Assignment On Protein Synthesis

Added on - 19 Sep 2019

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3.6PROTEIN SYNTHESISOBJECTIVEDescribe 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 inturn determine the physical and chemical characteristics of cells and, therefore, of theorganisms formed from them. Some proteins help assemble cellular structures such asthe plasma membrane, the cytoskeleton, and other organelles. Others serve ashormones, antibodies, and contractile elements in muscular tissue. Still others act asenzymes, 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 thegenes in an organism,proteome(PRŌteōm) refers to all of an organism's proteins.In the process calledgene expression,a gene's DNA is used as a template forsynthesis of a specific protein. First, in a process aptly namedtranscription,theinformation encoded in a specific region of DNA istranscribed(copied) to produce aspecific molecule of RNA (ribonucleic acid). In a second process, referred to astranslation, the RNA attaches to a ribosome, where the information contained in RNAistranslatedinto a corresponding sequence of amino acids to form a new proteinmolecule (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 acorresponding 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 ofthree such nucleotides in DNA is called abase triplet.Each DNA base triplet istranscribed as a complementary sequence of three nucleotides, called acodon. A givencodon specifies a particular amino acid. Thegenetic codeis the set of rules that relatethe base triplet sequence of DNA to the corresponding codons of RNA and the aminoacids 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 theinformation into a complementary sequence of codons. Three types of RNA are madefrom 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 aribosome until it is incorporated into a protein during translation. One end of thetRNA carries a specific amino acid, and the opposite end consists of a triplet ofnucleotides called ananticodon. By pairing between complementary bases, thetRNA anticodon attaches to the mRNA codon. Each of the more than 20 differenttypes of tRNA binds to only one of the 20 different amino acids.The enzymeRNA polymerase(poLIMerās) catalyzes transcription of DNA. However,the enzyme must be instructed where to start the transcription process and where toend it. Only one of the two DNA strands serves as a template for RNA synthesis. Thesegment of DNA where transcription begins, a special nucleotide sequence calledapromoter,is located near the beginning of a gene (Figure3.27a). This is where RNApolymerase attaches to the DNA. During transcription, bases pair in a complementarymanner: The bases cytosine (C), guanine (G), and thymine (T) in the DNA template pairwith 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:AUTAGCCGAUTATemplate DNA basesequenceComplementary RNA basesequence
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 bethe mRNA base sequence, and what enzyme would catalyze DNAtranscription?Transcription of the DNA strand ends at another special nucleotide sequence calledaterminator,which specifies the end of the gene (Figure3.27a). When RNApolymerase reaches the terminator, the enzyme detaches from the transcribed RNAmolecule and the DNA strand.Not all parts of a gene actually code for parts of a protein. Regions within a genecalledintronsdo notcode for parts of proteins. They are located between regionscalledexonsthatdocode for segments of a protein. Immediately after transcription,the transcript includes information from both introns and exons and is calledpremRNA.The introns are removed from premRNA bysmall nuclearribonucleoproteins(snRNPs, pronounced “snurps”; Figure3.27b). The snRNPs areenzymes that cut out the introns and splice together the exons. The resulting product isa functional mRNA molecule that passes through a pore in the nuclear envelope toreach the cytoplasm, where translation takes place.Although the human genome contains around 30,000 genes, there are probably500,000 to 1 million human proteins. How can so many proteins be coded for by so fewgenes? Part of the answer lies inalternative splicingof mRNA, a process in which the
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