Translation

1. Description of Translation

Genes contain the instructions a cell needs to make proteins.  Making proteins from DNA requires a 2-step process: 

1. Transcription: the process of copying the gene’s DNA into RNA.
2. Translation: the process of using RNA to synthesize protein.

Taken together, these two steps make up the “central dogma” of biology:

Figure \(\PageIndex{1}\). (CC BY-NC-SA)

Figure \(\PageIndex{2}\). (CC BY-NC-SA)

Transcription and processing of the newly made mRNA occurs in the nucleus of the cell.  
Once a mature mRNA transcript is made it is transported to the cytoplasm for translation into protein.

Figure \(\PageIndex{3}\). (CC BY-NC-SA)

Important Players in Translation 

messenger RNA(mRNA): RNA copy of DNA that contains the instructions to make a protein.

transfer RNA (tRNA): RNA molecule responsible for delivering amino acids to the ribosome.

Amino acid: The basic building block of a protein. There are 20 different amino acids, each differs in its R group.

Figure \(\PageIndex{4}\). (CC BY-NC-SA)

Protein: A chain of amino acids, also known as a polypeptide.

Ribosome: The organelle on which mRNA is translated into protein. Consists of a large (60S) and small subunit (40S) which are made up of ribosomal RNA (rRNA) and protein.

The Genetic Code: Translating RNA into Protein

How does the nucleotide sequence of RNA specify the specific order of amino acids in a protein? The answer lies in what is known as the genetic code. 

Consider RNA and Protein as different languages:

RNA consists of four different "letters" - A, U, G and C. 

Protein consists of 20 "letters" - the 20 amino acids

How can RNA code for protein?

• If each RNA base codes for just 1 amino acid, RNA could code for only 4 amino acids (not enough to include all 20 amino acids).

• If two RNA basescode for 1 amino acid, RNA could code for 16 amino acids (still not enough to include all 20 amino acids).

• If three RNA bases code for 1 amino acid, RNA could code for 64 amino acids (more than enough to include all 20 amino acids).

Thus, the genetic code is a triplet code in which three nucletides in RNA specify one amino acid in protein. 

Sets of three nucleotides that code for a specific amino acid are known as codons. These codons are recognized by, and basepair with, the tRNA molecule with the complementary anticodon. tRNA molecules act as translators because they are able to read the nucleic acid words (mRNA codons) and interpret them as protein words (amino acids). There is at least one tRNA for each of the 20 amino acids (some amino acids bind to 2 or 3 different tRNAs, so cells may contain as many as 32 different tRNAs).

Figure \(\PageIndex{5}\). (CC BY-NC-SA)

The codon "AUG" is the start signal for translation which places the amino acid, methionine (Met) at the beginning of each protein. Three codons, UAA, UAG, and UGA, act as signals to terminate translation. They are called STOP codons.

 

Figure \(\PageIndex{6}\). (CC BY-NC-SA)

Translation: RNA to Protein

Figure \(\PageIndex{7}\). (CC BY-NC-SA)

Translation Initiation: The small subunit binds to a site upstream (on the 5' side) of the start of the mRNA. It proceeds to scan the mRNA in the 5'-->3' direction until it encounters the START codon (AUG). The large subunit attaches and the initiator tRNA, which carries methionine (Met), binds to the P site on the ribosome.

Figure \(\PageIndex{8}\). (CC BY-NC-SA)

Elongation: A tRNA bound to its amino acid (known as an aminoacyl-tRNA) that is able to base pair with the next codon on the mRNA arrives at the A site. The preceding amino acid (Met at the start of translation) is covalently linked to the incoming amino acid with a peptide bond. The initiator tRNA moves to the E site and the ribosome moves one codon downstream. This shifts the more most recent tRNA from the A site to the P site, opening up the A site for the arrival of a new aminoacyl-tRNA. Polypeptide synthesis repeats, the tRNA residing in the E site is released from the complex, the tRNAs in the P site and A site shift over and the next amino acid is added to the growing polypeptide chain. This cycle repeats until a stop codon is reached.

Figure \(\PageIndex{9}\). (CC BY-NC-SA)

Termination: Translation ends when the ribosome reaches a STOP codon (UAA, UAG or UGA). There are no tRNA molecules with anticodons for stop codons, instead protein release factors recognize these codons when they arrive at the A site. Binding of a release protein causes the polypeptide (protein) to be released from the ribosome. The ribosome subunits dissociate (split) from each other and can be reassembled later for another round of protein synthesis.

Figure \(\PageIndex{10}\). (CC BY-NC-SA)

 

Translation Tutorial by Dr. Katherine Harris is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

Funded by the U.S. Department of Education, College Cost Reduction and Access (CCRAA) grant award # P031C080096.