reverse complement DNA string in Tcl

Tcl/Tk version 8.5:

%
% set s "ATGCATGCAATTGGCCAAAATTTTGGGGCCCC"
ATGCATGCAATTGGCCAAAATTTTGGGGCCCC
%
% set s [string reverse $s]
CCCCGGGGTTTTAAAACCGGTTAACGTACGTA
%
% set s [string map { A T G C C G T A } $s]
GGGGCCCCAAAATTTTGGCCAATTGCATGCAT
%

Tcl/Tk version 8.4:
[ http://wiki.tcl.tk/17187 ]

 proc string_reverse str {
    set res {}
    set i [string length $str]
    while {$i > 0} {append res [string index $str [incr i -1]]}
    set res
 }
% string_reverse "hello, world"
dlrow ,olleh