Figure During transcription, RNA nucleotides base-pair one by one with DNA

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Sydney Powers
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1 Objectives Describe the process of DNA transcription. Explain how an RNA message is edited. Describe how RNA is translated to a protein. Summarize protein synthesis. Key Terms messenger RNA (mrna) RNA polymerase intron exon RNA splicing transfer RNA (trna) anticodon ribosomal RNA (rrna) You have just learned how genetic information flows from DNA to RNA to protein. A sequence of DNA base triplets is transcribed into RNA codons, which are translated into a sequence of amino acids that form a polypeptide. In this section, you will learn the details of the mechanisms of transcription and translation. Transcription: DNA to RNA There are three types of ribonucleic acids (RNAs) involved in making proteins from the instructions carried in genes. Starting with transcription, the RNA molecule called messenger RNA (mrna) is transcribed from a DNA template. The transcription process resembles replication of a DNA strand. However, in transcription, only one of the DNA strands serves as a template for the newly forming mrna molecule. The two DNA strands first separate at the place where transcription will start. Then RNA bases pair with complementary DNA bases (Figure 11-14). Figure During transcription, RNA nucleotides base-pair one by one with DNA

2 nucleotides on one of the DNA strands (called the template strand). RNA polymerase links the RNA nucleotides together. The base-pairing during transcription is the same as when DNA replicates, except that RNA has uracil instead of thymine: the base U in RNA pairs with A in DNA. A transcription enzyme called RNA polymerase links the RNA nucleotides together. In the transcription of a gene, specific sequences of DNA nucleotides tell the RNA polymerase where to begin and end the transcribing process. Editing the RNA Message In prokaryotic cells, the mrna transcribed from a gene directly serves as the messenger molecule that is translated into a protein. But this is not the case in eukaryotic cells. In a eukaryotic cell, the RNA transcribed in the nucleus is modified or processed before it leaves the nucleus as mrna to be translated. The initial RNA transcripts have stretches of noncoding nucleotides that interrupt nucleotide sequences that actually code for amino acids. It is as if nonsense groups of letters were randomly scattered in an otherwise normal document. Such internal noncoding regions are called introns, and are found in most plant and animal genes. (Many researchers are now trying to determine the functions of introns and how they evolved.) The coding regions of the RNA transcript the parts of a gene that remain in the mrna and will be translated, or "expressed" are called exons. Before the RNA leaves the nucleus, the introns are removed and the exons are joined together, producing an mrna molecule with a continuous coding sequence (Figure 11-15). This process is called RNA splicing. With splicing completed, the "final draft" of eukaryotic mrna is ready for translation. Figure In eukaryotes, the RNA transcript is edited before it leaves the nucleus. Introns are removed and the exons are spliced together before the "final draft" transcript moves into the cytoplasm where it gets translated. Translation: RNA to Protein Translating the nucleic acid language to the protein language is an elaborate process. Like other cellular processes, the translation of mrna requires enzymes and sources of chemical energy such as ATP. The main players in the mrna translation process are ribosomes and another kind of RNA called transfer RNA. The Players Translating one language into another language requires an interpreter. Some person or device must recognize the words of one language and convert them into the other. For the cell, that interpreter is transfer

3 RNA. Transfer RNA (trna) translates the three-letter codons of mrna to the amino acids that make up proteins (Figure 11-16). Figure During translation, trnas transport and match amino acids to their appropriate codons on the mrna transcript. One end of the trna attaches to an amino acid. At the other end, a triplet of bases called the anticodon matches to the complementary mrna codon. To perform this task, a trna molecule must (1) become bound to the appropriate amino acid and (2) recognize the appropriate codon in the mrna. The unique structure of trna molecules enables them to perform both functions. There is a different version of trna molecule that matches each codon. At one end of the folded trna molecule is a specific triplet of bases called an anticodon. The three bases of the anticodon are complementary to a specific codon in the mrna. During translation, the anticodon on trna recognizes a particular codon on mrna by using base-pairing rules. At the other end of the trna molecule is a site where a particular amino acid can attach. An enzyme specific for each amino acid recognizes both a trna and its amino acid partner and links the two together, using energy from ATP. The ribosome, an organelle to which you were introduced in Chapter 6, coordinates the functioning of mrna and trna. The ribosome consists of two parts or subunits, each of which is made up of proteins and a considerable amount of yet another kind of RNA, ribosomal RNA (rrna). A complete ribosome has a binding site for mrna on its small subunit and two binding sites for trna on its large subunit (Figure 11-17). The subunits of the ribosome act like a vise, holding the mrna and trna molecules close together. Figure Ribosomes bring mrna and trnas together during translation. Each ribosome has an attachment

4 site for an mrna transcript, and two sites for trnas. One of the trna-binding sites, the "P" site, holds the trna carrying the growing polypeptide chain. The other site, the "A" site, holds a trna carrying the next amino acid to be added to the chain. (An easy way to remember which site is which is that "P" stands for "polypeptide" while "A" stands for "amino acid.") The ribosome connects the newly arrived amino acid to the growing polypeptide chain. The Process The first step in translation brings together all the pieces needed during translation: the mrna, the first trna with its attached amino acid, and the two subunits of a ribosome. The start codon AUG dictates where translation will begin, as shown in Figure Next, amino acids are added one by one to the growing chain of amino acids. Each addition occurs in a threestep process (Figure 11-18). This lengthening process continues until the ribosome reaches a stop codon UAA, UAG, or UGA. Remember that the three stop codons do not code for amino acids. When a new amino acid fails to arrive at the "A" site, the translation stops. The completed polypeptide, which is typically several hundred amino acids long, is set free by hydrolysis from the trna. A single ribosome can make an averagesized polypeptide in less than a minute. The whole process of translation is summarized in Figure

5 Figure During translation, the ribosome adds amino acids to the polypeptide chain. The ribosome moves down the transcript, codon by codon, until translation is completed.

6 Figure Translation begins with the attachment of a ribosome and the first trna to a "start" (AUG) codon. The ribosome then moves along the mrna transcript. The polypeptide elongates as an amino acid is added for each codon. When the ribosome arrives at a "stop" codon, the completed polypeptide is released. Review of Protein Synthesis What is the overall significance of transcription and translation? In learning these processes, you now understand how genes are responsible for the polypeptides and proteins that make up the structures and perform the activities of cells. Put more broadly, this is the way that genotype relates to phenotype. The chain of command originates with the information in the DNA of a gene. The DNA serves as a template, dictating transcription of a complementary strand of mrna. In turn, mrna specifies the sequence of amino acids in a polypeptide built with the assistance of trna and the rrna of a ribosome. Finally, the proteins that form from the polypeptides determine the appearance and functioning of the cell and of the whole organism. Concept Check What kind of nucleic acid is made during transcription? 2. How do introns and exons relate to RNA splicing? 3. List the three RNA types involved in transcription and translation, and describe the role of each. 4. Briefly describe the steps of protein synthesis.

Transcription and Translation of DNA

Transcription and Translation of DNA Genotype our genetic constitution ( makeup) is determined (controlled) by the sequence of bases in its genes Phenotype determined by the proteins synthesised when genes