Protein translation

Learning Objectives

  • Describe the genetic code and explain why it is considered almost universal
  • Explain the process of translation and the functions of the molecular machinery of translation
  • Compare translation in eukaryotes and prokaryotes

The synthesis of proteins consumes more of a cell’s energy than any other metabolic process. In turn, proteins account for more mass than any other macromolecule of living organisms. They perform virtually every function of a cell, serving as both functional (e.g., 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.

The Genetic Code

Translation of the mRNA template converts nucleotide-based genetic information into the “language” of amino acids to create a protein product. A protein sequence consists of 20 commonly occurring 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.

The three-nucleotide code means that there is a total of 64 possible combinations (43, 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. Typically, whereas the first two positions in a codon are important for 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.

Whereas 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 typically serves as the start codon to initiate translation. The reading frame, the way nucleotides in mRNA are grouped into codons, for translation is set 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 nearly universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all extant 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 (normally a stop codon). However, UGA can encode for selenocysteine using a stem-loop structure (known as the selenocysteine insertion sequence, or SECIS element), which is found at the 3′ untranslated region of the mRNA. Pyrrolysine uses a different stop codon, UAG. The incorporation of pyrrolysine requires the pylS gene and a unique transfer RNA (tRNA) with a CUA anticodon.

The codon table. On the left is the first letter of the codon (from top to bottom – U, C, A, G). On the top is the second letter (left to right U, C, A, G). On the right is the third letter (in each row, this is designated from top to bottom as U, C, A, G. UUU and UUC are Phe. UUA and UUG are Leu. UCU, UCC, UCA and UCG are Ser. UAU and UAC are Tyr. UAA and UAG are stop. UGU and UGC are Cys. UGA is stop. UGG is Trp. CUU, CUC, CUA, and CUG are Leu. CC, CCC, CCA, and CCG are Pro. CAU and CAC are his. CAA and CAG are Gln. CGU, CGC, CGA, CGG are Arg. AUU, AUC, AUA are Ile, AUG is Met and start. ACU, ACC, ACA, ACG is Thr. AAU AAc, is Asn. AAA, AAG is Lys. AGU, AGC is SEr. AGA, AG is ARg. GUU, GUC, GUA, GUG is Val. GCU, GCC, GCA, GCG, is ala. GAU, GAC is Asp. GAA, GAG is Glu. GGU, GGC, GGA, GGG is Gly.

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

Think about It

  • How many bases are in each codon?
  • What amino acid is coded for by the codon AAU?
  • What happens when a stop codon is reached?

The Protein Synthesis Machinery

In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation. The composition of each component varies across taxa; for instance, ribosomes may consist of different numbers of ribosomal RNAs (rRNAs) 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, tRNAs, and various enzymatic factors.


A ribosome is a complex macromolecule composed of catalytic rRNAs (called ribozymes) and structural rRNAs, as well as many distinct polypeptides. Mature rRNAs make up approximately 50% of each ribosome. Prokaryotes have 70S ribosomes, whereas 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 reassociate during the initiation of translation. In E. coli, the small subunit is described as 30S (which contains the 16S rRNA subunit), and the large subunit is 50S (which contains the 5S and 23S rRNA subunits), for a total of 70S (Svedberg units are not additive). Eukaryote ribosomes have a small 40S subunit (which contains the 18S rRNA subunit) and a large 60S subunit (which contains the 5S, 5.8S and 28S rRNA subunits), for a total of 80S. The small subunit is responsible for binding the mRNA template, whereas the large subunit binds tRNAs (discussed in the next subsection).

Each mRNA molecule is simultaneously translated 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 complete structure containing an mRNA with multiple associated ribosomes is called a polyribosome (or polysome). In both bacteria and archaea, before transcriptional termination occurs, each protein-encoding transcript is already being used to begin synthesis of numerous copies of the encoded polypeptide(s) because the processes of transcription and translation can occur concurrently, forming polyribosomes (Figure 2). The reason why transcription and translation can occur simultaneously is because both of these processes occur in the same 5′ to 3′ direction, they both occur in the cytoplasm of the cell, and because the RNA transcript is not processed once it is transcribed. This allows a prokaryotic cell to respond to an environmental signal requiring 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 is complete and the RNA molecule has been modified and transported out of the nucleus.

Diagram showing a double strand of DNA with RNA polymerase and a newly forming RNA strand. As the RNA elongates ribosomes bind and begin forming proteins. As the RNA gets longer, more and more ribosomes are bound in a row; this is called a polyribosome.

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

Transfer RNAs

Transfer RNAs (tRNAs) are structural RNA molecules and, depending on the species, many different types of tRNAs exist in the cytoplasm. Bacterial species typically have between 60 and 90 types. Serving as adaptors, each tRNA type binds to a specific codon on the mRNA template and adds the corresponding amino acid to the polypeptide chain. Therefore, tRNAs are the molecules that actually “translate” the language of RNA into the language of proteins. As the adaptor molecules of translation, it is surprising that tRNAs can fit so much specificity into such a small package. The tRNA molecule interacts with three factors: aminoacyl tRNA synthetases, ribosomes, and mRNA.

Mature tRNAs take on a three-dimensional structure when complementary bases exposed in the single-stranded RNA molecule hydrogen bond with each other (Figure 3). This shape positions the amino-acid binding site, called the CCA amino acid binding end, which is a cytosine-cytosine-adenine sequence at the 3′ end of the tRNA, and the anticodon at the other end. The anticodon is a three-nucleotide sequence that bonds with an mRNA codon through complementary base pairing.

An amino acid is added to the end of a tRNA molecule through the process of tRNA “charging,” during which each tRNA molecule is linked to its correct or cognate amino acid by a group of enzymes called aminoacyl tRNA synthetases. At least one type of aminoacyl tRNA synthetase exists for each of the 20 amino acids. During this process, the amino acid is first activated by the addition of adenosine monophosphate (AMP) and then transferred to the tRNA, making it a charged tRNA, and AMP is released.

