Textbook Question
Consider the sequence 3'-ACGCTACGTC-5'.
What is the total number of covalent bonds joining the nucleotides in each strand?
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7. DNA and Chromosome Structure
DNA Structure
2:06 minutesProblem 12
Textbook Question
Human adult hemoglobin is a tetramer containing two alpha (α) and two beta (β) polypeptide chains. The α gene cluster on chromosome 16 and the β gene cluster on chromosome 11 share amino acid similarities such that 61 of the amino acids of the α-globin polypeptide (141 amino acids long) are shared in identical sequence with the β-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?
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Understand the concept of gene duplication: Gene duplication is a process where a segment of DNA containing a gene is copied, resulting in two identical genes within the genome. Over time, these duplicated genes can accumulate mutations, leading to divergence in their sequences and functions. This is a key mechanism in the evolution of gene families.
Recognize the evolutionary relationship between the α-globin and β-globin genes: The α-globin and β-globin genes are part of the globin gene family, which likely arose from an ancestral globin gene that underwent duplication. This explains why the two genes are located on different chromosomes (chromosome 16 for α and chromosome 11 for β) and why their polypeptides share structural and functional similarities.
Consider the role of natural selection: After duplication, mutations in the duplicated genes can lead to slight differences in their protein products. If these differences provide an advantage (e.g., improved oxygen transport under different physiological conditions), natural selection will favor the retention of both genes, leading to the evolution of specialized functions.
Examine the shared amino acid sequences: The fact that 61 amino acids are identical between the α-globin and β-globin polypeptides suggests that these regions are critical for the protein's function (e.g., binding oxygen or maintaining the tetrameric structure). These conserved regions are less likely to accumulate mutations due to selective pressure to maintain their function.
Conclude with the concept of functional divergence: While the α-globin and β-globin genes share a common origin and some structural similarities, they have diverged to perform complementary roles in the hemoglobin tetramer. This functional divergence allows hemoglobin to efficiently transport oxygen in the blood, demonstrating the evolutionary advantage of gene duplication and specialization.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Gene Duplication
Gene duplication is a process where a segment of DNA is copied, resulting in two identical or similar genes. This can lead to the evolution of new functions as one gene may retain its original role while the other accumulates mutations that can result in a novel function. In the case of hemoglobin, the α and β globin genes likely arose from a common ancestral gene through duplication and subsequent divergence.
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Chromosomal Location and Evolution
The location of genes on different chromosomes can influence their evolutionary paths. Genes that are located on separate chromosomes can undergo independent mutations and selection pressures, leading to functional divergence. The α and β globin genes, located on chromosomes 16 and 11 respectively, illustrate how chromosomal location can contribute to the development of proteins with partially overlapping functions.
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Functional Redundancy
Functional redundancy occurs when two or more genes or proteins perform similar functions, allowing for compensation if one is mutated or lost. In the case of hemoglobin, the α and β chains provide a cooperative function in oxygen transport, which may explain why both are retained despite their partial sequence similarity. This redundancy can be advantageous for organisms, ensuring vital functions are maintained even under genetic variation.
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