How can organisms increase the range of protein products they produce without expanding genome size?

Organisms can increase the range of protein products they produce without expanding genome size primarily through mechanisms like alternative splicing and overlapping genes.

Alternative splicing allows a single gene to produce multiple mRNA variants by rearranging the order of exons, which can be included or excluded in different combinations. This process results in the production of various protein isoforms from the same gene, thereby expanding the functional diversity of proteins without needing to add more genes to the genome.

Overlapping genes contribute to this phenomenon as well; in some organisms, two or more genes can occupy the same region of DNA and share some sequences. This arrangement can lead to different proteins being produced from the same genetic material depending on the translational frame and start codon used. This further allows for the encoding of multiple proteins from a single stretch of DNA.

In contrast, mechanisms like gene amplification and horizontal gene transfer could lead to an increase in gene number or introduce new genes from other organisms, thereby expanding the genome size rather than preserving it. Gene deletion would actually reduce the genome size and limit the variety of proteins produced. Thus, only alternative splicing and overlapping genes provide a method to enhance protein diversity while maintaining a compact genome.