Three different drawings of tRNA. A) shows a single strand folded into a cross shape with intramolecular base pairing. The 3′ end at the top is labeled amino acid attachment site and has the sequence ACC. The 5′ end is also at the top. At the base of the cross is a three letter grouping called anticodon. This is complementary to a three letter set on the mRNA called a codon. B) shows a space filling 3-D model that is shaped like an L. One end is the amino acid attachment site and the other is the anticodon. C) is a ver simplified drawing shaped like zigzag; one end is the amino acid attachment site and the other is the anticodon.

Figure 3. (a) After folding caused by intramolecular base pairing, a tRNA molecule has one end that contains the anticodon, which interacts with the mRNA codon, and the CCA amino acid binding end. (b) A space-filling model is helpful for visualizing the three-dimensional shape of tRNA. (c) Simplified models are useful when drawing complex processes such as protein synthesis.

Think about It

  • Describe the structure and composition of the prokaryotic ribosome.
  • In what direction is the mRNA template read?
  • Describe the structure and function of a tRNA.

The Mechanism of Protein Synthesis

Translation is similar in prokaryotes and eukaryotes. Here we will explore how translation occurs in E. coli, a representative prokaryote, and specify any differences between bacterial and eukaryotic translation.


The initiation of protein synthesis begins with the formation of an initiation complex. In E. coli, this complex involves the small 30S ribosome, the mRNA template, three initiation factors that help the ribosome assemble correctly, guanosine triphosphate (GTP) that acts as an energy source, and a special initiator tRNA carrying N-formyl-methionine (fMet-tRNAfMet) (Figure 4). The initiator tRNA interacts with the start codon AUG of the mRNA and carries a formylated methionine (fMet). Because of its involvement in initiation, fMet is inserted at the beginning (N terminus) of every polypeptide chain synthesized by E. coli. In E. coli mRNA, a leader sequence upstream of the first AUG codon, called the Shine-Dalgarno sequence (also known as the ribosomal binding site AGGAGG), interacts through complementary base pairing with the rRNA molecules that compose the ribosome. This interaction anchors the 30S ribosomal subunit at the correct location on the mRNA template. At this point, the 50S ribosomal subunit then binds to the initiation complex, forming an intact ribosome.

In eukaryotes, initiation complex formation is similar, with the following differences:

  • The initiator tRNA is a different specialized tRNA carrying methionine, called Met-tRNAi
  • Instead of binding to the mRNA at the Shine-Dalgarno sequence, the eukaryotic initiation complex recognizes the 5′ cap of the eukaryotic mRNA, then tracks along the mRNA in the 5′ to 3′ direction until the AUG start codon is recognized. At this point, the 60S subunit binds to the complex of Met-tRNAi, mRNA, and the 40S subunit.
Diagram showing translation. At the start codon of the mRNA (AUG) the following attach: a tRNA with the anticodon UAC and containing the first amino acid, the large ribosomal subunit (a dome) and the small ribosomal subunit (a flat oval). During initiation, translational complex forms, and tRNA brings the first amino acid in polypeptide chain to bind to start codon om mRNA. At this point the tRNA is attached to the middle binding site (P) of the ribosome. The 3 sites from left to right are E, P, A. During elongation, tRNAs bring amino acids one by one to add to polypeptide chain. In the diagram, a tRNA with a long chain of circles is in the P site, a tRNA with a single circle is in the A site, and a tRNA without any circles is leaving from the E site. During termination, release factor recognizes stop codon, translational complex dissociates, and complete polypeptide is released. In the diagram a tRNA with a long strand is attached to the P site and a release factor (red shape) is attached to the stop codon in the mRNA which is now under the A site. Next the completed polypeptide leaves and all the other components dissociate from each other.

Figure 4. Translation in bacteria begins with the formation of the initiation complex, which includes the small ribosomal subunit, the mRNA, the initiator tRNA carrying N-formyl-methionine, and initiation factors. Then the 50S subunit binds, forming an intact ribosome.


In prokaryotes and eukaryotes, the basics of elongation of translation are the same. In E. coli, the binding of the 50S ribosomal subunit to produce the intact ribosome forms three functionally important ribosomal sites: The A (aminoacyl) site binds incoming charged aminoacyl tRNAs. The P (peptidyl) site binds charged tRNAs carrying amino acids that have formed peptide bonds with the growing polypeptide chain but have not yet dissociated from their corresponding tRNA. The E (exit) site releases dissociated tRNAs so that they can be recharged with free amino acids. There is one notable exception to this assembly line of tRNAs: During initiation complex formation, bacterial fMet−tRNAfMet or eukaryotic Met-tRNAi enters the P site directly without first entering the A site, providing a free A site ready to accept the tRNA corresponding to the first codon after the AUG.

Elongation proceeds with single-codon movements of the ribosome each called a translocation event. During each translocation event, the charged tRNAs enter at the A site, then shift to the P site, and then finally to the E site for removal. Ribosomal movements, or steps, are induced by conformational changes that advance the ribosome by three bases in the 3′ direction. Peptide bonds form between the amino group of the amino acid attached to the A-site tRNA and the carboxyl group of the amino acid attached to the P-site tRNA. The formation of each peptide bond is catalyzed by peptidyl transferase, an RNA-based ribozyme that is integrated into the 50S ribosomal subunit. The amino acid bound to the P-site tRNA is also linked to the growing polypeptide chain. As the ribosome steps across the mRNA, the former P-site tRNA enters the E site, detaches from the amino acid, and is expelled. Several of the steps during elongation, including binding of a charged aminoacyl tRNA to the A site and translocation, require energy derived from GTP hydrolysis, which is catalyzed by specific elongation factors. Amazingly, the E. coli translation apparatus takes only 0.05 seconds to add each amino acid, meaning that a 200 amino-acid protein can be translated in just 10 seconds.


The termination of translation occurs when a nonsense codon (UAA, UAG, or UGA) is encountered for which there is no complementary tRNA. On aligning with the A site, these nonsense codons are recognized by release factors in prokaryotes and eukaryotes that result in the P-site amino acid detaching from its tRNA, releasing the newly made polypeptide. The small and large ribosomal subunits dissociate from the mRNA and from each other; they are recruited almost immediately into another translation init iation complex.

