Complementary mRNA codon

Each tRNA has an amino acid arm with the terminal sequence CCA(3 ) to which an amino acid is esterified, an anticodon arm, a Ti//C arm, and a D arm some tRNAs have a fifth arm. The anticodon is responsible for the specificity of interaction between the aminoacyl-tRNA and the complementary mRNA codon. 

The actual information transfer is based on the interaction between the mRNA codons and another type of RNA, transfer RNA (tRNA see p. 82). tRNAs, of which there are numerous types, always provide the correct amino acid to the ribosome according to the sequence information in the mRNA. tRNAs are loaded with an amino acid residue at the 3 end. Approximately in the middle, they present the triplet that is complementary to each mRNA codon, known as the anticodon (GAA in the example shown). If the codon UUC appears on the mRNA, the anticodon binds a molecule of Phe-t-RNA to the mRNA (5) and thus brings the phenylalanine residue at the other end of the molecule into a position in which it can take over the growing polypeptide chain from the neighboring tRNA (6). 

Complementary, antiparallel bindinc of the anticodon for methionyl-tRNA (CAU) to the mRNA codon for methionine (AUG). 

Binding of the tRNA anticodon to the mRNA codon follows the rules of complementary and antiparallel binding, that is, the mRNA codon is "read" 5 ->3 by an anticodon pairing in the "flipped" (3 —>5 ) orientation (Figure 31.9). [Note When writing the sequences of bolh codons and anticodons, the nucleotide sequence must ALWAYS be listed in the 5 —>3 order.]... 

Among the 76 nucleotides of tRNAphe are two sets of three that are especially important. The first is a group of three bases called the anticodon, which is complementary to the mRNA codon for the amino acid being transferred. Table 28.3 lists two mRNA codons for phenylalanine, UUU and UUC (reading in the 5 —>3 direction). Because base-pairing requires the mRNA and tRNA to be antiparallel, the two anticodons are read in the 3 ->5 direction as AAA and AAG. 

Figure 25-28 Peptide-bond formation in protein biosynthesis showing how the amino-acid sequence is determined by complementary basepairing between messenger RNA and transfer RNA, The peptide chain is bound to tRNA, which is associated with mRNA through three bases in mRNA (codon) and three bases in tRNA (anticodon). In the diagram, the next codon A-A-G codes for lysine. Hence, Lys-tRNA associates with mRNA by codon-anticodon base-pairing and, under enzyme control, couples to the end of the peptide chain.

C represents a segment of the template strand of a molecule of DNA. Draw (a) the coding strand (b) the mRNA that is synthesized from C during transcription (c) the tRNA anticodons that are complementary to the mRNA codons (d) the amino acids (use one-letter codes) that form the peptide that C codes for. 

A-site The acceptor site on the ribosome into which an aminoacyl-charged transfer RNA (tRNA) is brought that has an anticodon that is complementary to the messenger RNA (mRNA) codon. The ribosome will then catalyze the formation of a peptide bond with the aminoacyl group on this tRNA and the growing peptide chain. 

The genetic information in DNA is converted into the linear sequence of amino acids in polypeptides in a two-phase process. During transcription, RNA molecules are synthesized from a DNA strand through complementary base pairing between the bases in DNA and the bases in free ribonucleoside triphosphate molecules. During the second phase, called translation, mRNA molecules bind to ribosomes that are composed of rRNA and ribosomal proteins. Transfer RNA-aminoacyl complexes position their amino acid cargo in the catalytic site within the ribosome in a process that involves complementary base pairing between the mRNA codons and tRNA anticodons. Once the amino acids are correctly positioned within the catalytic site, a peptide bond is formed. After the mRNA molecule moves relative to the ribosome, a new codon enters the ribosome s catalytic site and base pairs with the appropriate anticodon on another aminoacyl-tRNA complex. After a stop codon in the mRNA enters the catalytic site, the newly formed polypeptide is released from the ribosome. 

Translation begins as each ribosome binds an mRNA molecule and proceeds to convert its base sequence into a polymer of amino acids linked by peptide bonds. Each amino acid is specified by a code word, called a codon, that consists of three sequential bases. The actual transfer of information occurs when each mRNA codon interacts and forms complementary base pairs with a three-base sequence in a transfer RNA (tRNA) molecule called an anticodon. 

As shown in Figure 6-16, the activated amino acid is attached to the end of its specific /RNA molecule. The function of this aminoacyl /RNA molecule is to place the amino acid that it is carrying in the proper sequence position on the template. Triplets of nucleotides— called anticodons—in the /RNA molecule (Fig. 6-16) are attracted to the complementary codon of mRNA). The amino acid carried by its specific /RNA is thus brought to the correct position on the mRNA codon. Since the anticodon on the /RNA is identical to the codon on DNA (except thymine replaces uracil), the DNA directs the amino acids in the protein-forming chain. The process of protein biosynthesis can now be considered. 

