Answer
Mammalian genomes have a number of things which make them longer than prokaryotic genomes: introns in genes, transposons and retrotransposons, tandem repeats, multigene families, pseudogenes, and related items.
Work Step by Step
Review Concept 21.4 and make a list of the special items in multicellular eukarytoic genomes, including mammalian traits. All of these are the things that make eukaryotic genomes longer/larger than prokaryotic ones.