Most of the genes in general are discrete, non-overlapping units and do not share information with other genes. In contrast, some genes code for more than one protein or polypeptide. Such genes are called overlapping genes. Overlapping genes share some of the same sequences. These comprise of different genes whose nucleotide coding sequences overlap to some extent. The common nucleotide sequence is read in two or three different reading frames, thus specifying different polypeptides.
It has been found that the genes within the genome of an organism are generally not evenly distributed along the length of DNA. The genes of mitochondria, bacteria and viruses are often immediately adjacent to one another, and frequently overlap. In eukaryotes, however, there arc gene-rich and gene-poor regions within the genome where non-coding spacer regions span the distance between gene-rich areas.
Overlapping genes are common within the gene-rich areas. The nucleotide sequence of overlapping genes is read by RNA polymerases in two or more reading frames or from opposite strands of the DNA molecule, thus encoding different proteins within the same segment of DNA. Overlapping genes are found throughout nature, from bacteria and viruses to mammals, including humans. There are several examples of overlapping genes from the genomes of viruses, mitochondria and chromosomes sharing at least one nucleotide. Overlaps have been hypothesized to be involved in genome size minimization and as a regulatory mechanism of gene expression.
The nucleotide sequence of the DNAof ??174 phage has revealed that this phage has two overlapping genes in which the same stretch of DNA can code for two proteins that are translated in different reading frames. Once the overlapping expression is established, all mutations in one gene would also alter the other gene within the region of overlap. Some of these mutations, however, may affect one protein but not the other because of codon degeneracy.