Can someone explain to me the difference between motif and domain in protein structure. is a domain just repeated tertiary structural units that are made up of motifs. for example repeated beta-barrels in a protein, that are made up of greek keys?

A protein domain is a conserved part of a given protein sequence and (tertiary) structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded.

On the other hand, a motif is a distinctive sequence on a protein or DNA, having a three-dimensional structure that allows binding interactions to occur. Early on, clustering was used to detect common three-dimensional structural motifs in proteins.

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The proton has a positive charge while the neutron has no charge they both make up the nucleus of the atom while the elections have a negative charge and surround the nucleus of the atom. the protons and electrons are used to calculate the atom's mass whereas the neutrons don't affect the mass