Structure That Is The Site Of Protein Synthesis – Gene expression is the process by which genetic information flows from DNA to RNA to protein. The translation of DNA into RNA is called transcription; the synthesis of proteins from RNA templates is called translation. Details on gene expression and transcription can be found in a separate article.

Translation is carried out by ribosomes, which are large molecular complexes of ribosomal RNA (rRNA) and proteins. Ribosomes bind to RNA templates, also called messenger RNA (mRNA), and catalyze the formation of a polypeptide based on this template. In the process, a charged transfer RNA (tRNA) recognizes an mRNA nucleotide triplet that matches a specific amino acid (AA). The new AA is then linked to the next AA in the growing ribosome polypeptide. Translation ends when a specific nucleotide sequence in the mRNA (a stop codon) is reached. Subsequently, the ribosome dissociates and the newly synthesized mRNA and protein are released. Before proteins are functional, they need a proper shape and destination. Proteins begin to fold into their three-dimensional structure during translation according to the AA sequence and local chemical forces and reactions. Various specialized proteins (folding catalysts, chaperones) also help newly formed proteins to fold correctly and reach their correct destinations (eg, cytosol, organelles, extracellular matrix) through protein modifications. The rate of protein translation adjusts to current cell conditions and body demands, and is affected by the presence or absence of certain nutrients.

Structure That Is The Site Of Protein Synthesis

Translation occurs in three phases in a functional ribosome: initiation, elongation and termination; . It requires mRNA, tRNA and rRNA.

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Eukaryotes have ribosomal subunits numbered E ven (40S + 60S → 80S). PrO karyotes have ribosomal subunits numbered in O dd (30S + 50S → 70S).

For binding sites, think of a growing APE party: • Growth = GTP as energy source • A site: Arrival with Aminoacyl -tRNA • P site: peptide pool • E site: party over and empty ;tRNA E hits

ATP to activate (load) the tRNA and GTP for the G tRNA to be ripped off and passed through the ribosome (translocation) to grow a polypeptide.

Sugar attachment to the asparagine residue of proteins (ie, N-linked glycosylation) begins in the rough ER.

Brick Exchange • Brick

Enzymatic glycosylation should not be confused with non-enzymatic glycosylation. In glycation, aldoses (eg glucose) spontaneously attach to the amino groups of proteins and can influence their function. A classic example is HbA1c, whose function is not affected by glycation.

The reversible enzymatic modification of proteins alters the spatial structure (conformation) of the protein, thus allowing its activity to be regulated. For example, a protein may interact with other proteins and/or become recognizable as a substrate. Reversible protein modification essentially allows the protein to be turned on or off.

The predicted final destination of a protein depends on its signal sequence (if it has one) at the N-terminus and determines whether translation is terminated on free ribosomes or ribosomes in the rough ER.

If SRP is absent or dysfunctional, there will be an accumulation of proteins in the cytosol of the cell! Protein synthesis consumes more energy from a cell than any other metabolic process. In turn, proteins account for more mass than any other macromolecule in living organisms. They perform practically all the functions of a cell, serving as both functional (for example, enzymes) and structural elements. The process of translation, or protein synthesis, the second part of gene expression, involves the decoding by a ribosome of an mRNA message into a polypeptide product.

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Translation of the mRNA template converts the nucleotide-based genetic information into the “language” of amino acids to create a protein product. A protein sequence consists of 20 common amino acids. Each amino acid is defined within the mRNA by a triplet of nucleotides called a codon. The relationship between an mRNA codon and its corresponding amino acid is called the genetic code.

, with four different nucleotides possible at each of the three different positions within the codon). This number is greater than the number of amino acids, and a given amino acid is encoded by more than one codon ( Figure 1 ). This redundancy in the genetic code is called degeneracy. Normally, while the first two positions of a codon are important in determining which amino acid will be incorporated into a growing polypeptide, the third position, called the wobble position, is less critical. In some cases, if the nucleotide in the third position is changed, the same amino acid is still incorporated.

