The flow of genetic information within a cell, often described as the central dogma of molecular biology, dictates how the blueprints stored in DNA are ultimately converted into functional proteins. Protein synthesis is a fundamental process of life, responsible for creating the enzymes, structural components, and signaling molecules that govern cellular activity.
Deoxyribonucleic acid (DNA) remains stored within the nucleus, necessitating an intermediary molecule to deliver its instructions to the protein-making machinery outside. Messenger RNA (mRNA) acts as this molecular courier, transcribing the genetic instructions from the DNA template and carrying that coded message out to the cytoplasm, where the construction of a protein polypeptide chain takes place.
Creating the Messenger
The initial step in producing the mRNA molecule is transcription, a process occurring within the cell nucleus. The enzyme RNA polymerase is responsible for synthesizing a complementary pre-mRNA molecule by reading one strand of the DNA double helix, known as the template strand. This enzyme moves along the DNA, temporarily separating the two strands and adding ribonucleotides one by one to build the new RNA chain in a 5′ to 3′ direction.
Transcription begins at a specific DNA sequence called a promoter, which signals the RNA polymerase to bind and initiate the unwinding of the helix. As the polymerase advances during elongation, it uses base-pairing rules to dictate the sequence of the growing RNA molecule. For instance, a guanine (G) on the DNA template directs the addition of a cytosine (C) on the RNA, and an adenine (A) directs the addition of a uracil (U), as RNA contains uracil instead of thymine. The synthesis continues until the polymerase encounters a terminator sequence, which signals the completion of the transcript and its release from the DNA template.
The Coded Message
Once the initial RNA transcript is created, it must undergo several modifications to become a mature mRNA molecule capable of being translated. A modification is the addition of a 5′ cap, which is a modified guanine nucleotide attached to the beginning of the transcript. This cap serves to protect the molecule from degradation by enzymes, while also facilitating its binding to the ribosome. At the opposite end, the 3′ end is modified by the addition of a poly-A tail, a long chain of several hundred adenine nucleotides. This tail enhances the stability of the mRNA and aids in its export from the nucleus into the cytoplasm.
Before export, a final modification occurs through a process called RNA splicing, where specific non-coding sections called introns are removed from the pre-mRNA. The remaining coding segments, or exons, are then precisely joined together to form the continuous sequence that holds the instructions for the protein. The genetic code is a universal language where the sequence of nucleotides is read in groups of three, known as codons. Each codon specifies a single amino acid, and maintaining the correct reading frame—the proper grouping of three nucleotides—is necessary to ensure the resulting protein is built with the correct amino acid sequence.
Building the Protein
The mRNA molecule is now ready to leave the nucleus and travel into the cytoplasm, where the process of translation begins. This stage is orchestrated by the ribosome, a complex composed of protein and ribosomal RNA (rRNA), which serves as the factory for protein assembly. The ribosome binds to the mRNA and begins to read the nucleotide sequence codon by codon, starting with a specific initiation codon, typically AUG.
Transfer RNA (tRNA) molecules are the adaptors that translate the mRNA code into a chain of amino acids. Each tRNA carries a specific amino acid and possesses a three-nucleotide sequence called an anticodon, which is complementary to a specific mRNA codon. During the elongation phase, a tRNA carrying the next amino acid moves into the ribosome and base-pairs its anticodon with the exposed mRNA codon. The ribosome then catalyzes the formation of a peptide bond, connecting the newly delivered amino acid to the growing polypeptide chain.
The ribosome shifts forward one codon, moving the tRNAs and the mRNA through its structure, making the next codon available for decoding. This cyclical process continues until the ribosome encounters one of the three specific stop codons on the mRNA, which do not code for an amino acid. Instead, a release factor protein recognizes the stop codon, causing the complex to disassemble and the newly synthesized polypeptide chain to be freed.