On Saturday, December 6, Harvard’s School of Engineering and Applied Sciences will offer its free holiday lecture, DNA:  A Detective Story, in two sessions:  10-11AM and 1-2PM.  Professor Howard Stone of Princeton will speak about how scientists cracked the code of DNA, and kids will demonstrate the chemistry of DNA and how it replicates itself.  Audience participation is a big part of the show, and every kid gets a T-shirt.  It’s free and open to the public.  Register online to guarantee seating.  Location:  Harvard Science Center’s Lecture Hall B, 1 Oxford Street in Cambridge.