In summary, there are several key features that distinguish prokaryotic gene expression from that seen in eukaryotes. These are illustrated in Figure 6 and listed in Table 1.

a) Diagram of prokaryotic cell with a plasma membrane on the outside. The DNA is in the cytoplasm and the mRNA is being copied at the same time that ribosomes are building proteins of the developing mRNA. B) Diagram of a eukaryotic cell with a plasma membrane an a nucleus. The DNA is in the nucleus and pre-mRNA is made during transcription; this is then process into mature mRNA. The mature mRNA then leaves the nucleus and enters the cytoplasm where translation takes place. This is when ribosomes bind to the mRNA and make proteins.

Figure 6. (a) In prokaryotes, the processes of transcription and translation occur simultaneously in the cytoplasm, allowing for a rapid cellular response to an environmental cue. (b) In eukaryotes, transcription is localized to the nucleus and translation is localized to the cytoplasm, separating these processes and necessitating RNA processing for stability.

Table 1. Comparison of Translation in Bacteria versus Eukaryotes


30S (small subunit) with 16S rRNA subunit

50S (large subunit) with 5S and 23S rRNA subunits


40S (small subunit) with 18S rRNA subunit

60S (large subunit) with 5S, 5.8S, and 28S rRNA subunits

Amino acid carried by initiator tRNAfMetMet
Shine-Dalgarno sequence in mRNAPresentAbsent
Simultaneous transcription and translationYesNo

Protein Targeting, Folding, and Modification

During and after translation, polypeptides may need to be modified before they are biologically active. Post-translational modifications include:

  1. removal of translated signal sequences—short tails of amino acids that aid in directing a protein to a specific cellular compartment
  2. proper “folding” of the polypeptide and association of multiple polypeptide subunits, often facilitated by chaperone proteins, into a distinct three-dimensional structure
  3. proteolytic processing of an inactive polypeptide to release an active protein component, and
  4. various chemical modifications (e.g., phosphorylation, methylation, or glycosylation) of individual amino acids.

Think about It

  • What are the components of the initiation complex for translation in prokaryotes?
  • What are two differences between initiation of prokaryotic and eukaryotic translation?
  • What occurs at each of the three active sites of the ribosome?
  • What causes termination of translation?

Key Concepts and Summary

  • In translation, polypeptides are synthesized using mRNA sequences and cellular machinery, including tRNAs that match mRNA codons to specific amino acids and ribosomes composed of RNA and proteins that catalyze the reaction.
  • The genetic code is degenerate in that several mRNA codons code for the same amino acids. The genetic code is almost universal among living organisms.
  • Prokaryotic (70S) and cytoplasmic eukaryotic (80S) ribosomes are each composed of a large subunit and a small subunit of differing sizes between the two groups. Each subunit is composed of rRNA and protein. Organelle ribosomes in eukaryotic cells resemble prokaryotic ribosomes.
  • Some 60 to 90 species of tRNA exist in bacteria. Each tRNA has a three-nucleotide anticodon as well as a binding site for a cognate amino acid. All tRNAs with a specific anticodon will carry the same amino acid.
  • Initiation of translation occurs when the small ribosomal subunit binds with initiation factors and an initiator tRNA at the start codon of an mRNA, followed by the binding to the initiation complex of the large ribosomal subunit.
  • In prokaryotic cells, the start codon codes for N-formyl-methionine carried by a special initiator tRNA. In eukaryotic cells, the start codon codes for methionine carried by a special initiator tRNA. In addition, whereas ribosomal binding of the mRNA in prokaryotes is facilitated by the Shine-Dalgarno sequence within the mRNA, eukaryotic ribosomes bind to the 5′ cap of the mRNA.
  • During the elongation stage of translation, a charged tRNA binds to mRNA in the A site of the ribosome; a peptide bond is catalyzed between the two adjacent amino acids, breaking the bond between the first amino acid and its tRNA; the ribosome moves one codon along the mRNA; and the first tRNA is moved from the P site of the ribosome to the E site and leaves the ribosomal complex.
  • Termination of translation occurs when the ribosome encounters a stop codon, which does not code for a tRNA. Release factors cause the polypeptide to be released, and the ribosomal complex dissociates.
  • In prokaryotes, transcription and translation may be coupled, with translation of an mRNA molecule beginning as soon as transcription allows enough mRNA exposure for the binding of a ribosome, prior to transcription termination. Transcription and translation are not coupled in eukaryotes because transcription occurs in the nucleus, whereas translation occurs in the cytoplasm or in association with the rough endoplasmic reticulum.
  • Polypeptides often require one or more post-translational modifications to become biologically active.

Multiple Choice

Which of the following is the name of the three-base sequence in the mRNA that binds to a tRNA molecule?

  1. P site
  2. codon
  3. anticodon
  4. CCA binding site
Show Answer

Answer b. A codon is the name of the three-base sequence in the mRNA that binds to a tRNA molecule.

Which component is the last to join the initiation complex during the initiation of translation?

  1. the mRNA molecule
  2. the small ribosomal subunit
  3. the large ribosomal subunit
  4. the initiator tRNA
Show Answer

Answer c. The large ribosomal subunit is the component last to join the initiation complex during the initiation of translation.

During elongation in translation, to which ribosomal site does an incoming charged tRNA molecule bind?

  1. A site
  2. P site
  3. E site
  4. B site
Show Answer

Answer a. During elongation in translation an incoming charged tRNA molecule binds to the A site.

Which of the following is the amino acid that appears at the N-terminus of all newly translated prokaryotic and eukaryotic polypeptides?

  1. tryptophan
  2. methionine
  3. selenocysteine
  4. glycine
Show Answer

Answer b. Methionine appears at the N-terminus of all newly translated prokaryotic and eukaryotic polypeptides.

When the ribosome reaches a nonsense codon, which of the following occurs?

  1. a methionine is incorporated
  2. the polypeptide is released
  3. a peptide bond forms
  4. the A site binds to a charged tRNA
Show Answer

Answer b. When the ribosome reaches a nonsense codon the polypeptide is released.

Fill in the Blank

The third position within a codon, in which changes often result in the incorporation of the same amino acid into the growing polypeptide, is called the ________.

