The term fish is used for any any swimming vertebrate with fins. it includes a diverse group of animals, divided into four taxonomic classes, agnatha, chondrichthyes, osteichthyes, and placoderma. taxonomists first divided all "fish" into two classes, agnatha and placoderma. the placoderma are considered ancestors of all other fish. what fossil evidence taxonomists separate all "fish" into these two branches for classification?

You have just sequenced a new protein found in mice and observe that sulfur-containing cysteine residues occur at regular intervals. what is the significance of this finding? it will be important to include cysteine in the diet of the mice. cysteine residues are required for the formation of α helices and β pleated sheets. cysteine residues are involved in disulfide bridges that form tertiary structure. cysteine causes bends, or angles, to occur in the tertiary structure of proteins.