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I am currently studying the chapter called macromolecule. I get that a nucleic acid consists of repeating units of nucleotides, and I personally think "the repeating units" does not mean the nucleotdes are all the same, as nitrogen bases have four types either in DNA or RNA. How about the protein molecule. A protein molecule is made up of amino acids. However, I am sure about whether a protein molecule are made up of same and repeating units of amino acids. Theoretically, the R group in each protein is differed from one another. Therefore, does that mean we cannot state that a protein molecule is made up of repeating units of amino acids?

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They are repeating units in the sense that all of the units are amino acids. The R group is different in different amino acids (it is not just different theoretically). The different R groups contribute to the different shape, structure, function of proteins, and if all were the same, then we would have a very limited range of proteins.You will still say it is composed of repeating units, since all units are amino acids, and there are no foreign groups between them.

Similarly, all repeating units of DNA/RNA are nucleotides, even if they aren't the same nucleotides.

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Proteins are composed of repeated units of aminoacids with different R groups.

CO-CH(R1)-NH - CO-CH(R2)-NH - CO-CH(R3)-NH -

See how the aminoacid backbone is repeated again and again even if the R groups attached to that backbone is different.

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Proteins are heteropolymers of amino acids. Many Amino acids are joined together by peptide bond. During peptide bond formation C terminal of former amino acid & N terminal of successive amino acid join , that's why proteins always start with the N terminal and end with C terminal.This is a section of a hypothetical protein. 4 Amino acids are shown in the section. This order repeats itself many times. The image shown above is only a primary structure. Functional and structural proteins can never exist in this form.In reality a protein coils around itself or attains a fixed shape due to intermolecular and intermolecular interactions ; like the Beta pleated sheath in fibroin(silk protein) is due to hydrogen bonding.

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