Show Answer

The third position within a codon, in which changes often result in the incorporation of the same amino acid into the growing polypeptide, is called the wobble position.

The enzyme that adds an amino acid to a tRNA molecule is called ________.

Show Answer

The enzyme that adds an amino acid to a tRNA molecule is called aminoacyl-tRNA synthetase.


Each codon within the genetic code encodes a different amino acid.

Think about It

  1. Why does translation terminate when the ribosome reaches a stop codon? What happens?
  2. How does the process of translation differ between prokaryotes and eukaryotes?
  3. What is meant by the genetic code being nearly universal?
  4. Below is an antisense DNA sequence. Translate the mRNA molecule synthesized using the genetic code, recording the resulting amino acid sequence, indicating the N and C termini.Antisense DNA strand: 3′-T A C T G A C T G A C G A T C-5′
  5. Prior to the elucidation of the genetic code, prominent scientists, including Francis Crick, had predicted that each mRNA codon, coding for one of the 20 amino acids, needed to be at least three nucleotides long. Why is it not possible for codons to be any shorter?
  6. Label the following in the figure: ribosomal E, P, and A sites; mRNA; codons; anticodons; growing polypeptide; incoming amino acid; direction of translocation; small ribosomal unit; large ribosomal unit.
A long line at the bottom has many letters on it and is labeled G. Each set of 3 letters is circles and labeled D. Attached to this is a large dome (I) and a small oval (3). Attached to one group of 3 letters is a rectangle 9labeled C); the 3 letters at the bottom of the rectangle are labeled H. A circle at the end of the rectangle is labeled B. The rectangle in the center of the dome labeled I has a long chain of circles attached to it labeled A. An arrow at the bottom points to the right and is labeled F.

​Translation Play Audio

"Translation" literally means "to carry across"; that's what translation means. In this case, what is being carried across is information that originally was in the genome, enshrined in DNA, then gets transcribed into messenger RNA. And then that information is translated from the messenger RNA to a protein. So we're taking the same information, but it's going from one form to another; a nucleic acid code to an amino acid code in a protein. That translation is done not in individual letters. It's very much like the human language or any other language that, in this case, all the words are the same length. They're all three words long, and the reader in this case is something called a ribosome, which is this big, multi-subunit molecular machine that travels along the mRNA, and it reads much like a person reading Braille does. It reads along, detects what are these letters underneath it, and when it detects what those three letters are, it decides what the amino acid is supposed to be that it adds to the growing amino acid chain, polypeptide chain, to become a protein. Those mRNA letters are called a codon, and each codon codes for a different amino acid. And eventually those amino acids are all joined together to assemble a protein.

Christopher P. Austin, M.D.

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How do genes direct the production of proteins?

Most genes contain the information needed to make functional molecules called proteins. (A few genes produce regulatory molecules that help the cell assemble proteins.) The journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps: transcription and translation. Together, transcription and translation are known as gene expression.

During the process of transcription, the information stored in a gene's DNA is passed to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. Both RNA and DNA are made up of a chain of building blocks called nucleotides, but they have slightly different chemical properties. The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm.

Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. The mRNA interacts with a specialized complex called a ribosome, which "reads" the sequence of mRNA nucleotides. Each sequence of three nucleotides, called a codon, usually codes for one particular amino acid. (Amino acids are the building blocks of proteins.) A type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time. Protein assembly continues until the ribosome encounters a “stop” codon (a sequence of three nucleotides that does not code for an amino acid).

The flow of information from DNA to RNA to proteins is one of the fundamental principles of molecular biology. It is so important that it is sometimes called the “central dogma.”


The translation of mRNA begins with the formation of a complex on the mRNA (Figure 4). First, three initiation factor proteins (known as IF1, IF2, and IF3) bind to the small subunit of the ribosome. This preinitiation complex and a methionine-carrying tRNA then bind to the mRNA, near the AUG start codon, forming the initiation complex.
A schematic shows the formation of an initiation complex on an mRNA molecule in two stages. A summary diagram above the schematic shows the transcription and translation processes as two basic steps. The transcription step in the summary diagram is greyed out; the translation step is contained in a box to show it has been represented in more detail in the schematic below it. The schematic shows a segment of an mRNA molecule made up of 24 nucleotides. Each nucleotide is represented as a colored rectangle and is designated with the letter A, U, G, or C. The first stage shows the binding of the small ribosomal subunit, and the second stage shows the binding of an initiator tRNA carrying a methionine residue.

Figure 4: The translation initiation complex.

When translation begins, the small subunit of the ribosome and an initiator tRNA molecule assemble on the mRNA transcript. The small subunit of the ribosome has three binding sites: an amino acid site (A), a polypeptide site (P), and an exit site (E). The initiator tRNA molecule carrying the amino acid methionine binds to the AUG start codon of the mRNA transcript at the ribosome’s P site where it will become the first amino acid incorporated into the growing polypeptide chain. Here, the initiator tRNA molecule is shown binding after the small ribosomal subunit has assembled on the mRNA; the order in which this occurs is unique to prokaryotic cells. In eukaryotes, the free initiator tRNA first binds the small ribosomal subunit to form a complex. The complex then binds the mRNA transcript, so that the tRNA and the small ribosomal subunit bind the mRNA simultaneously.

Figure Detail

Although methionine (Met) is the first amino acid incorporated into any new protein, it is not always the first amino acid in mature proteins—in many proteins, methionine is removed after translation. In fact, if a large number of proteins are sequenced and compared with their known gene sequences, methionine (or formylmethionine) occurs at the N-terminusof all of them. However, not all amino acids are equally likely to occur second in the chain, and the second amino acid influences whether the initial methionine is enzymatically removed. For example, many proteins begin with methionine followed by alanine. In both prokaryotes and eukaryotes, these proteins have the methionine removed, so that alanine becomes the N-terminalamino acid (Table 1). However, if the second amino acid is lysine, which is also frequently the case, methionine is not removed (at least in the sample proteins that have been studied thus far). These proteins therefore begin with methionine followed by lysine (Flinta et al., 1986).

Table 1 shows the N-terminal sequences of proteins in prokaryotes and eukaryotes, based on a sample of 170 prokaryotic and 120 eukaryotic proteins (Flinta et al., 1986). In the table, M represents methionine, A represents alanine, K represents lysine, S represents serine, and T represents threonine.

