Translation is the process of protein synthesis in the cell. Proteins perform many functions and are vital for the normal functioning of the body as a whole. Therefore, it is important that the process of protein synthesis proceeds without errors.
Ribosome
Translation takes place in special non-membrane-bound organelles called ribosomes. Ribosomes are the molecular machines for protein building. Ribosomes are spherical in shape and consist of a large and a small subunit. The large subunit is approximately twice as large as the small subunit.
The function of ribosomes is to recognize messenger RNA codons and add the appropriate amino acid to the growing protein chain. The delivery of the correct amino acids is handled by tRNA.
Transfer RNA and amino acid coding
At the opposite end of the anticodon loop of the transport RNA, there is a binding site for the corresponding amino acid. Each tRNA can carry only one of 20 amino acids. At least one tRNA corresponds to each amino acid. Because the genetic code is redundant, there are 61 different tRNAs that correspond to all of the different amino acids in nature. The three stop codons do not have corresponding tRNAs.
The process of attachment (activation) of an amino acid to a tRNA takes place in the cytoplasm. Absence of errors at this step is very important, otherwise the tRNA will insert the wrong amino acids into the future protein. The activation reaction is carried out by special enzymes, of which there are 20, just as there are 20 amino acids. Each enzyme specifically recognizes an amino acid and its corresponding tRNA and binds them together.
During this process, energy is released and the resulting aminoacyl-tRNA contains the energy to allow the protein chain to grow, which is why this process is called amino acid activation.
Thus, tRNA acts as an adaptor between the nucleotide sequence of the messenger RNA and the sequence of amino acids in the protein; it transports the encoded amino acids to the site of synthesis and ensures the sequence of their connections.
Reading frame
The initiator of translation is the start codon AUG. The ribosome recognizes nucleotides 3 at a time, which establishes the reading frame for the nucleotide sequence of the gene, which begins with the start codon and ends with the stop codon.
But what if the sequence of 2 different amino acids accidentally add up to a start codon? For example, look at the sequence tyrosine-valine (UAU-GUA...). If instead of starting to read it from the first nucleotide, you start from the second, it will read as (...U-AUG-UA...). That is, we will get a start codon where it was not there originally. The same thing happens when an extra nucleotide is accidentally inserted or one of them "disappears." Because of the triplicity of the genetic code, there are three possible reading frames for each DNA strand, one starting from each of the first three nucleotides.
Usually a functional protein is synthesized from only one reading frame, but some viruses use two or three to encode a protein.
The longest reading frame (at least 100 amino acids) not interrupted by a stop codon is designated as the correct reading frame when researchers are analyzing sequences. This is called an open reading frame. Finding the end of an open reading frame is much easier than finding its beginning.
Initiation of translation
The translation process consists of three stages: initiation, elongation, and termination.
The mRNA synthesized during the transcription process leaves the nucleus and goes to the site of protein synthesis in the cytoplasm — the ribosome. The ribosome has specific translation sites. One of them, the A-site, supplies aminoacyl-tRNA, and the other, the P-site, increases the peptide chain.
Polypeptide chain synthesis starts only after the initiator tRNA encoding the start codon is attached to the ribosome. As a result of interaction of all components, a complex is formed: ribosome — mRNA — initiator tRNA — amino acid.
There are a number of essential nucleotide sequences that are present only in the region of start codon that distinguish it from the usual internal AUG codon. Such sequences include the Shain-Dalgarno sequence in prokaryotes and the Kozak sequence in eukaryotes. They are necessary to ensure that translation is not chaotic and begins in strictly defined places.
There are also special proteins that provide translation initiation, so they are called translation initiation factors.
Elongation
Elongation is the stage at which the polypeptide chain increases. At this stage, aminoacyl-tRNAs recognize their corresponding mRNA codons and attach the necessary amino acids to the chain being built. The ribosome moves along the mRNA and releases tRNA molecules from the amino acids they bring. The amino acids come together sequentially, and a peptide bond is formed between them.
To speed up the process and synthesize proteins more efficiently, several ribosomes synthesize a protein sequentially on a single mRNA. The structures consisting of one mRNA and several ribosomes working on it are called polysomes.
Termination
Termination is the process of ending translation. It occurs as soon as the ribosome bumps into one of the 3 stop codons. The place of tRNA is then taken by a specific enzyme called a termination factor. This protein hydrolyzes the bond between the last tRNA and the synthesized protein. After that, the ribosome splits into its two subunits, and the last tRNA is released into the cytoplasm.
The freshly-synthesized protein molecule enters the endoplasmic reticulum or cytoplasm, where it undergoes further changes and acquires the appropriate structure.
The entire process of protein synthesis lasts from 20 to 500 seconds and depends on the length of the polypeptide. For example, in E. coli, a ribosome can synthesize a protein of 300 amino acids in about 20 seconds.
Conclusion
Cells synthesize the proteins they need using a process called translation. tRNAs supply the appropriate amino acids to the ribosomes, where they attach to each other and release the tRNA. Translation consists of 3 stages: initiation (beginning of protein chain synthesis), elongation (successive increase of the chain), and termination (end of synthesis). The complex of mRNA with several ribosomes simultaneously leading protein synthesis is called a polysome.