Question

Human adult hemoglobin is a tetramer containing two alpha \((\alpha)\) 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?

Short answer

Answer: The existence of α and β polypeptides with partially shared function and structure on two separate chromosomes might be explained by a gene duplication event followed by diversification and specialization. This process resulted in the α and β gene clusters, located on chromosomes 16 and 11, having similarities in their amino acid sequences and functioning together as components of human adult hemoglobin.

Step-by-step solution

The α and β gene clusters are located on two different chromosomes (chromosome 16 and 11), but they share similarities in their amino acid sequences. Both of these polypeptides are components of human adult hemoglobin and have partially shared functions and structures. Therefore, it is important to examine how this relationship between the two gene clusters could have originated. #Step 2: Explore the concept of gene duplication#

Gene duplication is a process in which a region of a chromosome containing a gene is duplicated. A possible explanation for the similar amino acid sequences and partially shared functions of the α and β polypeptides is that they originated from a gene duplication event. This means that at some point in evolutionary history, the ancestral gene underwent duplication, resulting in two identical genes. #Step 3: Consider how gene duplication leads to diversification and specialization#

After gene duplication, the two identical genes could have undergone diversification, where random mutations and genetic drift caused them to acquire differences over time. This diversification process may have led the duplicated genes to separate from each other, becoming located on different chromosomes. As each gene accumulated more mutations, they were able to specialize in their functions, gaining subtle differences in structure and function to form the α and β polypeptide chains we observe today. #Step 4: Explain the shared function and structure of the α and β polypeptides#

Although the α and β gene clusters have diversified and specialized over time since the gene duplication event, they have maintained some shared similarities, such as their identical α-globin sequences and partially shared functions. This could be because these shared features are vital for the proper functioning of hemoglobin and have been preserved through evolutionary selection pressure. In conclusion, the existence of two polypeptides with partially shared function and structure on two different chromosomes might be explained by a gene duplication event followed by diversification and specialization of the two duplicated genes. This process has resulted in the α and β gene clusters located on chromosome 16 and 11, having similarities in their amino acid sequences and functioning together as components of human adult hemoglobin.

Key concepts

Chromosome Diversification

Understanding chromosome diversification is pivotal when explaining how genes for similar proteins, such as the alpha and beta globin chains of hemoglobin, can be found on different chromosomes. Chromosome diversification refers to the processes through which chromosomes change and evolve over time, leading to the distribution and rearrangement of genes across different chromosomes.

One key evolutive mechanism that contributes to chromosome diversification is gene duplication. When a gene duplicates, it can be subsequently inserted into a different location within the genome, sometimes ending up on an entirely different chromosome. Over time, the duplicated genes accumulate mutations independently, which means that while they may still have similar functions, the sequences can diverge, a concept commonly identified as neo- or sub-functionalization.

This process allows for increased genetic variability. Different versions of a gene can adapt to specific functions or be expressed differently based on cellular or developmental contexts. It is a cornerstone of evolutionary biology and helps explain the complexity and diversity of life on Earth.

Amino Acid Sequence Similarity

The concept of amino acid sequence similarity is critical in understanding the relationship between different proteins and tracing their evolutionary history. Proteins are made up of chains of amino acids, and the order of these amino acids determines the protein's structure and function. When two proteins share a high degree of similarity in their amino acid sequence, it suggests they may have a common ancestry.

In the case of the alpha and beta hemoglobin chains, their shared amino acid sequences hint at a common ancestral gene. The high degree of similarity (61 identical amino acids) indicates that they once were the same or very similar genes that diverged after a gene duplication event. Amino acid sequence similarity can be used as an evolutionary clue. It signifies which parts of the proteins are crucial for their function, as these segments tend to be more conserved throughout evolution due to evolutionary selection pressure.

Similarity in amino acid sequences is also a tool for predicting protein function. If two proteins have comparable sequences, they might interact with similar molecules or take part in related biological pathways. This is particularly true for sequences that have resisted change, which usually are integral to the protein's role.

Evolutionary Selection Pressure

The concept of evolutionary selection pressure involves the external factors that favor certain traits over others, influencing which organisms are more likely to survive and reproduce. This is how nature 'selects' for beneficial genes, which then become more common in a population over time.

When it comes to proteins like the alpha and beta chains of hemoglobin, parts of their amino acid chains have been conserved because they are critical for the protein's function. For example, the structure of the hemoglobin protein is vital for its ability to carry oxygen effectively. Evolutionary selection pressure has maintained the integrity of these important sequences even while other parts of the gene may have diverged dramatically, allowing for optimization of function and efficiency. If a mutation alters these critical areas of the protein, the change is often detrimental, and the organism carrying it is less likely to survive and pass on its genes.

In context to the alpha and beta globin chains, these preserved sequences indicate their shared structural characteristics are essential for hemoglobin's function in oxygen transport. The persistence of these similarities despite being on different chromosomes exemplifies the strength of evolutionary selection pressures in maintaining critical features necessary for survival and highlighting the importance of conserved sequences in the study of molecular evolution.