PORTLAND — Portland Ovations presents “Porgy and Bess” at 8 p.m. Friday at Merrill Auditorium. This year marks the 75th anniversary of the opera, written by George Gershwin, with lyrics by Ira Gershwin and DuBose and Dorothy Heyward.

The score includes such classics as “It Ain’t Necessarily So,” “I Got Plenty of Nuttin’ ” and “Summertime.”

A free pre-performance lecture by Ellen Chickering of the University of Southern Maine’s School of Music will explore the life of George Gershwin and his combination of tragic opera themes with contemporary issues of African-American life. The lecture begins at 6:30 p.m. in the Merrill rehearsal hall.

Tickets range from $38 to $53 and are available at PortTix, the Merrill Auditorium box office; by phone at 842-0800; and online at www.portlandovations.org.