For 10 points, name this letter, which denotes a line separating adjacent sarcomeres in muscles.
Left-handed helices formed from alternating purine-pyrimidine nucleotides form a type of DNA denoted by this letter. The sex chromosomes of birds and platypuses are denoted by this letter and W. A disk denoted by this letter uses desmin-synemin heteropolymers to keep desmin fibers lined up, and a series of those disks occurs in the I band and is a line denoted by this letter. For 10 points, name this letter, which denotes a line separating adjacent sarcomeres in muscles.
Left-handed helices formed from alternating purine-pyrimidine nucleotides form a type of DNA denoted by this letter.
Z [accept ZDNA before "letter"; accept zigzag DNA before "letter"]