There's question in Hardt's Essential Genetics that bugs me, because I'm not sure if the answer is very simple or if I just don't get the question right. If you have a sequence of 4 ribonucleotides (with equal frequency), what is the probability to have a start codon/stop codon and what would be the average distance between stop codons in a DNA sequence... The probability of a start codon is 1/64, it's 3/64 for stop codons (of course it depends on the organism but in this case it must be the "convenctional" start/stop codons). A codon is made of 3, not 4 nucleotides. Within a sequence of 4 codons, there could be 2 codons (XXXY or YXXX), so it seems the probability would be 2/64 and 6/64, but I'm not sure if it's that simple... For the average distance it's easier; 64/3 codons or 64 nucleotides.