Catalyzes the attachment of serine to tRNA(Ser). Is also able to aminoacylate tRNA(Sec) with serine, to form the misacylated tRNA L-seryl-tRNA(Sec), which will be further converted into selenocysteinyl-tRNA(Sec). ATP + L-serine + tRNA(Ser) = AMP + diphosphate + H(+) + L-seryl-tRNA(Ser) ATP + L-serine + tRNA(Sec) = AMP + diphosphate + H(+) + L-seryl-tRNA(Sec) Aminoacyl-tRNA biosynthesis; selenocysteinyl-tRNA(Sec) biosynthesis; L-seryl-tRNA(Sec) from L-serine and tRNA(Sec): step 1/1. Homodimer. The tRNA molecule binds across the dimer. Consists of two distinct domains, a catalytic core and a N-terminal extension that is involved in tRNA binding. Belongs to the class-II aminoacyl-tRNA synthetase family. Type-1 seryl-tRNA synthetase subfamily.