Single-stranded DNA (ssDNA) Genomes – DNA Viruses

00:00 Finally, the last of the DNA viruses, these are viruses with single-stranded DNA genomes, and these viruses, curiously, can package either strand, either the minus or the plus strand of DNA. So when these viruses infect a cell, this single-stranded DNA, can be circular or linear, gets into the cell, but again remember, the only molecule that can make mRNA is a double-stranded DNA molecule, among the DNA genomes. So you can immediately predict that this single-strand has to be repaired to make it double-stranded, and in fact that's what happens, it becomes double-stranded and now mRNA can be made from this double-stranded DNA. That mRNA is shown in green with the shaded background. It gives rise to proteins, which go on to build new virus particles. At the same time of course, the viral genome has to be replicated and that double-stranded DNA is replicated to single-stranded products, either plus or minus, which both get packaged into the particles. And I want to tell you about two different virus families within this category of single-stranded DNA genomes, the one on the left of the slide called the Circoviridae. These are very interesting viruses, well they are all interesting, but this is particularly interesting. Circo viruses, first of all the genome is a circle of single-stranded DNA. They are very small, between 1.7 and 2.2 kb in length, they don't code for many proteins at all, but they infect everyone on the planet. We all have these viruses in our blood, at most times in our lives, in fact the blood supply is full of these circo viruses. But apparently they are of no consequence, at least not negative consequence. They may help us but we don't know yet. Alright, those are the circular single-stranded genomes of the circle viruses, and then we have the parvoviruses.

02:03 They have linear single-stranded DNA genomes, these cause an important human disease called fifth disease, if you happen to have a cat or a dog, you know that you need to immunize them against parvovirus infection, because infection with those viruses can kill them.