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PROTEIN SYNTHESIS Part I: Transcription and Translation The synthesis of proteins occurs in two steps: 1) the transcription of the genetic information from DNA into RNA (mRNA) and 2) the translation of the mRNA into a polypeptide chain. Transcription occurs only at specific sites (loci) on the DNA. Only the genes transcribed will be expressed in the cells, but at any one moment in time only a fraction of the genes on the DNA are being expressed. The process of transcription begins with a partial denaturation of the DNA. The enzyme RNA polymerase then binds to one strand of the DNA and makes a complementary RNA copy of the DNA. When the RNA copy has been made it diffuses away from the DNA and the two strands of the DNA renature to the double helix formation. The nitrogenous bases found in RNA are the same as those found in DNA except that thymine is replaced by uracil. The product of transcription, called messenger RNA (mRNA), is then translated. A ribosome binds to the mRNA and "reads" it three bases at a time. Each sequence of three bases (codon) code for a specific amino acid. These amino acids are attached to transfer RNA (tRNA). The tRNAs interact with the mRNA codon by hydrogen bonding. Thus the tRNA that will bind the mRNA in the ribosome will be the one that has a base sequence (anticodon) that is complementary to the codon. The amino acids attached to two adjacent tRNAs are then covalently bonded together by an enzyme in the ribosome. A dipeptide has now been formed. The process repeats itself until a complete polypeptide, usually around 300 amino acids long, has been produced. In summary, the base sequence of the DNA determines the base sequence of the RNA, which determines the sequence of binding of the tRNAs, which determine the sequence of the amino acids hooked together, which determines the primary structure of a protein. Questions: Work individually. 1. Describe the differences between DNA and RNA. There are a couple differences between DNA and RNA. Firstly, the “D” in DNA stands for deoxyribose, while the “R” in RNA stands for ribose. Therefore, RNA has more oxygen than DNA. Secondly, the bases for DNA are adenine, thymine, guanine, and cytosine, while the bases for RNA are adenine, uracil, guanine, and cytosine.
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This document was uploaded on 11/02/2011 for the course BIO 117 at Rollins.

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