Question

Human adult hemoglobin is a tetramer containing two alpha (a) and two beta ( $\beta$ ) polypeptide chains. The a 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?

Short answer

Answer: Alpha and beta-globin polypeptides can have partially shared function and structure due to gene duplication followed by chromosomal translocations and divergent evolution. These processes result in the evolution of the polypeptides from a common ancestor, while still retaining some shared characteristics.

Step-by-step solution

Understanding the Polypeptides

First, let's review the information given in the exercise. Human adult hemoglobin is a tetramer made up of two alpha (a) and two beta ( $\beta$ ) polypeptide chains. The alpha-globin polypeptide has 141 amino acids and is located on chromosome 16, while the beta-globin polypeptide has 146 amino acids and is located on chromosome 11. Despite being on separate chromosomes, 61 amino acids in their sequences are identical, suggesting some shared function and structure.

Recognizing Gene Duplication

One possible explanation for the existence of two similar polypeptides on different chromosomes is gene duplication, which is a common mechanism for the evolution of new genes. During this process, a gene is accidentally copied, and as a result, the organism has two copies of the same gene. Over time, the copies can accumulate mutations and potentially evolve new functions while retaining some similarities to the original gene.

Understanding Chromosomal Translocation

Another possible explanation for the similar polypeptides on separate chromosomes is chromosomal translocation. This occurs when a segment of one chromosome breaks off and attaches to another chromosome, possibly carrying the gene for one of the polypeptides with it. This would result in the two similar genes ending up on different chromosomes, while still retaining some shared function and structure.

Divergent Evolution

After two similar genes have been separated on different chromosomes by processes such as gene duplication and chromosomal translocation, they can undergo divergent evolution. This means that the genes accumulate different mutations over time and slowly start to differ from each other functionally and structurally. However, since they share a common ancestry, they may still retain a partially shared function and structure.

Conclusion

In conclusion, the existence of two similar polypeptides with partially shared function and structure on different chromosomes can be explained by gene duplication and chromosomal translocations, followed by divergent evolution. As a result, the alpha and beta-globin polypeptides have evolved from a common ancestor, accumulating distinct mutations over time, while still retaining some shared characteristics.

Key concepts

Gene Duplication

Gene duplication is a vital process in genetic diversity and evolution. It occurs when a gene is copied within the genome, resulting in two or more identical or nearly identical copies. Imagine you have a recipe for a delicious pie, and you accidentally make a second copy of the recipe. Now you have the freedom to tweak one recipe without losing the original, which is essentially what nature does with genes.

Over time, the duplicated genes can evolve independently from one another. This might mean that one copy retains the original function while the other copy accumulates mutations that could confer a new function or improve an existing one. Imagine one of your pie recipes stays the same, ensuring you can always bake your tried-and-true pie, while the other recipe is modified to create a whole new flavor.

In the context of hemoglobin, gene duplication could help explain why we have alpha and beta polypeptides. If an ancestral gene duplicated, one copy could have led to the alpha chain while the other evolved into the beta chain, each one modifying slightly over eons but still retaining core similarities essential for hemoglobin's function.

Chromosomal Translocation

Chromosomal translocation is another genetic rearrangement with significant evolutionary consequences. It occurs when a segment of one chromosome breaks off and reattaches to a different chromosome. This can be compared to cutting a chapter from one book and pasting it into another. If that chapter carries genetic information, such as a gene, that gene is now in a new location, with potential new regulatory elements affecting its expression.

Translocation can lead to genetic diversity but also to diseases, depending on where and how the genetic material is reattached. In the story of hemoglobin evolution, a translocation event could have moved one of the globin genes from one chromosome to another. This accidental cut-and-paste might be why the alpha and beta chains, while related and sharing part of their sequences, are found on different chromosomes.

Divergent Evolution

Divergent evolution refers to the process by which two or more related entities (like species, genes, or protein structures) grow increasingly different over time. Think of it as two paths branching off from the same starting point but heading in different directions through a forest. Although they start together, each path encounters different obstacles and landscapes, leading them to diverge from one another.

In genetic terms, after gene duplication or translocation, the once identical copies of a gene begin to diverge due to mutations and different selective pressures. In the case of the hemoglobin alpha and beta chains, divergent evolution has allowed these sequences to drift apart in some areas, but since they continue to perform similar functions in the body—carrying oxygen—they have retained enough similarities to work together effectively within the hemoglobin protein.

Polypeptide Chains

Polypeptide chains are the building blocks of proteins. A polypeptide is a chain of amino acids linked together by peptide bonds. These chains fold into specific three-dimensional shapes to form proteins, which carry out a vast array of functions in living organisms.

For hemoglobin, the tetramer structure with two alpha and two beta polypeptide chains is essential for its role in transporting oxygen. Each polypeptide chain in hemoglobin has a similar but slightly different role, which reflects the subtle variations in their amino acid compositions and structures. Despite evolving on different chromosomes, through gene duplication and divergent evolution, the alpha and beta chains have maintained the ability to come together, forming the crucial oxygen-carrying hemoglobin protein.