Table 1: N-Terminal Sequences of Proteins

N-Terminal SequencePercent of Prokaryotic Proteins with This SequencePercent of Eukaryotic Proteins with This Sequence
MA* 28.24% 19.17%
MK** 10.59% 2.50%
MS* 9.41% 11.67%
MT* 7.65% 6.67%

* Methionine was removed in all of these proteins

** Methionine was not removed from any of these proteins

Once the initiation complex is formed on the mRNA, the large ribosomal subunit binds to this complex, which causes the release of IFs (initiation factors). The large subunit of the ribosome has three sites at which tRNA molecules can bind. The A (amino acid) site is the location at which the aminoacyl-tRNA anticodon base pairs up with the mRNA codon, ensuring that correct amino acid is added to the growing polypeptide chain. The P (polypeptide) site is the location at which the amino acid is transferred from its tRNA to the growing polypeptide chain. Finally, the E (exit) site is the location at which the "empty" tRNA sits before being released back into the cytoplasm to bind another amino acid and repeat the process. The initiator methionine tRNA is the only aminoacyl-tRNA that can bind in the P site of the ribosome, and the A site is aligned with the second mRNA codon. The ribosome is thus ready to bind the second aminoacyl-tRNA at the A site, which will be joined to the initiator methionine by the first peptide bond (Figure 5).

A schematic diagram shows the fully assembled translation initiation complex, which includes the ribosome and an initiator tRNA, bound to an mRNA molecule. The ribosome has two subunits, which are shown in gray. The large subunit is depicted as an oval bound to the top of the mRNA. The small subunit is shown as an elongated oval approximately one-third the width of the large subunit, and is bound to the bottom of the mRNA. The ribosome has three binding sites from left to right: an E (exit) site, a P (polypeptide) site, and an A (amino acid) site. An initiator tRNA molecule is shown in the P site, where its UAC nucleotide sequence is bound to the AUG nucleotide sequence on the mRNA. The mRNA is shown with its five-prime end to the left, and the three-prime end to the right. This means that the E site of the ribosome is closer to the five-prime end of the mRNA and the A site of the ribosome is closer to the three-prime end of the mRNA. Translation will proceed from the five-prime to the three-prime end of the mRNA.

Figure 5: The large ribosomal subunit binds to the small ribosomal subunit to complete the initiation complex.

The initiator tRNA molecule, carrying the methionine amino acid that will serve as the first amino acid of the polypeptide chain, is bound to the P site on the ribosome. The A site is aligned with the next codon, which will be bound by the anticodon of the next incoming tRNA.


Translation protein

Translation (biology)

Cellular process of protein synthesis

Overview of the translation of eukaryotic messenger RNA
Diagram showing the translation of mRNA and the synthesis of proteins by a ribosome
Initiation and elongation stages of translation as seen through zooming in on the nitrogenous bases in RNA, the ribosome, the tRNA, and amino acids, with short explanations.
The three phases of translation initiation polymerase binds to the DNA strand and moves along until the small ribosomal subunit binds to the DNA. Elongation is initiated when the large subunit attaches and termination end the process of elongation.

In molecular biology and genetics, translation is the process in which ribosomes in the cytoplasm or endoplasmic reticulum synthesize proteins after the process of transcription of DNA to RNA in the cell's nucleus. The entire process is called gene expression.

In translation, messenger RNA (mRNA) is decoded in a ribosome, outside the nucleus, to produce a specific amino acid chain, or polypeptide. The polypeptide later folds into an active protein and performs its functions in the cell. The ribosome facilitates decoding by inducing the binding of complementarytRNAanticodon sequences to mRNA codons. The tRNAs carry specific amino acids that are chained together into a polypeptide as the mRNA passes through and is "read" by the ribosome.

Translation proceeds in three phases:

  1. Initiation: The ribosome assembles around the target mRNA. The first tRNA is attached at the start codon.
  2. Elongation: The last tRNA validated by the small ribosomal subunit (accommodation) transfers the amino acid it carries to the large ribosomal subunit which binds it to the one of the precedingly admitted tRNA (transpeptidation). The ribosome then moves to the next mRNA codon to continue the process (translocation), creating an amino acid chain.
  3. Termination: When a stop codon is reached, the ribosome releases the polypeptide. The ribosomal complex remains intact and moves on to the next mRNA to be translated.

In prokaryotes (bacteria and archaea), translation occurs in the cytosol, where the large and small subunits of the ribosome bind to the mRNA. In eukaryotes, translation occurs in the cytoplasm or across the membrane of the endoplasmic reticulum in a process called co-translational translocation. In co-translational translocation, the entire ribosome/mRNA complex binds to the outer membrane of the rough endoplasmic reticulum (ER) and the new protein is synthesized and released into the ER; the newly created polypeptide can be stored inside the ER for future vesicle transport and secretion outside the cell, or immediately secreted.

Many types of transcribed RNA, such as transfer RNA, ribosomal RNA, and small nuclear RNA, do not undergo translation into proteins.

A number of antibiotics act by inhibiting translation. These include anisomycin, cycloheximide, chloramphenicol, tetracycline, streptomycin, erythromycin, and puromycin. Prokaryotic ribosomes have a different structure from that of eukaryotic ribosomes, and thus antibiotics can specifically target bacterial infections without any harm to a eukaryotic host's cells.

Basic mechanisms[edit]

Further information: Bacterial translation, Archaeal translation, and Eukaryotic translation

A ribosome translating a protein that is secreted into the endoplasmic reticulum. tRNAs are colored dark blue.
Tertiary structure of tRNA. CCA tailin yellow, Acceptor stemin purple, Variable loopin orange, D armin red, Anticodon armin blue with Anticodonin black, T armin green.

