Transcription and Translation
The function of DNA in the living world is at once strangely complex and surprisingly simple. The genetic code carried in molecules of DNA is responsible for a spectacular variety of life forms. Plant life in the Amazon, creatures on the ocean floor, and animals in the Serengeti have all arisen, and survive and thrive in their respective environments, because of the function of DNA. Inside every living cell, DNA directs vital activities, such as growth, division, movement, respiration, and even death, by providing the instructions that cells use to build proteins. DNA gives rise to the physical and behavioral adaptations that make organisms unique. And yet, the chemical language in which DNA's instructions are written is extremely simple. It consists of just four letters, which correspond to the four functional molecules, called nucleotides, from which DNA is built: adenine (A), thymine (T), cytosine (C), and guanine (G). Particular sequences of these four molecules function as genes by providing the instructions required to build one or more proteins. The DNA molecule resembles a twisted ladder, with pairs of nucleotides forming the ladder's rungs. As a rule, adenine always pairs with thymine and cytosine with guanine. When a cell requires a particular protein, an activation signal stimulates the release of an enzyme called RNA polymerase, which causes the DNA to "unzip" between nucleotide pairs in the region of the appropriate gene. As the RNA polymerase molecule moves along one of the unzipped DNA strands, it assembles a similar nucleic acid molecule, known as messenger RNA (mRNA), using free nucleotides found inside the nucleus. The mRNA molecule is a mirror image of the DNA strand that is being read, except that the nucleotide uracil (U) is substituted for thymine. This is called transcription. After this process is complete, the mRNA is transported outside the nucleus to the cytoplasm, where it can be translated into a...
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