Under physiological conditions, a polypeptide folds up into secondary structure. what are the most common secondary structure elements?

The folding of polypeptide begin during translation of the polypeptide chain. Amino acids interact with each other to form three dimensional tertiary structure and secondary structure elements formed spontaneously as an intermediate before the formation of tertiary structure.

There are two common secondary structure elements formed that are  alpha helices(α helix) and beta pleated sheets ( β pleated sheet).  Both the structures are attcahed by a hydrogen bond which is present between amino hydrogen and carboxyl oxygen atoms in the peptide backbone.

Hence, the two common elements of secondary structure are alpha helices and beta sheets.

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands

b a mature ovary of a plant.