The basic process of protein production is addition of one amino acid at a time to the end of a protein. This operation is performed by a ribosome. A ribosome is made up of two subunits, a small subunit and a large subunit. These subunits come together before translation of mRNA into a protein to provide a location for translation to be carried out and a polypeptide to be produced.[1] The choice of amino acid type to add is determined by an mRNA molecule. Each amino acid added is matched to a three nucleotide subsequence of the mRNA. For each such triplet possible, the corresponding amino acid is accepted. The successive amino acids added to the chain are matched to successive nucleotide triplets in the mRNA. In this way the sequence of nucleotides in the template mRNA chain determines the sequence of amino acids in the generated amino acid chain.[2] Addition of an amino acid occurs at the N-terminus of the peptide and thus translation is said to be carboxyl-to-amino directed.[3]

The mRNA carries genetic information encoded as a ribonucleotide sequence from the chromosomes to the ribosomes. The ribonucleotides are "read" by translational machinery in a sequence of nucleotide triplets called codons. Each of those triplets codes for a specific amino acid.

The ribosome molecules translate this code to a specific sequence of amino acids. The ribosome is a multisubunit structure containing rRNA and proteins. It is the "factory" where amino acids are assembled into proteins. tRNAs are small noncoding RNA chains (74–93 nucleotides) that transport amino acids to the ribosome. tRNAs have a site for amino acid attachment, and a site called an anticodon. The anticodon is an RNA triplet complementary to the mRNA triplet that codes for their cargo amino acid.

Aminoacyl tRNA synthetases (enzymes) catalyze the bonding between specific tRNAs and the amino acids that their anticodon sequences call for. The product of this reaction is an aminoacyl-tRNA. In bacteria, this aminoacyl-tRNA is carried to the ribosome by EF-Tu, where mRNA codons are matched through complementary base pairing to specific tRNA anticodons. Aminoacyl-tRNA synthetases that mispair tRNAs with the wrong amino acids can produce mischarged aminoacyl-tRNAs, which can result in inappropriate amino acids at the respective position in protein. This "mistranslation"[4] of the genetic code naturally occurs at low levels in most organisms, but certain cellular environments cause an increase in permissive mRNA decoding, sometimes to the benefit of the cell.

The ribosome has two binding sites for tRNA. They are the aminoacyl site (abbreviated A), the peptidyl site/ exit site (abbreviated P/E). With respect to the mRNA, the three sites are oriented 5’ to 3’ E-P-A, because ribosomes move toward the 3' end of mRNA. The A-site binds the incoming tRNA with the complementary codon on the mRNA. The P/E-site holds the tRNA with the growing polypeptide chain. When an aminoacyl-tRNA initially binds to its corresponding codon on the mRNA, it is in the A site. Then, a peptide bond forms between the amino acid of the tRNA in the A site and the amino acid of the charged tRNA in the P/E site. The growing polypeptide chain is transferred to the tRNA in the A site. Translocation occurs, moving the tRNA in the P/E site, now without an amino acid; the tRNA that was in the A site, now charged with the polypeptide chain, is moved to the P/E site and the tRNA leaves and another aminoacyl-tRNA enters the A site to repeat the process.[5]

After the new amino acid is added to the chain, and after the tRNA is released out of the ribosome and into the cytosol, the energy provided by the hydrolysis of a GTP bound to the translocaseEF-G (in bacteria) and a/eEF-2 (in eukaryotes and archaea) moves the ribosome down one codon towards the 3' end. The energy required for translation of proteins is significant. For a protein containing n amino acids, the number of high-energy phosphate bonds required to translate it is 4n-1[citation needed]. The rate of translation varies; it is significantly higher in prokaryotic cells (up to 17–21 amino acid residues per second) than in eukaryotic cells (up to 6–9 amino acid residues per second).[6]

Even though the ribosomes are usually considered accurate and processive machines, the translation process is subject to errors that can lead either to the synthesis of erroneous proteins or to the premature abandonment of translation. The rate of error in synthesizing proteins has been estimated to be between 1/105 and 1/103 misincorporated amino acids, depending on the experimental conditions.[7] The rate of premature translation abandonment, instead, has been estimated to be of the order of magnitude of 10−4 events per translated codon.[8] The correct amino acid is covalently bonded to the correct transfer RNA (tRNA) by amino acyl transferases. The amino acid is joined by its carboxyl group to the 3' OH of the tRNA by an ester bond. When the tRNA has an amino acid linked to it, the tRNA is termed "charged". Initiation involves the small subunit of the ribosome binding to the 5' end of mRNA with the help of initiation factors (IF). In bacteria and a minority of archaea, initiation of protein synthesis involves the recognition of a purine-rich initiation sequence on the mRNA called the Shine-Delgarno sequence. The Shine-Delgarno sequence binds to a complementary pyrimidine-rich sequence on the 3' end of the 16S rRNA part of the 30S ribosomal subunit. The binding of these complementary sequences ensures that the 30S ribosomal subunit is bound to the mRNA and is aligned such that the initiation codon is placed in the 30S portion of the P-site. Once the mRNA and 30S subunit are properly bound, an initiation factor brings the initiator tRNA-amino acid complex, f-Met-tRNA, to the 30S P site. The initiation phase is completed once a 50S subunit joins the 30 subunit, forming an active 70S ribosome.[9] Termination of the polypeptide occurs when the A site of the ribosome is occupied by a stop codon (UAA, UAG, or UGA) on the mRNA. tRNA usually cannot recognize or bind to stop codons. Instead, the stop codon induces the binding of a release factor protein.[10] (RF1 & RF2) that prompts the disassembly of the entire ribosome/mRNA complex by the hydrolysis of the polypeptide chain from the peptidyl transferase center of the ribosome[11] Drugs or special sequence motifs on the mRNA can change the ribosomal structure so that near-cognate tRNAs are bound to the stop codon instead of the release factors. In such cases of 'translational readthrough', translation continues until the ribosome encounters the next stop codon.[12]

The process of translation is highly regulated in both eukaryotic and prokaryotic organisms. Regulation of translation can impact the global rate of protein synthesis which is closely coupled to the metabolic and proliferative state of a cell. In addition, recent work has revealed that genetic differences and their subsequent expression as mRNAs can also impact translation rate in an RNA-specific manner.[13]

Clinical significance[edit]

