In sickle-cell disease, as a result of a single amino acid change, the mutant hemoglobin tetramers associate with each other and assemble into large fibers. Based on this information alone, we can conclude that sickle-cell hemoglobin exhibits:

a. altered primary structure.
b. altered secondary structure.
c. altered tertiary structure.
d. altered quaternary structure.

a. altered primary structure.

d. altered quaternary structure.

Explanation:

The primary structure of a protein represents the amino acid sequence of its polypeptide chains. Change in the single amino acid in the beta chain of hemoglobin represents the changed primary structure of this polypeptide chain. This altered primary structure also changes the quaternary structure of the protein.

The quaternary structure represents an arrangement of two or more polypeptide subunits of a protein. Substitution of a negatively charged amino acid (glutamic acid) for valine (uncharged and nonpolar amino acid) creates a hydrophobic sticky point at the surface of beta chains of hemoglobin and facilitates their aggregation.

1. amino acid refers to the fundamental building block of proteins, comprising of an amino group and a carboxyl group.

2. catalyst refers to any substance, which increases the rate of chemical reaction without being used up in the reaction.

3. a complex molecule containing he genetic code is dna or deoxyribonucleic acid.

4. an enzyme refers to a molecule of protein, which is unique in accelerating up a particular chemical reaction in the biological systems.

5. nucleic acid refers to an sequence of sugars, phosphates, and nitrogenous bases, for example, dna and rna.

6. a protein refers to a large, composite organic molecule comprising of a huge number of amino acids associated together with the of peptide bonds.

7. rna refers to a nucleic acid that takes part in the production of proteins.

8. substrate refers to a compound, which takes part in a chemical reaction, and is chemically modified by the reaction.