Which type of genes contain introns that must be spliced out of the original RNA transcript?

Mammalian genes are characterized by the presence of introns within their coding sequences. Introns are segments of DNA that do not code for proteins and need to be removed from the initial RNA transcript during a process known as splicing. This splicing occurs in the nucleus of eukaryotic cells, where the primary transcript (pre-mRNA) is processed to yield a mature mRNA molecule that includes only the exons—those segments that encode the actual protein.

In contrast, bacterial genes typically do not contain introns and are directly transcribed into mRNA, which is then translated into proteins without the need for splicing. Viral genes vary widely in their organization but often lack introns, particularly in the case of many RNA viruses. Plant genes do contain introns, similar to mammalian genes, but since the question specifically asks about the most recognized and well-studied example of intron-containing genes, mammalian genes are the most representative.

Thus, the choice of mammalian genes stands out as the best answer due to their well-documented processing requirements that include splicing out introns after transcription.