# Concept 15 DNA and proteins are key molecules of the cell nucleus.

### Cross pure-bred pea plants to identify dominant flower color.

HI! Phoebus Levene's tetranucleotide block has a fixed nucleotide sequence where each nucleotide appears only once. Let's see what happens if we change the order of the nucleotides. What if each nucleotide is used only once, but the order of the nucleotides can be random? How many different combinations, blocks, would there be? 24 That is correct. 1 No, this is Levene's tetranucleotide theory. 4 No, there are more than 4 sequence combinations. 256 No, there are 256 combinations only if the nucleotides are used more than once. In a tetranucleotide block where each nucleotide is used only once, and the order is random, there can be 24 different combinations. 4 X 3 X 2 X 1 = 24 If each of these 24 tetranucleotide blocks is a "letter" in the DNA alphabet, how many different four letter "words" can be made? over 300,000 1 No, there is more than one combination. 4 No, there are more than four different "words." 24 No, this is the number of different "letters." over 300,000 That is correct. Using 24 different "letters," there are 331,776 different combinations of four letter "words." x = 331,776 Using 24 different "letters," there are 331,776 different combinations of four letter "words." Now, let's go back to the tetranucleotide block. How many different combinations would there be if the nucleotides can appear more than once and the order of the nucleotides is random? 1 No, this is Levene's tetranucleotide theory. 4 No, there are more than 4 sequence combinations. 24 No, there are 24 combinations only if each nucleotide appears once. 256 That is correct. In a tetranucleotide block where the nucleotides can appear more than once and the order is random, there can be 256 possible combinations. 256 is a large number of possible DNA letters. So, if Phoebus Levene had considered randomizing the order of the nucleotides in his tetranucleotide blocks, he would have seen that DNA wasn't as stupid as he thought.