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Deoxyribonucleic acid (DNA) located in a eukaryotic cell's nucleus contains discrete sections known as genes and these contain the genetic code to manufacture proteins. In protein synthesis, messenger RNA(mRNA) is able to make a complementary strand using either side of DNA as the template. In that way, it has a copy of the genetic instructions needed to manufacture a protein molecule. Although DNA remains in the nucleus, mRNA can travel to the cytoplasm and attach to a ribosome. Ribosomes are the site of protein synthesis in a cell. The genetic code contains nitrogenous bases in a specific order as dictated by the DNA code. mRNA copies the code using base-pairing rules. The bases of DNA are adenine, thymine, guanine and cytosine. RNA has the bases adenine, guanine, cytosine and uracil instead of thymine which is found in DNA. In DNA, base-pairing rules are adenine pairs to thymine and cytosine to guanine. In RNA, substitute a uracil for the thymine when pairing with adenine. Therefore, if the original DNA strand starts with TAC, the complementary would read AUG. Each triplet is known as a codon. These codons are instructions to insert a particular amino acid into position in a growing polypeptide chain. In the genetic code chart, AUG stands for the amino acid methionine and it also is a "start" codon to get protein synthesis underway. Special transfer RNA(tRNA)molecules bring the appropriate amino acids into position into the growing polypeptide chain at the ribosome. As each triplet is added to the growing chain, eventually, a stop codon is reached. This signal causes protein synthesis to cease and the polypeptide will be transported to the Golgi apparatus to be further processed and folded into a functional protein to be transported where needed.
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