AS WE SAW IN CHAPTER 1, DNA is a very long, thin molecule. This molecule contains the genes that determine what an organism is and does. Since the only thing that varies among DNA molecules is the sequence of base pairs, not the sugar-phosphate backbone, the "program" encoded by DNA must reside in the order of base pairs. And indeed, it is helpful to think of DNA as an old-fashioned ribbon-like computer tape (now replaced by CDs) containing the software of the cell, this software being the base sequence. As in any computer, the software can only be executed if the proper hardware decodes the instructions present in that software. This is in fact how the genes present in DNA carry out their commands: the language of DNA base pairs is interpreted and executed by the cell's hardware. The decoding of DNA takes place in two separate steps, called transcription and translation, respectively.
Proteins are the end products of the flow of genetic information in living cells. They directly determine how an organism looks and acts. The function of transcription is to produce a ribonucleic acid (RNA) copy of the DNA, while the function of translation is to decode this RNA copy and direct the production of proteins. Proteins can be thought of as agents that determine the phenotype of cells and organisms, while the genotype is stored in DNA.
Figure 4.1 The Structures of DNA and RNA Contrasted. DNA is a double-stranded molecule with deoxyribose as the sugar, while RNA is a single-stranded molecule with ribose as the sugar. The thymine in DNA is replaced by a similar structure, uracil, in RNA.
Was this article helpful?