BERLIN (AP).- The Berlin Wall’s construction 50 years ago must be a constant reminder to citizens today to stand up for freedom and democracy, the city’s mayor said Saturday as a united Germany commemorated the bitter anniversary. Seeing Berlin divided by the wall tore apart the country as well as separating the city’s streets, neighbors and families, mayor Klaus Wowereit said at a televised ceremony. “It is our shared responsibility to keep the memory alive and to pass it on to the coming generations as a reminder to stand up for freedom and democracy to ensure that such injustice may never happen again,” Wowereit said. German Chancellor Angela Merkel — who grew up behind the wall in Germany’s communist eastern part — also attended the commemoration in Berlin, where parts of the wall and an attached surveillance tower now form a museum. The road where the museum is located was divided in two on August 13, 1961, and some 2,000 east German residents were expelled from t