n=input('how long do you want the string to be?')
M=rand(n,1)
D='X'
for ii =[1:n]
    if M(ii)<0.25
        D(ii)='A';
    elseif M(ii)<0.5
        D(ii)='C';
    elseif M(ii)<0.75
        D(ii)='G';
    else
        D(ii)='U';
    end
end
D