The formation of a peptide bond is an example of a condensation reaction. This is because, when two amino acids come together, a water molecule is let go. 

Because an amino acid has been altered in sickle cell anemia, we can say that the amino acid sequence for hemoglobin has been changed. The amino acid sequence is defined as the primary structure for a protein, so that is the level that has been altered. It should be noted that the subsequent levels of protein structure would be altered as well, but the manipulation of the amino acid sequence is a changing of the primary structure first.

The primary structure of a protein is defined by the sequence of amino acid residues. It is this sequence that lays the foundation for all other higher levels of structures in a protein. Secondary structure is defined by the hydrogen bonding between the carboxyl and amino backbone of the amino acids. Tertiary is defined by amino acid side chain interactions. Finally, quaternary structure is defined by the assembly of subunits of a protein into the overall larger protein structure.

Alpha-helices are formed by hydrogen bonding involving an alpha carbon-bound amine group's hydrogen and the carbonyl group attached to the amino acid four amino acids upstream.

A protein motif (aka supersecondary structure) is a defined arrangement of secondary structures within a protein. It is commonly occurring enough to have an identified structure. An example would be the beta-alpha-beta loop. While the arrangement is made up of secondary structure, the overall motif itself can be considered supersecondary or possibly even tertiary, though its components are secondary structures. Motif's do not necessarily have a defined function across different proteins. Protein domains on the other hand, do.

Hydrogen bonds stabilize interactions among the amide and carboxyl groups in the main chain of the polypeptide. These interactions may induce the formation of alpha-helices and/or beta-pleated sheets.

Proline is bound to two alkyl groups thus giving it a planar configuration, giving the nitrogen only the ability to accept hydrogen bonds not donate them. While this is not a problem at the beginning of an alpha helix this can disturb the bonds if place further down the chain. Thus proline is often referred to as the "alpha helix buster."

Alpha helices and beta sheet, the dominant secondary structural motifs in polypeptides are formed by hydrogen bonds between the carbonyl and amino groups of the amino acid backbone.

In an antiparallel beta sheet, the hydrogen bonding angle is 180 degrees and optimal; this is the most stable angle. In parallel sheets, it is a less stable 150 degrees. Whether a sheet is parallel or antiparallel does not tell us anything about what amino acids it is composed of, so each of the other answers is incorrect.

In an alpha helix, hydrogen bonding occurs every four amino acids, starting from the 1st binding to the 4th in the sequence; the 2nd amino acid binds to the 6th, the 3rd to the 7th, and so on.