  function y = Strand(x)
% x is a string consisting of the characters A, C, T, and G.
% y is a string obtained by replacing A by T, T by A, C by G
%       and G by C.
y = '';
for k=1:length(x)
    if strcmp(x(k),'A')
        y = [y 'T'];
    elseif strcmp(x(k),'T')
        y = [y 'A'];
    elseif strcmp(x(k),'C')
        y = [y 'G'];
    else
        y = [y 'C'];
    end
end
