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From Wikipedia:

For example, introns are extremely common within the nuclear genome of higher vertebrates (e.g. humans and mice), where protein-coding genes almost always contain multiple introns, while introns are rare within the nuclear genes of some eukaryotic microorganisms,[8] for example baker's/brewer's yeast (Saccharomyces cerevisiae). In contrast, the mitochondrial genomes of vertebrates are entirely devoid of introns, while those of eukaryotic microorganisms may contain many introns. Introns are well known in bacterial and archaeal genes, but occur more rarely than in most eukaryotic genomes.

In my biochemistry course we learned that bacteria have no introns and eukaryots nearly always have them. What's correct?

Do viruses have introns as the Wikipedia article claims?

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As a general rule, a blanket statement in Biology should always be considered false.

It so happens that bacteria do have 'introns' (or at least the intron genomic sequence has been determined) in the sense of the cis-acting autocatalytic Group I and II introns. These are sequences of RNA that adopt such a conformation to catalyze their own excision from an mRNA transcript, and thus may be considered ribozymes, enzyme made of ribonucleic acid.

Although not generally considered 'introns' segments of RNA are removed during the course of tRNA in both prokaryotes and eukaryotes.

Now introns that are removed by the action of the spliceosome are strictly found in eukaryotes, as bacteria lack the spliceosome.

There seem to be a considerable number of papers regarding the question of intron (particularly spliceosomal) phylogeny that should be relatively easy to find if you are so interested.

One example paper in which viral introns (Group I) are reported: http://nar.oxfordjournals.org/content/22/13/2532.abstract

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    $\begingroup$ As a general rule, a blanket statement in Biology should always be considered false. I could not agree more. $\endgroup$ – Amory Sep 28 '13 at 23:59

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