Question

Human adult hemoglobin is a tetramer containing two alpha (a) and two beta ( $\beta$ ) polypeptide chains. The $\alpha$ gene cluster on chromosome 16 and the $\beta$ gene cluster on chromosome 11 share amino acid similarities such that 61 of the amino acids of the $\alpha$ -globin polypeptide ( 141 amino acids long) are shared in identical sequence with the $\beta$ -globin polypeptide $(146$ amino acids long. How might one explain the existence of two polypeptides with partially shared function and structure on two different chromosomes? Include in your answer a link to Ohno's hypothesis regarding the origin of new genes during evolution.

Short answer

Question: Explain the existence of two polypeptides, alpha-globin and beta-globin, with partially shared function and structure encoded on two different chromosomes. Answer: The existence of alpha-globin and beta-globin polypeptides with partially shared function and structure on two different chromosomes can be explained as a result of a gene duplication event followed by functional divergence during evolution, as suggested by Ohno's hypothesis. A common ancestral globin gene might have been duplicated and separated onto different chromosomes, leading to the formation of the alpha-globin on chromosome 16 and beta-globin on chromosome 11. Over time, these duplicated genes diverged in their function and structure, while still retaining some shared similarities, allowing for the development of a more complex and efficient hemoglobin molecule in humans.

Step-by-step solution

Understand the alpha and beta globin polypeptide similarities

The alpha-globin polypeptide is 141 amino acids long and is encoded on chromosome 16. The beta-globin polypeptide is 146 amino acids long and is encoded on chromosome 11. They share a similar amino acid sequence, with 61 of the amino acids being identical between the two polypeptides.

Understand the Ohno's hypothesis

Ohno's hypothesis suggests that new genes with similar or partially shared functions can originate through gene duplication events during evolution. This process involves the accidental duplication of a gene and subsequent divergence of its function, leading to the formation of a new gene.

Explain the possible origin of two polypeptides on different chromosomes

The similarities between the alpha-globin and beta-globin polypeptides can be explained by the occurrence of a gene duplication event in their evolutionary past. A common ancestral globin gene might have been duplicated, and these duplicated genes got separated onto different chromosomes, chromosome 16 for the alpha-globin and chromosome 11 for the beta-globin.

Discuss the divergence of the alpha and beta globin polypeptides

Over time, due to random mutations and selection pressures, the duplicated globin genes diverged in their function and structure, while still retaining some shared similarities. The divergence between these genes has led to distinct, yet partially shared, roles of the alpha-globin and beta-globin polypeptides in forming the adult hemoglobin tetramer.

Connect the explanation with Ohno's hypothesis

In accordance with Ohno's hypothesis, the existence of alpha and beta-globin polypeptides with partially shared function and structure on two different chromosomes can be explained as a result of gene duplication and functional divergence during evolution. This process has allowed for the development of a more complex and efficient hemoglobin molecule in humans.

Key concepts

Hemoglobin

Hemoglobin is a critical protein in human physiology, vital for transporting oxygen throughout our bodies. The structure of hemoglobin is a tetramer, meaning it consists of four polypeptide chains. In adults, these are typically two alpha ($\alpha$) and two beta ($\beta$) chains. The gene for the $\alpha$-globin is located on chromosome 16, and the $\beta$-globin gene is on chromosome 11. Despite being on different chromosomes, these chains share significant amino acid sequences: 61 out of the 141 amino acids in $\alpha$-globin are common with $\beta$-globin's 146 amino acids.

These similarities suggest a shared evolutionary history, supported by a concept known as gene duplication. In the context of hemoglobin, gene duplication may have allowed a common ancestral gene to replicate and then evolve separately on different chromosomes. This process enabled hemoglobin's functional adaptations, crucial for its role in oxygen transport. The separate evolution of $\alpha$ and $\beta$ polypeptides provided the complex and highly efficient hemoglobin molecule we see in humans today, able to fine-tune oxygen delivery in various physiological conditions.

Ohno's Hypothesis

Ohno's hypothesis posits that new genes are mostly generated through the duplication of existing ones. This evolutionary process begins with an accidental duplication of a gene, which then provides material for evolutionary innovation as the duplicate may mutate without compromising the original gene's function.

In many cases, this duplicated gene can evolve new functions or even partially shared ones. This is observed in the hemoglobin gene clusters. The $\alpha$ and $\beta$ globin genes are believed to have originated from a single ancestral gene. Following duplication, these genes diverged but retained some similar features due to their shared origin.

According to Ohno's theory, the split between $\alpha$-globin and $\beta$-globin genes is a prime example of gene duplication driving evolution, reinforcing the diversity and adaptability of organisms over time. This concept shows how complex proteins, such as hemoglobin, might retain certain functional similarities while showing functional divergence, supporting sophisticated biological functions.

Evolutionary Biology

Evolutionary biology examines the processes that produced the diversity of life, emphasizing mechanisms like mutation, selection, and genetic drift. Gene duplication is a powerful mechanism in this context, fostering genetic diversity and eventually leading to new functions. This is a critical factor in understanding the complexity of life's evolutionary tapestry.

Specifically, the gene duplication event that formed $\alpha$ and $\beta$-globin chains helped diversify hemoglobin function, enhancing the physiological capabilities necessary for more advanced systems, like the human respiratory system.

During evolution, mutations followed by natural selection could mold duplicated genes for specialized functions, each helping organisms adapt to new environments or biological needs. The hemoglobin protein is a testament to this, illustrating how functional and structural diversification within a single protein family can result from evolutionary forces. This adaptability is essential for meeting varying oxygen demands across different tissues and environmental conditions, a concept fundamental to evolutionary biology.