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Unformatted text preview: molecules. Ribosomal RNA (rRNA) helps form the ribosomes, where proteins are built. Messenger RNA (mRNA) molecules are long, single nu- cleotide strands that resemble half of a DNA mole- cule and carry the message containing instructions for protein synthesis from the DNA gene in the nu- cleus to the ribosomes in the cytoplasm. Protein synthesis involves two major phases: transcription, when complementary mRNA is made at the DNA gene, and translation, when the information carried in mRNA molecules is decoded and used to assemble proteins. These steps are summarized simply in Figure 3.16, and described in more detail next. Transcription The word transcription often refers to one of the jobs done by a secretary converting notes from one form (shorthand notes or an audiotape record- ing) into another form (a typewritten letter, for ex- ample). In other words, the same information is transformed from one form or format to another. In cells, transcription involves the transfer of information from DNAs base sequence into the complementary base sequence of mRNA (Figure 3.16, step 1). Only DNA and mRNA are involved in transcription. Whereas each three-base sequence specifying a particular amino acid on the DNA gene is called a triplet, the corresponding three- base sequences on mRNA are called codons. The form is different, but the same information is being conveyed. Thus, if the (partial) sequence of DNA triplets is AAT-CGT-TCG, the related codons on mRNA would be UUA-GCA-AGC. Translation A translator takes words in one language and re- states them in another language. In the trans- lation phase of protein synthesis, the language of nucleic acids (base sequence) is translated into the language of proteins (amino acid sequence). Translation occurs in the cytoplasm and involves three major varieties of RNA. As illustrated in Figure 3.16, steps 25, translation consists of the following events. Once the mRNA attaches to the ribosome (step 2), tRNA comes into the picture. Its job is to transfer, or ferry, amino acids to the ribo- some, where they are bound together by enzymes in the exact sequence specified by the gene (and its mRNA). There are about 45 common types of tRNAs, each capable of carrying one of the 20 or so common types of amino acid to the protein syn- thesis sites. But that is not the only job of the tiny tRNAs. They also have to recognize the mRNA codons calling for the amino acid they are toting. They can do this because they have a special three-base sequence called an anticodon on their head that can bind to the complementary codons (step 3). Once the first tRNA has maneuvered itself into the correct position at the beginning of the mRNA message, the ribosome moves the mRNA strand along, bringing the next codon into position to be read by another tRNA. As amino acids are brought to their proper positions along the length of mRNA, they are joined together by enzymes (step 4). As an amino acid is bonded to the chain, its4)....
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