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What Controls the Synthesis of Proteins? Nucleic acids contain the information, in a coded form, which specifies the sequence in which amino acids will be put together to form a protein. The monomer of a nucleic acid is a nucleotide, which has three parts itself: a 5-carbon sugar (either ribose, in RNA, or deoxyribose, in DNA), a base (there are 5, which we'll call A,T,C,G, and U), and a phosphate group. Stick model of a nucleotide with the base A Simplified diagram of the same nucleotide Guess what kind of reaction combines nucleotides to form a polynucleotide? As usual, synthesis is used, breaking an -OH and an -H from the phosphate of one nucleotide and the sugar of another, and forming a bond between them. When many nucleotides are connected together by synthesis reactions, they form a polynucleotide strand of DNA. The noteworthy thing about nucleic acids is that the bases are attracted to each other (due to opposite charges arranged in matching patterns). A is attracted to T or U; C is attracted to G. These pairs are called complementary bases. A DNA molecule consists of two polynucleotide strands bound together by the bonds between the complementary bases. The sequence of the bases in a strand can be any combination of the bases A,T,C and G (U is used instead of T in RNA); the other strand, though, has to have complementary bases. The order of the bases in a strand contains the information that specifies the order of the amino acids in a protein. Suppose you have a DNA molecule, and one strand in it has the sequence AAGGCATGT. What will be the sequence of the other strand in the DNA molecule? How many different 10-nucleotide DNA strands could be made? Here are two possibilities: AAAAAAAATA, AAAAAAAATG. How many different sequences are possible (the two ends are chemically different, so mirror images are different sequences)? Let's start with an easier question. How many 2-nucleotide strands are possible? List all the two-nucleotide combinations and count them. Here are a few to get you started. Finish the list, and write down the total number of many you get. AA, AC, AG, AT, Total number of possible 2-nucleotide strands = ___________________ Okay, now back to the original question. How many 10-nucleotide DNA strands could be made, then? How would you calculate the answer? As you can see, that's a lot. The human genome (or that of most any mammal, for that matter) contains about 3 billion nucleotides. Imagine how many different sequences of 3 billion nucleotides are possible. This means that there's room in your genome for a whole lot of information, specifying a lot of different proteins. Protein synthesis requires two steps: transcription, in which an RNA molecule complementary to one strand of a DNA molecule is synthesized; and translation, in which coded information in an RNA molecule is used to direct the synthesis of a polypeptide (protein).