When {HEAD ~ a position} in the compared sequences is occupied by {TAIL ~ the same nucleotide base} or amino acid, then the molecules are homologous, or identical, at that position 
is occupied by
When { HEAD ~ a position } in the compared sequences is occupied by { TAIL ~ the same nucleotide base } or amino acid , then the molecules are homologous , or identical , at that position
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH