But over time, we all accumulate mutations that are not corrected. Some DNA changes have no effect at all. Others may have profound effects. As a result, no protein would be made. This is analogous to reading a book where someone tore out a whole chapter, except that in the case of the DNA mutation the information for all the other amino acids is still present but unrecognizable.
All cancers are due to accumulated mutations that affect genes controlling cell division.
In bacteria, genes are organized as linear strings of information. In order for eukaryote genes to be correctly read, the introns have to be removed from RNA before the messages can be exported from the nucleus and sent to ribosomes for decoding. Perhaps because it allows new proteins to evolve rapidly by shuffling different exon regions. But each cell expresses only a small fraction of genes. Most genes are turned off most or all of the time, and activated only when needed.