Ribosomes, tRNA, amino acids, cofactors mRNA

Translation f

Ribosomes, tRNA, amino acids, cofactors

COOH Polypeptide f|gi :fi 2-5 Overview of gene expression components; transcription for production of mRNA and translation for production of polypeptide (protein).

(a2PP') functions to open double-stranded DNA at the promoter sequence and use the DNA strand as a template to sequentially add ribonucleotides (ATP, GTP, UIP, and CTP) to form the growing mRNA strand.

Transcription proceeds in a 5' to 3' direction. However, in mRNA the thymine triphosphate (TIP) of DNA is replaced with uracil triphosphate (OTP). Synthesis of the single-stranded mRNA product ends when specific nucleotide base sequences on the DNA template are encountered. In some instances, termination of transcription may be facilitated by a rho cofactor, which can disrupt the mRNA-RNA polymerase-template DNA complex.

In bacteria, the mRNA molecules that result from the transcription process are polycistronic, that is, they encode for several gene products. Frequently, polycistronic mRNA may encode several genes whose products (proteins) are involved in a single or closely related cellular function. When a cluster of genes is under the control of a single promoter sequence, the gene group is referred to as an operon.

The transcription process not only produces mRNA but also tRNA and rRNA. All three types of RNA have key roles in protein synthesis.

Translation. The next phase in gene expression, translation, involves protein synthesis. By this process the genetic code within mRNA molecules is translated into specific amino acid sequences that are responsible for protein structure, and hence, function (see Figure 2-5).

Before discussing translation, an understanding of the genetic code that is originally transcribed from DNA to mRNA and then translated from mRNA to protein is warranted. The code consists of triplets of nucleotide bases, referred to as codons; each codon encodes for a specific amino acid. Because there are 64 different codons for 20 amino acids, an amino acid can be encoded by more than one codon (Table 2-1). However, each codon specifies only one amino acid. Therefore, through translation the codon sequences in mRNA direct which amino acids are added and in what order. Translation ensures that proteins with proper structure and function are produced. Errors in the process can result in aberrant proteins that are nonfunctional, underscoring the need for translation to be well controlled and accurate.

To accomplish the task of translation, intricate interactions between mRNA, tRNA, and rRNA are required. Sixty different types of tRNA molecules are responsible for transferring different amino acids from intracellular reservoirs to the site of protein synthesis. These molecules, whose structure resembles an inverted t, contain one sequence recognition site for binding to specific three-base sequences (codons) on the mRNA molecule (Figure 2-6), A second site binds specific amino acids, the building blocks of proteins. Each amino acid is joined to a specific tRNA molecule via the enzymatic activity of aminoacyl-tRNA synthetases. Therefore, tRNA molecules have the primary function of using the codons of the mRNA molecule as the template for precisely delivering a specific amino acid for polymerization. Ribosomes, composed of rRNA and proteins, also are central to translation and provide the site at which translation occurs.

Translation, diagrammatically shown in Figure 2-6, involves three steps: initiation, elongation, and termination. Following termination, bacterial proteins often undergo posttranslational modifications as a final step in protein synthesis.

Initiation begins with the association of ribo-somal subunits, mRNA, formylmethionine tRNA (f-met; carrying the initial amino acid of the protein to be synthesized), and various initiation factors (see Figure 2-6, A). Assembly of the complex begins at a specific 3- to 9-base sequence on the mRNA referred to as the ribosomal binding site, or RBS. After the initial complex is formed, addition of individual amino acids begins.

Elongation involves tRNAs mediating the sequential addition of amino acids in a specific sequence that is dictated by the codon sequence of the mRNA molecule (see Figure 2-6, B to C, and Table 2-1). As the mRNA molecule threads through the ribosome in a 5r to 3' direction, peptide bonds are formed between adjacent amino acids still bound by their respective

Table 2-1 The Genetic Code as Expressed by Triplet Base Sequences of mRNA*

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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