Protein transfer RNA

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. 

There are many different kinds of RNA manufactured in a cell. Ales-senger RNA (mRNA) is produced to take the information contained in a specific segment of DNA and then use it to make proteins. Ribosomal RNA (rRNA) is part of a large RNA protein complex called the ribosome that binds mRNA and joins amino acids to make a protein. Transfer RNA (tRNA) brings amino acids to the ribosome and ensures that the amino acid used is in the order specified by mRNA. Many other kinds of RNA are also present in a cell. RNA plays an important role in the proper ftmctioning of a cell. SEE ALSO DNA Replication Nucleic Acids Proteins. 

Messenger RNA molecules are synthesized to be complementary to a portion (gene) of the DNA molecule in the cell. They serve as the template or pattern on which a protein will be constructed (a particular group of nitrogen bases on mRNA is able to accommodate and specify a particular amino acid in a particular location in the protein). Transfer RNA molecules are much smaller than mRNA, and their structure accommodates only a single specific amino acid molecule. They "find" their specific amino acid in the cellular fluids and bring it to mRNA, where it is added to the protein molecule being synthesized. 

FIGURE 15.24 Protein synthesis. A section of DNA unwinds and transcription results in production of messenger RNA. Outside the cell nucleus, at a ribosome, the code carried by messenger RNA is translated into the amino acid seguence of a newly synthesized protein. Transfer RNA brings the amino acids into position one at a time. 

Messenger RNA provides the code needed to determine the sequence of amino acids to be added to the protein.Transfer RNA brings the necessary amino acids to the mRNA in the ribosome.The tRNA bonds to the mRNA,and the amino acid is transferred to the growing protein chain. After transfer of the amino acid, the tRNA is released from the mRNA. 

Cellular protein biosynthesis involves the following steps. One strand of double-stranded DNA serves as a template strand for the synthesis of a complementary single-stranded messenger ribonucleic acid (mRNA) in a process called transcription. This mRNA in turn serves as a template to direct the synthesis of the protein in a process called translation. The codons of the mRNA are read sequentially by transfer RNA (tRNA) molecules, which bind specifically to the mRNA via triplets of nucleotides that are complementary to the particular codon, called an anticodon. Protein synthesis occurs on a ribosome, a complex consisting of more than 50 different proteins and several stmctural RNA molecules, which moves along the mRNA and mediates the binding of the tRNA molecules and the formation of the nascent peptide chain. The tRNA molecule carries an activated form of the specific amino acid to the ribosome where it is added to the end of the growing peptide chain. There is at least one tRNA for each amino acid. 

RNA structures, compared to the helical motifs that dominate DNA, are quite diverse, assuming various loop conformations in addition to helical structures. This diversity allows RNA molecules to assume a wide variety of tertiary structures with many biological functions beyond the storage and propagation of the genetic code. Examples include transfer RNA, which is involved in the translation of mRNA into proteins, the RNA components of ribosomes, the translation machinery, and catalytic RNA molecules. In addition, it is now known that secondary and tertiary elements of mRNA can act to regulate the translation of its own primary sequence. Such diversity makes RNA a prime area for the study of structure-function relationships to which computational approaches can make a significant contribution. 

Three-base codons on die mRNA corresponding to specific amino acids direct the sequence of building a protein. These codons are recognized by tRNAs (transfer RNAs) carrying die appropriate amino acids. Ribosomes are the machinery for protein syn diesis. 

Transfer RNA (tRNA) serves as a carrier of amino acid residues for protein synthesis. Transfer RNA molecules also fold into a characteristic secondary structure (marginal figure). The amino acid is attached as an aminoacyl ester to the 3 -terminus of the tRNA. Aminoacyl-tRNAs are the substrates for protein biosynthesis. The tRNAs are the smallest RNAs (size range—23 to 30 kD) and contain 73 to 94 residues, a substantial number of which are methylated or otherwise unusually modified. Transfer RNA derives its name from its role as the carrier of amino acids during the process of protein synthesis (see Chapters 32 and 33). Each of the 20 amino acids of proteins has at least one unique tRNA species dedicated to chauffeuring its delivery to ribosomes for insertion into growing polypeptide chains, and some amino acids are served by several tRNAs. For example, five different tRNAs act in the transfer of leucine into... 

