Translation

Translation is a key process in biological lifeforms. It is this set of events that transforms the code contained in DNA, and later mRNA, into the proteins necessary for cellular life.

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Translation is the synthesis of a protein from an mRNA template. This process involves several key molecules including mRNA, the small and large subunits of the ribosome, tRNA, and finally, the release factor. The process is broken into three stages: initiation, elongation, and termination. Let’s see the process in action…

Eukaryotic mRNA, the substrate for translation, has a unique 3′-end called the poly-A tail. mRNA also contains codons that will encode specific amino acids.

A methylated cap is found at the 5′-end. Translation initiation begins when the small subunit of the ribosome attaches to the cap and moves to the translation initiation site.

tRNA is another key molecule. It contains an anticodon that is complementary to the mRNA codon to which it binds. The first codon is typically AUG. Attached to the end of tRNA is the corresponding amino acid. Methionine corresponds to the AUG codon.

The large subunit now binds to create the peptidyl (or P) site and the aminoacyl (or A) site. The first tRNA occupies the P-site. The second tRNA enters the A-site and is complementary to the second codon.

The methionine is transferred to the A-site amino acid, the first tRNA exits, the ribosome moves along the mRNA, and the next tRNA enters. These are the basic steps of elongation.

As elongation continues, the growing peptide is continually transferred to the A-site tRNA, the ribosome moves along the mRNA, and new tRNAs enter.

When a stop codon is encountered in the A-site, a release factor enters the A-site and translation is terminated. When termination is reached, the ribosome dissociates, and the newly formed protein is released.

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1.) mRNA

Eukaryotic mRNA has unique features. Before it can be translated, it must be processed and modified.
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2.) Poly A Tail

One of the specific features it must have is a Poly A tail.
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3.) Codons

The "readable" portion of mRNA consists of codons for individual amino acids, shown here with colors. Each color represents a codon for a unique amino acid.
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4.) Cap

A cap is also added to the mRNA. This cap is necessary for the stability of the mRNA molecule.
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5.) Small Subunit

To begin initiation, the small subunit of a ribosome attaches to the mRNA. The area where it first attaches is an untranslated section of mRNA located near the cap.
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6.) First Codon

The small subunit then moves along the mRNA until it reaches the first readable codon. This codon always codes for the amino acid methionine.
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7.) tRNA

The tRNA molecules are the next to enter. These charged molecules of RNA bring the amino acids to the ribosome.
8.) Coding Sequence

Each tRNA has a triplet coding sequence that corresponds with the amino acid it carries along.
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9.) Complementary Codon

The triplet sequence of the tRNA is complementary to the triplet codon sequence in the mRNA.
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10.) Specific Amino Acid

Each triplet codon sequence in the mRNA and the tRNA corresponds to a specific amino acid.
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11.) Large Subunit

After the first tRNA moves into place, the large subunit of the ribosome attaches to the small subunit.
12.) Two Ribosome Sites

The complete ribosome consists of two sites: petidyl (left) and aminoacyl (right).
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13.) Peptidyl & Aminoacyl

After the first tRNA has attached to the peptidyl site, a second tRNA enters the complete ribosome and attaches to its complementary mRNA codon in the aminoacyl site.
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14.) From P Site to A Site

With the two tRNA in place, the amino acid from the peptidyl tRNA, moves and attaches to the tRNA in the aminoacyl site.
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15.) tRNA Exits

No longer bearing an amino acid, the tRNA from the peptidyl site leaves the ribosome.
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16.) Ribosome Moves

The ribosome then moves along the strand of mRNA, and the former aminoacyl site becomes the new peptidyl site. A new tRNA, based upon the triplet coding sequence of the mRNA, fills the new aminoacyl site.
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17.) Transferred Again

Again, the growing peptide chain of amino acids is transferred from the petidyl tRNA to the amino acid of the tRNA in the aminoacyl site.
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18.) A Site Becomes P Site

The ribosome moves along the mRNA again, and another charged (with its amino acid) tRNA fills the aminoacyl site.
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19.) And Transferred Again

The growing peptide chain is again transferred from the peptidyl tRNA to the amino acid attached to the tRNA in the aminoacyl site.
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20.) Peptide Synthesis

This process of peptide synthesis continues as the ribosome moves along the mRNA, and the future protein grows longer.
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21.) Stop Codon

No tRNA exists that recognizes the triplet sequence in mRNA known as a stop codon. Instead, a release factor enters the ribsome.
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22.) Release Factor

The release factor interferes with peptide elongation, and the ribosome moves no further.
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23.) Peptide Chain Leaves

The peptide chain is released from the tRNA and leaves the ribosome.
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24.) Subunits Dissosciate

With the peptide chain gone, the ribosome dissociates into its individual large and small subunits.
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25.) Synthesis Complete

Protein synthesis is now complete. The peptide chain is ready to act as a protein or be combined with other chains to form larger, polypeptide proteins.
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