Translational control is critical for the development and survival of cancer. Cancer cells must frequently regulate the translation phase of gene expression, though it is not fully understood why translation is targeted over steps like transcription. While cancer cells often have genetically altered translation factors, it is much more common for cancer cells to modify the levels of existing translation factors.[14] Several major oncogenic signaling pathways, including the RAS–MAPK, PI3K/AKT/mTOR, MYC, and WNT–β-catenin pathways, ultimately reprogram the genome via translation.[15] Cancer cells also control translation to adapt to cellular stress. During stress, the cell translates mRNAs that can mitigate the stress and promote survival. An example of this is the expression of AMPK in various cancers; its activation triggers a cascade that can ultimately allow the cancer to escape apoptosis (programmed cell death) triggered by nutrition deprivation. Future cancer therapies may involve disrupting the translation machinery of the cell to counter the downstream effects of cancer.[14]

Mathematical modeling of translation[edit]

Figure M0. Basic and the simplest model M0of protein synthesis. Here, *M – amount of mRNA with translation initiation site not occupied by assembling ribosome, *F – amount of mRNA with translation initiation site occupied by assembling ribosome, *R – amount of ribosomes sitting on mRNA synthesizing proteins, *P – amount of synthesized proteins.[16]
Figure M1'. The extended model of protein synthesis M1with explicit presentation of 40S, 60S and initiation factors (IF) binding.[16]

The transcription-translation process description, mentioning only the most basic ”elementary” processes, consists of:

  1. production of mRNA molecules (including splicing),
  2. initiation of these molecules with help of initiation factors (e.g., the initiation can include the circularization step though it is not universally required),
  3. initiation of translation, recruiting the small ribosomal subunit,
  4. assembly of full ribosomes,
  5. elongation, (i.e. movement of ribosomes along mRNA with production of protein),
  6. termination of translation,
  7. degradation of mRNA molecules,
  8. degradation of proteins.

The process of amino acid building to create protein in translation is a subject of various physic models for a long time starting from the first detailed kinetic models such as[17] or others taking into account stochastic aspects of translation and using computer simulations. Many chemical kinetics-based models of protein synthesis have been developed and analyzed in the last four decades.[18][19] Beyond chemical kinetics, various modeling formalisms such as Totally Asymmetric Simple Exclusion Process (TASEP),[19]Probabilistic Boolean Networks (PBN), Petri Nets and max-plus algebra have been applied to model the detailed kinetics of protein synthesis or some of its stages. A basic model of protein synthesis that took into account all eight 'elementary' processes has been developed,[16] following the paradigm that "useful models are simple and extendable".[20] The simplest model M0 is represented by the reaction kinetic mechanism (Figure M0). It was generalised to include 40S, 60S and initiation factors (IF) binding (Figure M1'). It was extended further to include effect of microRNA on protein synthesis.[21] Most of models in this hierarchy can be solved analytically. These solutions were used to extract 'kinetic signatures' of different specific mechanisms of synthesis regulation.

Genetic code[edit]

Main article: Genetic code

Whereas other aspects such as the 3D structure, called tertiary structure, of protein can only be predicted using sophisticated algorithms, the amino acid sequence, called primary structure, can be determined solely from the nucleic acid sequence with the aid of a translation table.

This approach may not give the correct amino acid composition of the protein, in particular if unconventional amino acids such as selenocysteine are incorporated into the protein, which is coded for by a conventional stop codon in combination with a downstream hairpin (SElenoCysteine Insertion Sequence, or SECIS).

There are many computer programs capable of translating a DNA/RNA sequence into a protein sequence. Normally this is performed using the Standard Genetic Code, however, few programs can handle all the "special" cases, such as the use of the alternative initiation codons which are biologically significant. For instance, the rare alternative start codon CTG codes for Methionine when used as a start codon, and for Leucine in all other positions.

Example: Condensed translation table for the Standard Genetic Code (from the NCBI Taxonomy webpage).


The "Starts" row indicate three start codons, UUG, CUG, and the very common AUG. It also indicates the first amino acid residue when interpreted as a start: in this case it is all methionine.

Translation tables[edit]

Main articles: List of genetic codes and Genetic code § List of alternative codons

Even when working with ordinary eukaryotic sequences such as the Yeast genome, it is often desired to be able to use alternative translation tables—namely for translation of the mitochondrial genes. Currently the following translation tables are defined by the NCBI Taxonomy Group for the translation of the sequences in GenBank:[22]

See also[edit]


