Which sequences in DNA are described as non-coding?

Introns are sequences within a gene that are transcribed into RNA but are not translated into protein. These non-coding regions are essential for several biological processes, including alternative splicing, where different combinations of exons (the coding sequences) are joined together to produce various proteins from a single gene.

While exons do code for proteins and are therefore referred to as coding sequences, introns serve as non-coding sequences. Nucleosomes are structures that package DNA into a compact form and do not refer to specific DNA sequences that are coding or non-coding. Gene promoters are regions located upstream of the coding sequence that play a critical role in initiating transcription, but they are not themselves non-coding regions in terms of the sequences that directly contribute to RNA coding. Thus, the distinction of introns as non-coding is key in understanding gene structure and expression.