A gene is commonly defined as the fundamental unit of inheritance, carrying the instructions for building specific molecules, such as proteins or functional RNA. While genes are unquestionably located on DNA, precisely defining a gene based on DNA sequence can be complex due to variations in gene structures.

 1. Gene Variability:

  • Genes can encode various types of functional molecules, including protein-coding genes, transfer RNA (tRNA), and ribosomal RNA (rRNA) genes.
  • Therefore, defining a gene solely by its DNA sequence is challenging because different types of genes can have distinct structural characteristics.

 2. Monocistronic and Polycistronic Genes:

  • Genes within transcription units can be categorized as monocistronic or polycistronic, depending on whether they code for one or multiple polypeptides (proteins).
  • In eukaryotes, structural genes in transcription units are often monocistronic, meaning they code for a single polypeptide. This arrangement is common for genes with interrupted coding sequences (split genes).
  • In contrast, bacteria (prokaryotes) often have polycistronic genes within transcription units, where multiple genes code for different polypeptides.



3. Exons and Introns:

  • Eukaryotic genes, particularly monocistronic ones, commonly exhibit a split-gene arrangement with coding sequences known as exons.
  • Exons are DNA sequences that appear in mature or processed RNA and correspond to parts of the gene that code for functional products, such as protein segments.
  • Introns, or intervening sequences, are also present in eukaryotic genes. However, they do not appear in mature or processed RNA.
  •  Introns can pose challenges when defining genes based solely on DNA segments because they are non-coding regions.



4. Promoters and Regulatory Sequences:

  • The inheritance of traits or characteristics can be influenced by not only the structural gene but also by other DNA regions, such as promoters and regulatory sequences.
  •  Promoters play a vital role in initiating transcription by providing binding sites for RNA polymerase.
  •   Regulatory sequences, while not encoding RNA or proteins, can impact the expression of nearby genes. They are often loosely referred to as regulatory genes.