Figure 3.14 DNA molecule section showing the phosphate-sugar ''backbone.''

Figure 3.14 DNA molecule section showing the phosphate-sugar ''backbone.''

is, if one strand has the bases ATGCCACTA, the other strand must be TACGGTGAT, to form the following pairings:

What does DNA code for? Very simply, by specifying the sequence of amino acids, it contains instructions for the construction of all the proteins that the organism can make. Recall that proteins can have 20 different amino acids. So how can four nucleotide bases code for all 20? The answer is that the DNA bases code in groups of three.

The sequence for the bottom strand in the example above, and the corresponding amino acid sequence, is

As mentioned above, RNA uses uracil in place of thymine and ribose sugar instead of deoxyribose. In addition, it is a single strand. The major function of RNA is to communicate the DNA code from the cell nucleus to the cytoplasm, where proteins are synthesized. More details on the mechanisms involved are provided in Section 6.2.1. RNA has another function, recently discovered. It can act as a catalyst, similar to protein enzymes. RNA with this capability are called ribozymes. One school of thought holds that because RNA can act as both a genetic template and as a catalyst, it may be that when life originated, it was based on RNA for both of those functions.

Several nucleotide monomers are important participants in biochemical reactions. More is said in Section 5.1.3 about adenosine triphosphate (ATP) and its central role in energy metabolism. The cell uses a number of other nucleotides. Cyclic adenosine monophosphate (cAMP) is involved in regulation of cell metabolism. Adenine is combined with other organic molecules to form a number of coenzymes, including:

flavin adenine dinucleotide (FAD) nicotinamide adenine dinucleotide (NAD) nicotinamide adenine dinucleotide phosphate (NADP) guanosine triphosphate (GTP)

These compounds are important in the mechanisms for many biochemical processes, including photosynthesis and respiration, as discussed below.

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