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.

Key concepts

Understanding Protein-Coding Genes

Protein-coding genes are segments of DNA that hold the instructions for making proteins. These genes go through a process called gene expression, which involves two key steps: transcription and translation.
During transcription, the DNA sequence of a protein-coding gene is copied into a messenger RNA (mRNA) molecule. This step is crucial as it begins the process of turning genetic information into tangible protein structures.
Translation follows this, where ribosomes read the mRNA sequence and construct a chain of amino acids that folds into a functioning protein. It's fascinating to think of these genes as blueprints. Each one contains distinct plans for building proteins that our bodies need for various functions.
Without protein-coding genes, cells wouldn't have the instructions necessary to create the proteins that support everything from cellular structure to complex biochemical interactions.

The Role of Proteomic Analysis

Proteomic analysis is like taking a detailed inventory of all the proteins in a cell, tissue, or organism. This field of study allows scientists to explore the variety and abundance of proteins expressed in a given biological context.
Through proteomic analysis, researchers can identify and quantify thousands of different proteins, providing insights into how proteins contribute to the physiological functions and processes in the body. It also helps in identifying how proteins interact with each other within cells.
This type of analysis is particularly important in understanding discrepancies between the number of protein-coding genes and the actual proteins found in cells. It highlights the dynamic and complex world of proteins that can't be fully explained merely by counting genes.

• Reveals protein abundance and variety
• Uncovers protein functions and interactions
• Aids in understanding complex biological processes

Proteomic studies are indispensable for gaining a fuller picture of the proteome—the entire set of proteins produced by an organism—and for clarifying how gene expression plays out at the protein level.

Deciphering Gene Expression

Gene expression is the process by which the genetic code in our DNA is used to produce proteins. This process is not as straightforward as one might think, especially due to a mechanism known as alternative splicing.
Alternative splicing allows a single gene to produce multiple protein variants. This occurs during mRNA processing, where different combinations of exons are joined together, leading to the production of diverse proteins from a single gene. It is a form of gene expression regulation that adds versatility to the genetic code.

• Enables one gene to produce different proteins
• Increases the number of possible proteins beyond the number of genes
• Contributes to organismal complexity without increasing genome size

By using alternative splicing, organisms like humans can produce far more proteins than there are genes, thereby resolving the discrepancy between the number of protein-coding genes and the wide array of proteins expressed. This flexibility is key to the adaptation and functionality in complex organisms.