Gene expression – the method by which instructions present in DNA are converted into functional products (like proteins) – consists of two main processes, transcription and translation. This follows the Central Dogma of Molecular Biology. Translation involves the conversion of mRNA to proteins, and it occurs in ribosomes.
To understand translation, we need to know few important terms -
mRNA: It stands for messenger RNA and is produced during the process of transcription. It is complementary to the sequence of DNA. mRNA travels out of the nucleus and into the cytoplasm to the ribosomes.
Codon: A group of three nucleotide bases of mRNA is called a codon. Most codons correspond to an amino acid, as outlined by the genetic code.
tRNA: It stands for transfer RNA and is found in the cytoplasm of the cell. During protein synthesis, it is present in ribosomes.
Amino Acid: The building blocks which make up a protein when arranged in a particular manner.
Anticodon: Sequence of nucleotides complementary to the ones present in a codon, present in tRNA. The tRNA binds to the corresponding amino acid depending on the anticodon – called ‘cognate tRNA’.
The codon gives instructions regarding which amino acids to be added. The ribosomes bind to the incoming mRNA. The anticodon in the tRNA binds to corresponding codon in the mRNA, and the attached amino acids form peptide bonds, catalysed by the rRNA present in the ribosome. This facilitates the formation of a polypeptide which later folds to form a protein. Translation initiates at the start codon (AUG) and terminates at the stop codons (UAA, UAG, UGA).
The process of adding amino acids continues till the complete mRNA is translated (that is, a stop codon is reached). Subsequently, the mRNA and the sequence of amino acids (polypeptide) is released from the ribosome. The polypeptide undergoes folding and/or modification in the cytosol or endoplasmic reticulum, forming a functional protein. After this, the protein can stay in the cytosol (liquid portion of cytoplasm) or can go to any of the cell organelles like mitochondria, chloroplasts, etc.