  1. ^Brooker RJ, Widmaier EP, Graham LE, Stiling PD (2014). Biology (Third international student ed.). New York, NY: McGraw Hill Education. p. 249. ISBN .
  2. ^Neill C (1996). Biology (Fourth ed.). The Benjamin/Cummings Publishing Company. pp. 309–310. ISBN .
  3. ^Stryer L (2002). Biochemistry (Fifth ed.). W. H. Freeman and Company. p. 826. ISBN .
  4. ^Moghal A, Mohler K, Ibba M (November 2014). "Mistranslation of the genetic code". FEBS Letters. 588 (23): 4305–10. doi:10.1016/j.febslet.2014.08.035. PMC 4254111. PMID 25220850.
  5. ^Griffiths A (2008). "9". Introduction to Genetic Analysis (9th ed.). New York: W.H. Freeman and Company. pp. 335–339. ISBN .
  6. ^Ross JF, Orlowski M (February 1982). "Growth-rate-dependent adjustment of ribosome function in chemostat-grown cells of the fungus Mucor racemosus". Journal of Bacteriology. 149 (2): 650–3. doi:10.1128/JB.149.2.650-653.1982. PMC 216554. PMID 6799491.
  7. ^Wohlgemuth I, Pohl C, Mittelstaet J, Konevega AL, Rodnina MV (October 2011). "Evolutionary optimization of speed and accuracy of decoding on the ribosome". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 366 (1580): 2979–86. doi:10.1098/rstb.2011.0138. PMC 3158919. PMID 21930591.
  8. ^Sin C, Chiarugi D, Valleriani A (April 2016). "Quantitative assessment of ribosome drop-off in E. coli". Nucleic Acids Research. 44 (6): 2528–37. doi:10.1093/nar/gkw137. PMC 4824120. PMID 26935582.
  9. ^Nakamoto T (February 2011). "Mechanisms of the initiation of protein synthesis: in reading frame binding of ribosomes to mRNA". Molecular Biology Reports. 38 (2): 847–55. doi:10.1007/s11033-010-0176-1. PMID 20467902. S2CID 22038744.
  10. ^Baggett NE, Zhang Y, Gross CA (March 2017). Ibba M (ed.). "Global analysis of translation termination in E. coli". PLOS Genetics. 13 (3): e1006676. doi:10.1371/journal.pgen.1006676. PMC 5373646. PMID 28301469.
  11. ^Mora L, Zavialov A, Ehrenberg M, Buckingham RH (December 2003). "Stop codon recognition and interactions with peptide release factor RF3 of truncated and chimeric RF1 and RF2 from Escherichia coli". Molecular Microbiology. 50 (5): 1467–76. doi:10.1046/j.1365-2958.2003.03799.x. PMID 14651631.
  12. ^Schueren F, Thoms S (August 2016). "Functional Translational Readthrough: A Systems Biology Perspective". PLOS Genetics. 12 (8): e1006196. doi:10.1371/JOURNAL.PGEN.1006196. PMC 4973966. PMID 27490485.
  13. ^Cenik C, Cenik ES, Byeon GW, Grubert F, Candille SI, Spacek D, et al. (November 2015). "Integrative analysis of RNA, translation, and protein levels reveals distinct regulatory variation across humans". Genome Research. 25 (11): 1610–21. doi:10.1101/gr.193342.115. PMC 4617958. PMID 26297486.
  14. ^ abXu Y, Ruggero D (March 2020). "The Role of Translation Control in Tumorigenesis and Its Therapeutic Implications". Annual Review of Cancer Biology. 4 (1): 437–457. doi:10.1146/annurev-cancerbio-030419-033420.
  15. ^Truitt ML, Ruggero D (April 2016). "New frontiers in translational control of the cancer genome". Nature Reviews. Cancer. 16 (5): 288–304. doi:10.1038/nrc.2016.27. PMC 5491099. PMID 27112207.
  16. ^ abcGorban AN, Harel-Bellan A, Morozova N, Zinovyev A (July 2019). "Basic, simple and extendable kinetic model of protein synthesis". Mathematical Biosciences and Engineering. 16 (6): 6602–6622. doi:10.3934/mbe.2019329. PMID 31698578.
  17. ^MacDonald CT, Gibbs JH, Pipkin AC (1968). "Kinetics of biopolymerization on nucleic acid templates". Biopolymers. 6 (1): 1–5. doi:10.1002/bip.1968.360060102. PMID 5641411. S2CID 27559249.
  18. ^Heinrich R, Rapoport TA (September 1980). "Mathematical modelling of translation of mRNA in eucaryotes; steady state, time-dependent processes and application to reticulocytes". Journal of Theoretical Biology. 86 (2): 279–313. doi:10.1016/0022-5193(80)90008-9. PMID 7442295.
  19. ^ abSkjøndal-Bar N, Morris DR (January 2007). "Dynamic model of the process of protein synthesis in eukaryotic cells". Bulletin of Mathematical Biology. 69 (1): 361–93. doi:10.1007/s11538-006-9128-2. PMID 17031456. S2CID 83701439.
  20. ^Coyte KZ, Tabuteau H, Gaffney EA, Foster KR, Durham WM (April 2017). "Reply to Baveye and Darnault: Useful models are simple and extendable". Proceedings of the National Academy of Sciences of the United States of America. 114 (14): E2804–E2805. Bibcode:2017PNAS..114E2804C. doi:10.1073/pnas.1702303114. PMC 5389313. PMID 28341710.
  21. ^Morozova N, Zinovyev A, Nonne N, Pritchard LL, Gorban AN, Harel-Bellan A (September 2012). "Kinetic signatures of microRNA modes of action". RNA. 18 (9): 1635–55. doi:10.1261/rna.032284.112. PMC 3425779. PMID 22850425.
  22. ^Elzanowski A, Jim Ostell (7 Jan 2019). "The Genetic Codes". National Center for Biotechnology Information. Retrieved 28 March 2019.

Further reading[edit]

  • Champe PC, Harvey RA, Ferrier DR (2004). Lippincott's Illustrated Reviews: Biochemistry (3rd ed.). Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN .
  • Cox M, Nelson DR, Lehninger AL (2005). Lehninger principles of biochemistry (4th ed.). San Francisco...: W.H. Freeman. ISBN .
  • Malys N, McCarthy JE (March 2011). "Translation initiation: variations in the mechanism can be anticipated". Cellular and Molecular Life Sciences. 68 (6): 991–1003. doi:10.1007/s00018-010-0588-z. PMID 21076851. S2CID 31720000.

External links[edit]


Translation is the process by which a protein is synthesized from the information contained in a molecule of messenger RNA (mRNA). During translation, an mRNA sequence is read using the genetic code, which is a set of rules that defines how an mRNA sequence is to be translated into the 20-letter code of amino acids, which are the building blocks of proteins. The genetic code is a set of three-letter combinations of nucleotides called codons, each of which corresponds with a specific amino acid or stop signal. Translation occurs in a structure called the ribosome, which is a factory for the synthesis of proteins. The ribosome has a small and a large subunit and is a complex molecule composed of several ribosomal RNA molecules and a number of proteins. Translation of an mRNA molecule by the ribosome occurs in three stages: initiation, elongation, and termination. During initiation, the small ribosomal subunit binds to the start of the mRNA sequence. Then a transfer RNA (tRNA) molecule carrying the amino acid methionine binds to what is called the start codon of the mRNA sequence. The start codon in all mRNA molecules has the sequence AUG and codes for methionine. Next, the large ribosomal subunit binds to form the complete initiation complex. During the elongation stage, the ribosome continues to translate each codon in turn. Each corresponding amino acid is added to the growing chain and linked via a bond called a peptide bond. Elongation continues until all of the codons are read. Lastly, termination occurs when the ribosome reaches a stop codon (UAA, UAG, and UGA). Since there are no tRNA molecules that can recognize these codons, the ribosome recognizes that translation is complete. The new protein is then released, and the translation complex comes apart.


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