While 61 of the 64 possible triplets code for amino acids, three of the 64 codons do not code for an amino acid; they terminate protein synthesis, releasing the polypeptide from the translation machinery. These are called stop codons or nonsense codons. Another codon, AUG, also has a special function. In addition to specifying the amino acid methionine, it also normally serves as the start codon to initiate translation. The reading frame, the way mRNA nucleotides are grouped into codons, for translation, is defined by the AUG start codon near the 5′ end of the mRNA. Each set of three nucleotides following this start codon is a codon in the mRNA message.

The genetic code is almost universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all life on Earth shares a common origin. However, unusual amino acids such as selenocysteine ​​and pyrrolysine have been observed in archaea and bacteria. In the case of selenocysteine, the codon used is UGA (usually a stop codon). However, UGA can encode selenocysteine ​​via a stem-loop structure (known as a selenocysteine ​​insertion sequence or SECIS element), which is located in the 3′ untranslated region of the mRNA. Pyrrolysine uses a different stop codon, UAG. The incorporation of pyrrolysine requires the

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Figure 1. This figure shows the genetic code for translating each triplet of nucleotides in mRNA to an amino acid or termination signal in a nascent protein. The first letter of a codon is displayed vertically on the left, the second letter of a codon is displayed horizontally on top, and the third letter of a codon is displayed vertically on the right. (credit: work modified by the National Institutes of Health)

In addition to the mRNA template, many molecules and macromolecules contribute to the translation process. The composition of each component varies according to taxa; for example, ribosomes can consist of different numbers of ribosomal RNA (rRNA) and polypeptides depending on the organism. However, the general structures and functions of the protein synthesis machinery are comparable from bacteria to human cells. Translation requires the input of an mRNA template, ribosomes, tRNA, and various enzymatic factors.

A ribosome is a complex macromolecule made up of catalytic rRNAs (called ribozymes) and structural rRNAs, as well as many different polypeptides. Mature rRNAs make up approximately 50% of each ribosome. Prokaryotes have 70S ribosomes, while eukaryotes have 80S ribosomes in the cytoplasm and rough endoplasmic reticulum, and 70S ribosomes in mitochondria and chloroplasts. Ribosomes dissociate into large and small subunits when they are not synthesizing proteins and reassemble during the initiation of translation. In

, the small subunit is described as 30S (containing the 16S rRNA subunit) and the large subunit is 50S (containing the 5S and 23S rRNA subunits), for a total of 70S (Svedberg units are not additives). Eukaryotic ribosomes have a small 40S subunit (containing the 18S rRNA subunit) and a large 60S subunit (containing the 5S, 5.8S, and 28S rRNA subunits), for a total of 80S. The small subunit is responsible for binding the mRNA template, while the large subunit binds tRNAs (discussed in the next subsection).

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Each mRNA molecule is translated simultaneously by many ribosomes, all synthesizing protein in the same direction: reading the mRNA from 5′ to 3′ and synthesizing the polypeptide from the N terminus to the C terminus. The structure complete containing an mRNA with multiple associated ribosomes is called a polyribosome (or polysome). In both bacteria and archaea, before transcription termination occurs, each protein-coding transcript is already being used to begin the synthesis of numerous copies of the encoded polypeptides because the processes of transcription and translation can occur simultaneously, forming polyribosomes (Figure 2). . The reason transcription and translation can occur simultaneously is because both processes occur in the same 5′ to 3′ direction, both occur in the cytoplasm of the cell, and because RNA transcription does not it is processed once transcribed. This allows a prokaryotic cell to respond to an environmental signal that requires new proteins very quickly. In contrast, in eukaryotic cells, simultaneous transcription and translation is not possible. Although polyribosomes also form in eukaryotes, they cannot do so until RNA synthesis has been completed and the RNA molecule has been modified and transported out of the nucleus.

Figure 2. In prokaryotes, multiple RNA polymerases can transcribe a single bacterial gene while numerous ribosomes simultaneously translate mRNA transcripts into polypeptides. In this way, a specific protein can quickly reach a high concentration in the bacterial cell.

Transfer RNAs (tRNAs) are structural RNA molecules and, depending on the

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