自己做的話：
#List是妳的序列
unlist(strsplit(List,""))->sep.letter;#把每個字母都單獨分開
#遍歷所有字母
count_a=0; count_g=0; count_t=0; count_c=0;
for(i in 1:length(sep.letter)){
if(sep.letter[i]=="a"){count_a=count_a+1;}
if(sep.letter[i]=="g"){count_g=count_g+1;}
.........................
}