Soon after the structure of DNA was discovered, Francis Crick hypothesized that an adaptor molecule, such as the one shown in Figure 26.2, would be required for protein synthesis. It was discovered that tRNA serves as the adaptor molecule by linking the information stored in mRNA to the primary sequence of the polypeptide. The adaptor function of tRNA is mediated by base pairing between mRNA sequences called codons, and complementary sequences on tRNA called anticodons. For every codon on the mRNA, a single amino acid is delivered by the tRNA to the growing polypeptide chain. The term genetic code refers to the specific sequences in the mRNA codon that determine which tRNA molecule is going to have the complementary anticodon and, therefore, what amino acid is required at that position in the final polypeptide chain. 

Codon Three consecutive nucleotides in mRNA that specify a given amino acid during protein synthesis the anticodon of tRNA is complementary to the mRNA codon. 

Wobble hypothesis As proposed by Francis Crick, the Wobble hypothesis explains why one tRNA anticodon can be complementary to more than one mRNA codon. 

The pioneering molecular biologists recognized that, because amino acids cannot bind directly to the sets of three nucleotides that form their codons, adapters are required. The adapters were found to be tRNA molecules. Each tRNA molecule contains an anticodon and covalently binds a specific amino acid at its 3 -end (see Chapters 12 and 14). The anticodon of a tRNA molecule is a set of three nucleotides that can interact with a codon on mRNA (Fig. 15.2). To interact, the codon and anticodon must be complementary (i.e., they must be able to form base pairs in an antiparallel orientation). Thus, the anticodon of a tRNA serves as the link between an mRNA codon and the amino acid that the codon specifies. 

Some aminoacyl-tRNA synthetases use the anticodon of the tRNA as a recognition site as they attach the amino acid to the hydroxyl group at the 3 -end of the tRNA (Fig. 15.6). However, other synthetases do not use the anticodon but recognize only bases located at other positions in the tRNA. Nevertheless, insertion of the amino acid into a growing polypeptide chain depends solely on the bases of the anticodon, through complementary base-pairing with the mRNA codon. 

When Met-tRNAj (or a peptidyl-tRNA) is bound to the P site, the mRNA codon in the A site determines which aminoacyl-tRNA will bind to that site. An aminoacyl-tRNA binds when its anticodon is antiparallel and complementary to the mRNA codon. In eukaryotes, the incoming aminoacyl-tRNA first combines with elongation... 

It is not at all important for you to learn the genetic code, but it is interesting to see how it works. The code is written in terms of the mRNA codons that will encode each amino acid. Thus, the corresponding DNA sequence in the gene is the complementary base-pairing sequence, e.g. AAG in the mRNA encodes lysine, and it will itself have been encoded by TTC in the DNA since T base-pairs with A and C base-pairs with G. 

Section 26.11 Three RNAs are involved in gene expression. In the transcription phase, a strand of messenger RNA (mRNA) is synthesized from a DNA template. The four bases A, G, C, and U, taken three at a time, generate 64 possible combinations called codons. These 64 codons comprise the genetic code and code for the 20 amino acids found in proteins plus start and stop signals. The mRNA sequence is translated into a prescribed protein sequence at the ribosomes. There, small polynucleotides called transfer RNA (tRNA), each of which contains an anticodon complementary to an mRNA codon, carries the correct amino acid for incorporation into the growing protein. Ribosomal RNA (rRNA) is the main constituent of ribosomes and appears to catalyze protein biosynthesis. 

Anticodon A three-base sequence in tRNA that is complementary to one of the mRNA codons, forming a link between each codon and the corresponding amino acid. 

Figure 25.16 illustrates this process of protein biosynthesis. It shows a tRNA molecule about to add to the mRNA codon AAU. Note that the tRNA molecule has a triplet base sequence TTA at one end of the molecule and the amino add asparagine (Asn) attached to the otho- end. The triplet TTA, called an anticodon, is the complementary base sequence to the codon AAU. The tRNA anticodon pairs up with the mRNA codon. The amino acid asparagine is then transferred to the end of the protein chain, where it links to valine (Val). Once the amino add has been transferred, the tRNA molecule is released from mRNA. 

The decoding process involves antiparallel base pairing between the three bases of mRNA codons and the complementary anticodons of transfer RNA (tRNA) during peptide bond formation (Fig. 6). The first and middle bases... 

Anticodon The trinucleotide sequence at the end of one arm of tRNA that base pairs with a complementary messenger RNA (mRNA) codon. 

New Core Chemical Skills are added Writing the Complementary DNA Strand (17.3), Writing the mRNA Segment for a DNA Template (17.4), and Writing the Amino Acid for an mRNA Codon (17.4). 

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