DNA translation: what is it and what are its phases

DNA translation is the second process of protein synthesis. It occurs in all living beings and takes place in the cytoplasm, where the ribosomes are found, which acquire a fundamental role in the process.

Translation does not happen suddenly. It is necessary that a first step, transcription, has been taken beforehand, in which the genetic material in the form of DNA is transcribed into the previously mentioned RNA molecule. Let's see how it happens and what is necessary for it to happen.

• Related article: "Differences between DNA and RNA"

What is DNA translation?

It is well known that DNA, specifically its stretches, the genes, contain the genetic information about how we are. However, for genes to be able to encode information and make proteins synthesize, it is necessary a whole process of reading and coding DNA, RNA of different types, in addition to the involvement of ribosomes.

There are two steps necessary to transform the information hidden in a gene into a well-elaborated protein:

The first is the transcription of DNA.

A DNA sequence, that is, a gene, is made up of nucleotides, which are adenine, thymine, guanine, and cytosine (A, T, G, and C, respectively).

During transcription, the piece of DNA is transcribed into an RNA molecule (ribonucleic acid), which differs from DNA in that, instead of containing the nucleotide thymine (T), it has uracil (U). A is complementary to T, and C to U. This RNA is processed and trimmed, becoming a messenger RNA (mRNA).

After the transcription comes the translation, which is the step in which the RNA is read to form a polypeptide chain, which is basically a protein but with a very linear structure. For this to happen, it is necessary to join amino acids, which will depend on the nucleotides in the RNA.

The genetic code

As we were already saying, during translation the information contained in the mRNA is read, using as if they were the instruction manual to form a chain of amino acids, that is, a polypeptide. It is in this phase that what could be considered as the structure immediately prior to the protein will be obtained., which is basically a chain of amino acids but with a three-dimensional structure.

Each sequence of three nucleotides, called codons, of the mRNA (A, G, C and U) corresponds to a specific amino acid, or to a start or stop signal. The triplets that encode the end of polypeptide synthesis are UGA, UAG, and UAA, while the AUG codon encodes the start signal and also the amino acid methionine.

Together, codon-amino acid relationships are what make up the genetic code. It is what allows cells to decode, through mRNA, a chain of nucleotides to a chain of amino acids. To understand it better, below we have a strand of mRNA, with nucleotides. Next to it, we have the amino acids that correspond to each nucleotide triplet, as well as the start and stop signals.

• 5'
• AUG - methionine / start
• GAG - Glutamate
• CUU - Leucine
• AGC - Serine
• UAG - STOP
• 3'

The role of ribosomes and tRNA

Before going into detail with how DNA translation occurs, we are going to talk about the two elements that allow the mRNA to be read and a string to be synthesized: ribosomes and transfer RNA.

Transfer RNA (tRNA)

Transfer RNA (tRNA) is a type of RNA that serves as a molecular bridge to connect the codons of the mRNA with the amino acids for which they code. Without this type of RNA, it would not be possible to relate an amino acid to the triplet of nucleotides present in the mRNA..

In each tRNA there is an end that has a sequence of three nucleotides, called anticodon, that is complementary to the triplet of nucleotides of mRNA. At the other end they carry the amino acid.

Ribosomes

Ribosomes are organelles made up of two subunits similar in appearance to two hamburger buns.: the large subunit and the small subunit. In the ribosome, in addition, there are three hollow places where the tRNA binds with the mRNA: the A, P and E sites. It is in ribosomes that polypeptides are built.

The large and small subunits gather around the mRNA and, through enzymatic action, the ribosome catalyzes a chemical reaction that joins the amino acids of tRNA into a chain polypeptide.

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DNA translation: the process

Every second, our cells are producing hundreds of proteins. It is for this reason that translation is such an important process for life, since without it we would be left without the ability to transform the information contained in genes into something useful. DNA translation occurs in three stages: initiation, elongation, and termination.

Initiation

The initiation of DNA translation occurs in the ribosome. This organelle is assembled around an mRNA molecule, where a tRNA will come.

This last type of RNA must carry the amino acid methionine, encoded by the codon AUG, which is the signal for the initiation of the synthesis of the polypeptide chain.

This ribosome-tRNA-mRNA-methionine complex is known as an initiation complex, and is necessary for translation to take place.

Elongation

Elongation, as the name suggests, is the stage in which amino acids are added to the polypeptide chain, making it longer and longer. As more nucleotide triplets of the mRNA are translated, the more amino acids the polypeptide will have.

Every time a new codon is exposed, a corresponding tRNA binds. The existing chain of amino acids is attached to the amino acid of the tRNA through a chemical reaction. The mRNA shifts a codon onto the ribosome, exposing a new codon to be read.

Within the elongation we can distinguish three stages:

In the first, an anticodon, that is, a triplet of tRNA that contains complementary bases to those of a triplet of mRNA, "pairs" with an exposed codon of the mRNA at the A site.

A peptide bond is formed, through the catalytic action of aminoacyl-tRNA synthetase, between the newly introduced amino acid and the one immediately before it. The new amino acid is found in the A site of the ribosome, while the old one is in P. After the link is formed, the polypeptide is transferred from the P site to the A.

The ribosome advances a codon in the mRNA. The tRNA at the A site that carries the polypeptide moves to the P site. Then it moves to site E and exits the ribosome.

This process is repeated many times, as many as new amino acids are placed if a signal has not appeared before indicating that the continuation of the polypeptide chain must be stopped.

Termination

Termination is the moment when the polypeptide chain is released, ceasing to grow. It begins when a stop codon (UAG, UAA, or UGA) appears in the mRNA. This, when it is introduced into the ribosome, it triggers a series of events that result in the separation of the strand from its tRNA, allowing it to float towards the cytosol.

It may be the case that, despite termination, the polypeptide still needs to take the correct three-dimensional shape in order for it to become a well-formed protein.

Although, in essence, proteins are polypeptide chains, their difference from the newly manufactured polypeptide chains in the complex ribosomal is that they have a three-dimensional shape, while the new trinca polypeptide chain is basically a very linear chain of amino acids.

Bibliographic references:

• Pamela C Champe, Richard A Harvey and Denise R Ferrier (2005). Lippincott's Illustrated Reviews: Biochemistry (3rd ed.). Lippincott Williams & Wilkins. ISBN 0-7817-2265-9
• David L. Nelson and Michael M. Cox (2005). Lehninger Principles of Biochemistry (4th ed.). W. H. Freeman. ISBN 0-7167-4339-6
• Hirokawa et al. (2006). The Ribosome Recycling Step: Consensus or Controversy? Trends in Biochemical Sciences, 31 (3), 143-149.