Question

Annotation of the human genome sequence reveals a discrepancy between the number of protein-coding genes and the number of predicted proteins actually expressed by the genome. Proteomic analysis indicates that human cells are capable of synthesizing more than 100,000 different proteins and perhaps three times this number. What is the discrepancy, and how can it be reconciled?

Short answer

Question: Explain how alternative splicing can help explain the discrepancy between the number of protein-coding genes and the number of proteins expressed by the human genome. Answer: Alternative splicing helps explain the discrepancy between the number of protein-coding genes and the number of proteins expressed by the human genome by allowing a single gene to code for multiple proteins. This process increases the variety of proteins an organism can produce, expanding its functional capabilities without needing a larger number of genes. As a result, alternative splicing bridges the gap between the number of protein-coding genes and the number of actual proteins expressed in human cells.

Step-by-step solution

Defining protein-coding genes and proteomic analysis

Protein-coding genes are sequences of DNA that, during the process of gene expression, are transcribed into mRNA and then translated into proteins. The protein-coding genes provide the blueprint for building the proteins essential for the organism's structure and function. Proteomic analysis refers to the study of the entire protein composition of an organism, tissue, or cell, and it helps in understanding the variety of proteins produced and their functions.

Transcription and translation

Transcription is the process by which an enzyme called RNA polymerase reads the DNA sequence and creates a complementary RNA molecule called messenger RNA (mRNA). This mRNA is then transported out of the cell nucleus and into the cytoplasm, where the process of translation takes place. Translation is the process where ribosomes interpret the mRNA sequence and build the corresponding amino acid chain, which eventually folds into a functional protein.

Alternative splicing

Alternative splicing is a process that occurs during transcription where different segments of the pre-mRNA (a precursor molecule to mRNA) are selectively included or excluded from the final mRNA molecule. This means that a single gene can give rise to multiple mRNA molecules, each coding for a different protein isoform. This process is essential for creating a diverse range of proteins from a limited number of genes.

Discrepancy explanation

The human genome consists of approximately 20,000-25,000 protein-coding genes, while proteomic analysis suggests that human cells can synthesize over 100,000 different proteins or even more. This discrepancy can be explained by the process of alternative splicing. Through alternative splicing, a single gene can code for multiple proteins, which greatly increases the diversity of proteins that can be produced from a limited number of protein-coding genes.

Reconciling the discrepancy

The discrepancy between the number of protein-coding genes and the number of proteins produced can be reconciled by understanding that alternative splicing allows for a single gene to code for multiple proteins. This process increases the variety of proteins an organism can produce and expand its functional capabilities without needing a larger number of genes. Thus, alternative splicing helps to bridge the gap between the number of protein-coding genes and the number of actual proteins expressed in human cells.