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3.6PROTEIN SYNTHESISOBJECTIVE• Describe the sequence of events in protein synthesis.Although cells synthesize many chemicals to maintain homeostasis, much of the cellularmachinery is devoted to synthesizing large numbers of diverse proteins. The proteins in turn determine the physical and chemical characteristics of cells and, therefore, of the organisms formed from them. Some proteins help assemble cellular structures such as the plasma membrane, the cytoskeleton, and other organelles. Others serve as hormones, antibodies, and contractile elements in muscular tissue. Still others act as enzymes, regulating the rates of the numerous chemical reactions that occur in cells, ortransporters, carrying various materials in the blood. Just as genome means all of the genes in an organism,proteome(PRŌ‐te‐ōm) refers to all of an organism's proteins.In the process calledgene expression,a gene's DNA is used as a template for synthesis of a specific protein. First, in a process aptly namedtranscription,the information encoded in a specific region of DNA istranscribed(copied) to produce a specific molecule of RNA (ribonucleic acid). In a second process, referred to as translation, the RNA attaches to a ribosome, where the information contained in RNA istranslatedinto a corresponding sequence of amino acids to form a new protein molecule (Figure3.26).Figure3.26Overview of gene expression.Synthesis of a specific protein requires transcription of a gene's DNA into RNA and translation of RNA into a corresponding sequence of amino acids.Transcription occurs in the nucleus; translation occurs in the cytoplasm.ImagineeringWhy are proteins important in the life of a cell?DNA and RNA store genetic information as sets of three nucleotides. A sequence of three such nucleotides in DNA is called abase triplet.Each DNA base triplet is transcribed as a complementary sequence of three nucleotides, called acodon. A givencodon specifies a particular amino acid. Thegenetic codeis the set of rules that relate the base triplet sequence of DNA to the corresponding codons of RNA and the amino acids they specify.ExamplesAnimation: Protein Synthesis
TranscriptionDuringtranscription, which occurs in the nucleus, the genetic information representedby the sequence of base triplets in DNA serves as a template for copying the information into a complementary sequence of codons. Three types of RNA are made from the DNA template:1.Messenger RNA (mRNA)directs the synthesis of a protein.2.Ribosomal RNA (rRNA)joins with ribosomal proteins to make ribosomes.3.Transfer RNA (tRNA)binds to an amino acid and holds it in place on a ribosome until it is incorporated into a protein during translation. One end of the tRNA carries a specific amino acid, and the opposite end consists of a triplet of nucleotides called ananticodon. By pairing between complementary bases, the tRNA anticodon attaches to the mRNA codon. Each of the more than 20 different types of tRNA binds to only one of the 20 different amino acids.The enzymeRNA polymerase(po‐LIM‐er‐ās) catalyzes transcription of DNA. However, the enzyme must be instructed where to start the transcription process and where to end it. Only one of the two DNA strands serves as a template for RNA synthesis. The segment of DNA where transcription begins, a special nucleotide sequence called apromoter,is located near the beginning of a gene (Figure3.27a). This is where RNA polymerase attaches to the DNA. During transcription, bases pair in a complementary manner: The bases cytosine (C), guanine (G), and thymine (T) in the DNA template pair with guanine, cytosine, and adenine (A), respectively, in the RNA strand (Figure3.27b). However, adenine in the DNA template pairs with uracil (U), not thymine, in RNA:AUTAGC→CGAUTATemplate DNA base sequenceComplementary RNA base sequence
Figure3.27Transcription.DNA transcription begins at a promoter and ends at a terminator.During transcription, the genetic information in DNA is copied to RNA.ImagineeringIf the DNA template had the base sequence AGCT, what would be the mRNA base sequence, and what enzyme would catalyze DNA transcription?Transcription of the DNA strand ends at another special nucleotide sequence called aterminator,which specifies the end of the gene (Figure3.27a). When RNA polymerase reaches the terminator, the enzyme detaches from the transcribed RNA molecule and the DNA strand.Not all parts of a gene actually code for parts of a protein. Regions within a gene calledintronsdo notcode for parts of proteins. They are located between regions calledexonsthatdocode for segments of a protein. Immediately after transcription, the transcript includes information from both introns and exons and is calledpre‐mRNA.The introns are removed from pre‐mRNA bysmall nuclear ribonucleoproteins(snRNPs, pronounced “snurps”; Figure3.27b). The snRNPs are enzymes that cut out the introns and splice together the exons. The resulting product is a functional mRNA molecule that passes through a pore in the nuclear envelope to reach the cytoplasm, where translation takes place.Although the human genome contains around 30,000 genes, there are probably 500,000 to 1 million human proteins. How can so many proteins be coded for by so few genes? Part of the answer lies inalternative splicingof mRNA, a process in which the
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