In the cytoplasm, the mRNA attaches to a ribosome and acts as a template for the construction of a protein with the proper amino acid sequence (a process known as translation ). Single amino acids are brought to the ribosome by transfer RNA molecules (tRNA) and added to the growing amino acid chain in the order instructed by the mRNA. Each time a nucleotide is added to the growing RNA strand, one molecule of ATP is broken down to ADP. Each time a tRNA binds an amino acid and each time the amino acid is added to the protein, additional ATP is broken down to ADP. Because proteins can contain many hundreds of amino acids, the cell must expend the energy in 1,000 or more ATP molecules to build each protein molecule. 

Transfer RNA (tRNA) transports amino acids to the ribosomes, where they are joined together to make proteins. 

Figure 28.7 A representation of protein biosynthesis. The codon base sequences on mRNA are read by tRNAs containing complementary anticodon base sequences. Transfer RNAs assemble the proper amino acids into position for incorporation into the growing peptide.

H Translation is the process by which mRNA directs protein synthesis. Each mRNA is divided into codons, ribonucleotide triplets that are recognized by small amino acid-carrying molecules of transfer RNA (tRNA), which deliver the appropriate amino acids needed for protein synthesis. 

Mitochondria are unique organelles in man and higher animals in that they contain their own genome. Mitochondrial DNA (mtDNA) in humans is a small (16.5 kb), circular genome that encodes only 13 proteins, 22 transfer RNA (tRNA), and 2 ribosomal RNA (rRNA) molecules. mtDNA is inherited only from the mother and is present in multiple copies within one mitochondrion. 

The majority of the peptides in mitochondria (about 54 out of 67) are coded by nuclear genes. The rest are coded by genes found in mitochondrial (mt) DNA. Human mitochondria contain two to ten copies of a smaU circular double-stranded DNA molecule that makes up approximately 1% of total ceUular DNA. This mtDNA codes for mt ribosomal and transfer RNAs and for 13 proteins that play key roles in the respiratory chain. The linearized strucmral map of the human mitochondrial genes is shown in Figure 36-8. Some of the feamres of mtDNA are shown in Table... 

All eukaryotic cells have four major classes of RNA ri-bosomal RNA (rRNA), messenger RNA (mRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). The first three are involved in protein synthesis, and snRNA is involved in mRNA splicing. As shown in Table 37-1, these various classes of RNA are different in their diversity, stability, and abundance in cells. 

Several different types of RNA, including ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA), are involved in protein synthesis. 

Figure 8.4 outlines the proeess of protein synthesis involving the ribosome, ruRNA, a series of aminoacyl transfer RNA (tRNA) moleeules (at least one for eaeh amino aeid)... 

As we have noted, the outcome of a virus infection is the synthesis of viral nucleic acid and viral protein coats. In effect, the virus takes over the biosynthetic machinery of the host and uses it for its own synthesis. A few enzymes needed for virus replication may be present in the virus particle and may be introduced into the cell during the infection process, but the host supplies everything else energy-generating system, ribosomes, amino-acid activating enzymes, transfer RNA (with a few exceptions), and all soluble factors. The virus genome codes for all new proteins. Such proteins would include the coat protein subunits (of which there are generally more than one kind) plus any new virus-specific enzymes. 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

What is the importance of this enzyme family for the biogenesis problem These enzymes form the link between the protein world and the nucleic acid world . They catalyse the reaction between amino acids and transfer RNA molecules, which includes an activation step involving ATR The formation of the peptide bond, i.e., the actual polycondensation reaction, takes place at the ribosome and involves mRNA participation and process control via codon-anticodon interaction. 

Sidney Altman discovered this property of RNA in the course of studies on precursor transfer RNA. It was realized that the catalytic properties of RNA are not exactly the same as those of protein enzymes, since the ribozyme is itself active and thus undergoes change during the catalytic reaction. This does not correspond to the generally accepted definition of an enzyme. Later studies, however, showed that some ribozymes are capable of acting catalytically at other RNA molecules. The ribozymes remain completely unchanged in this process, and thus fulfil the definition of a real enzyme. 

Ribonucleic acid (RNA) Molecules including messenger RNA, transfer RNA, ribosomal RNA, or small RNA. RNA serves as a template for protein synthesis and other biochemical processes of the cell. The structure of RNA is similar to that of DNA except for the base thymidine being replaced by uracil. 

Transfer RNA (tRNA) RNA with a triplet nucleotide sequence that is complementary to the triplet nucleotide coding sequences of mRNA. tRNAs in protein synthesis bond with amino acids and transfer them to the ribosomes, where proteins are assembled according to the genetic code carried by mRNA... 

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