capsid is left outside attached to the cell wall of the bacteria. This DNA from this virus enters into these cells always in the same orientation.
And what I mean by that, it's always this end entering and not this end here. So this end right here is referred to as the left end. And so that comes in first into the cell and immediately that these particular genes start entering that cell.
So right here, we don't wait for these to come into the cell, this DNA that encodes these genes to come into the cell. But these genes right here are transcribed by the host's RNA polymerase. So we transcribe these, translate them, and you can see a couple of really interesting molecules in here. The first one, if transcribed, is going to inhibit the host's restriction system, so turning off the immune system of that bacterial cell.
Then the viral DNA will have a transcription of a gene that encodes tRNA polymerase, so now it's making its own RNA polymerase, which is used to transcribe all of these. And then we have another enzyme here that inactivates the host's RNA polymerase. So what that means is now that the host RNA polymerase is unable to transcribe anything else.
And the whole system is now turned over to transcribing and then obviously translating these particular proteins. Other things of interest in here. that you'll see things like lysozyme these are all proteins remember these are going to be our late proteins middle to late proteins and then these are definitely our late proteins down here that are causing release of this bacteriophage so it comes in a particular orientation