Preexisting rock that is subsequently altered to form a metamorphic rock is termed a

The right answer is protolith.

The protolith is an original rock of a metamorphic rock.

Metamorphism is the set of processes that induce mineralogical and textural modifications of a rock, in the solid state and under the effect of temperature, pressure and fluids.

When there is no longer any indication of the nature of the original rock, it is sometimes possible to identify it from the overall chemical composition of the rock (provided that the chemical changes are not too large).

a community that experiences major changes undergoes progress

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.