Question

Describe three ways that the number of proteins exceeds the number of protein- encoding genes in the human genome.

Short answer

The number of proteins exceeds the number of protein-encoding genes in the human genome through the processes of alternate splicing, post-translational modifications, and a single gene coding for multiple polypeptides.

Step-by-step solution

Way 1: Alternate Splicing

During transcription process of a gene, the pre-mRNA can be alternatively spliced, which means certain exons of a gene may be included within or excluded from the final processed mRNA, producing different variations of the protein (isoforms) encoded by that gene.

Way 2: Post-translational Modifications

After translation, proteins undergo several post-translational modifications. These modifications add to the diversity of the protein structure and function. They include phosphorylation, acetylation, methylation, and ubiquitination, altering the behavior and function of the final protein product.

Way 3: Single Gene Coding for Multiple Polypeptides

A single gene can code for multiple distinct polypeptides. This is possible through a process called alternative open reading frames (ORFs). An ORF is a sequence of DNA or RNA that has the potential to be translated into a protein. A gene can have more than one ORF, hence translating to different polypeptides, increasing the number of resultant proteins.

Key concepts

Alternative Splicing

One of the marvels of genetic expression is the process known as alternative splicing, which allows a single gene to produce multiple protein isoforms. Essentially, during the transcription of DNA into mRNA, segments of the pre-mRNA, called introns, are removed, and the remaining segments, known as exons, are pieced together. However, the inclusion or exclusion of certain exons can vary, resulting in different mRNA transcripts from the same DNA sequence.

Imagine a single gene as a set of instructions for constructing a model airplane. Alternative splicing is like choosing different pieces or configurations from this set to create variations of the airplane model. Each variation still stems from the same core instructions, yet the end products serve different purposes or functions. In biological terms, these 'variations' translate to different proteins that may fulfill diverse roles within the organism, enhancing its ability to adapt and function efficiently.

Post-Translational Modifications

Proteins are not finished products as soon as they are translated from mRNA. Post-translational modifications (PTMs) are crucial alterations that occur after a protein has been produced during translation. These modifications can be thought of as the finishing touches or customizations that alter a protein's properties and regulate its activity, stability, location, and interactions with other molecules.

PTMs can involve the addition or removal of various functional groups or proteins. Phosphorylation, the addition of a phosphate group, can turn enzymes on or off, changing the protein's function. Acetylation can alter the interaction a protein has with DNA, influencing gene expression, while ubiquitination signals for a protein's degradation. These modifications essentially multiply the functional capabilities of a protein, enabling the vast array of complex and nuanced biological processes required for life.

Alternative Open Reading Frames

Beyond alternative splicing, the concept of alternative open reading frames (ORFs) further broadens the diversity of proteins. An open reading frame is a segment of nucleotides that contains a sequence of bases that could potentially be translated into a protein. Intriguingly, a single stretch of DNA can contain multiple ORFs, meaning it can give rise to various polypeptides depending on where translation begins and ends.

These alternative ORFs might be likened to reading a sentence by starting at different words; the overall set of letters is the same, but the meaning changes with different starting points. Cells use this ingenious mechanism to expand the repertoire of proteins they can produce from a single gene, thus enabling a limited number of genes to support a vast array of cellular functions necessary for the organism's survival and adaptation to its environment.