Complexity_ A Guided Tour - Melanie Mitchell [45]
FIGURE 6.1. Illustration of the double helix structure of DNA. (From the National Human Genome Research Institute, Talking Glossary of Genetic Terms [http://www.genome.gov/glossary.cfm.])
FIGURE 6.2. Illustration of transcription of DNA into messenger RNA. Note that the letter U is RNA’s version of DNA’s letter T.
The process of transcription continues until the gene is completely transcribed as mRNA.
The second step is translation (figure 6.3), which happens in the cell cytoplasm. The newly created mRNA strand moves from the nucleus to the cytoplasm, where it is read, one codon at a time, by a cytoplasmic structure called a ribosome. In the ribosome, each codon is brought together with a corresponding anticodon residing on a molecule of transfer RNA (tRNA). The anticodon consists of the complementary bases. For example, in figure 6.3, the mRNA codon being translated is UAG, and the anticodon is the complementary bases AUC.A tRNA molecule that has that anticodon will attach to the mRNA codon, as shown in the figure. It just so happens that every tRNA molecule has attached to it both an anticodon and the corresponding amino acid (the codon A U C happens to code for the amino acid isoleucine in case you were interested). Douglas Hofstadter has called tRNA “the cell’s flash cards.”
FIGURE 6.3. Illustration of translation of messenger RNA into amino acids.
The ribosome cuts off the amino acids from the tRNA molecules and hooks them up into a protein. When a stop-codon is read, the ribosome gets the signal to stop, and releases the protein into the cytoplasm, where it will go off and perform whatever function it is supposed to do.
The transcription and translation of a gene is called the gene’s expression and a gene is being expressed at a given time if it is being transcribed and translated.
All this happens continually and simultaneously in thousands of sites in each cell, and in all of the trillions of cells in your body. It’s amazing how little energy this takes—if you sit around watching TV, say, all this subcellular activity will burn up fewer than 100 calories per hour. That’s because these processes are in part fueled by the random motion and collisions of huge numbers of molecules, which get their energy from the “ambient heat bath” (e.g., your warm living room).
The paired nature of nucleotide bases, A with T and C with G, is also the key to the replication of DNA. Before mitosis, enzymes unwind and separate strands of DNA. For each strand, other enzymes read the nucleotides in the DNA strand, and to each one attach a new nucleotide (new nucleotides are continually manufactured in chemical processes going on in the cell), with A attached to T, and C attached to G, as usual. In this way, each strand of the original two-stranded piece of DNA becomes a new two-stranded piece of DNA, and each cell that is the product of mitosis gets one of these complete two-stranded DNA molecules. There are many complicated processes in the cell that keep this replication process on track. Occasionally (about once every 100 billion nucleotides), errors will occur (e.g., a wrong base will be attached), resulting in mutations.
It is important to note that there is a wonderful self-reference here: All this complex cellular machinery—the mRNA, tRNA, ribosomes, polymerases, and so forth—that effect the transcription, translation, and replication of DNA are themselves encoded in that very DNA. As Hofstadter remarks: “The DNA contains coded versions of its own decoders!” It also contains coded versions of all the proteins that go into synthesizing the nucleotides the DNA is made up of. It’s a self-referential circularity