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while cytosine on one chain is always paired with guanine on the other. Other researchers had shown that DNA has a helical structure, so Watson and Crick called the structure a double helix — a shape that looks like a spiral staircase. One of the best outcomes of the identification of the double helix was that scientists understood how the genes copy themselves, or replicate. The helix can break apart into two strands. The two individual strands then each act as a template so that a new strand forms on each in the only way it can, by connecting to the only nucleic acid it can combine with — namely, a nucleic acid containing adenine connects to a nucleic acid containing thymine, and a nucleic acid containing cytosine connects with a nucleic acid containing guanine. The result is two new double helixes. The DNA has to somehow control the creation of the animal or plant and the ongoing processes that allow it to live. The DNA does so by producing ribonucleic acid, or RNA. RNA comprises nucleic acids just like DNA. But the sugar in RNA is ribose, and the bases are adenine, cytosine, and guanine, with uricil replacing thymine. In the same way that the double helix can break apart to reproduce itself, it can break apart and construct a complementary RNA molecule. This process is called transcription. The RNA remains a single strand, not a double helix. The RNA is called messenger RNA because it carries the message from the DNA to the next level of control, the enzyme. An enzyme is a protein that acts as a facilitator for a chemical reaction — for example, the breakdown of a complex carbohydrate like glycogen (the storage form of glucose — the body’s source of immediate energy) into small glucose molecules that the body can instantly use. The messenger RNA accomplishes its task by acting as a template in its turn for the production of the enzyme or protein in the process of translation. Proteins consist of amino acids. Every three bases in the messenger RNA, called a triplet, causes one particular amino acid to line up opposite them. Each group of three is a codon, because it codes for a specific amino acid. By making up artificial messenger RNA that contained the same codon again and again, determining which amino acid each codon selected became possible. With 4 different bases in sets of 3, the maximum number of codons is 64, but only 20 amino acids exist. Different codons select the same amino acid, and some codons act as the code for the end of a protein without selecting an amino acid. Just to complicate the issue a little further, the amino acids don’t actually line up opposite the codons but are carried at one end of another RNA molecule, called transfer RNA. At its other end, transfer RNA has the bases that are complementary to the codon. So the transfer RNA lines up neatly against the messenger RNA, while the amino acids line up next to one another at the other

Thyroid for dummies  
Thyroid for dummies  
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