Protein synthesis is the fundamental cellular process by which cells create proteins. This intricate mechanism transforms genetic information, encoded in DNA, into a sequence of amino acids that form a protein. Proteins are essential for a vast array of cellular functions, including structural support, catalyzing metabolic reactions as enzymes, and regulating gene expression. This process underpins the growth, maintenance, and repair of all living organisms.
Copying the Blueprint: Transcription
Transcription is the initial step in protein synthesis, where genetic information stored in DNA is copied into a messenger RNA (mRNA) molecule. This process occurs within the nucleus of eukaryotic cells. RNA polymerase unwinds the DNA double helix to create an RNA copy.
Transcription proceeds through three distinct phases. Initiation begins when RNA polymerase binds to a specific DNA region called the promoter, signaling the start of a gene. This binding allows the DNA strands to separate, exposing the template strand.
During elongation, RNA polymerase moves along the DNA template, adding complementary RNA nucleotides to build a single-stranded mRNA molecule. An adenine (A) in the DNA template pairs with a uracil (U) in the newly forming RNA. Termination occurs when RNA polymerase encounters a specific stop sequence in the gene, prompting the release of the new mRNA strand.
Building the Protein: Translation
Following transcription, the mRNA molecule carries genetic instructions from the nucleus to the cytoplasm, where translation takes place. This complex process is carried out by ribosomes, cellular structures composed of ribosomal RNA (rRNA) and proteins. Ribosomes provide the site for protein synthesis, organizing translation and catalyzing the reactions that link amino acids.
Transfer RNA (tRNA) molecules play a critical role as adaptors, connecting mRNA codons to the specific amino acids they encode. Each tRNA has an anticodon, a sequence of three nucleotides that binds to a complementary codon on the mRNA. The other end of the tRNA carries the corresponding amino acid, ensuring correct delivery to the ribosome. The genetic code dictates which three-nucleotide codon specifies each of the 20 common amino acids.
Translation also occurs in three main phases: initiation, elongation, and termination. Initiation begins when the small ribosomal subunit attaches to the mRNA molecule and an initiator tRNA, typically carrying the amino acid methionine, binds to the start codon (AUG) on the mRNA. The large ribosomal subunit then joins this complex, forming a functional ribosome.
During elongation, amino acids are added one by one to form a polypeptide chain. The ribosome moves along the mRNA, reading each codon. A new tRNA carrying its specific amino acid enters the ribosome’s A site, and a peptide bond forms between the newly arrived amino acid and the growing polypeptide chain. The ribosome then translocates, moving three nucleotides along the mRNA, shifting the tRNAs to different sites within the ribosome and exposing a new codon for the next amino acid. This process repeats, extending the amino acid chain.
Termination occurs when the ribosome encounters one of three specific stop codons (UAA, UAG, or UGA) on the mRNA. These stop codons do not code for an amino acid but instead signal the release of the completed polypeptide chain from the ribosome.
Beyond the Assembly Line: Protein Folding
After the polypeptide chain is synthesized during translation, it undergoes a crucial process called protein folding. This involves the newly formed chain coiling and bending into a specific three-dimensional (3D) structure. The precise 3D shape is essential for the protein to perform its intended biological function. For instance, an enzyme must fold correctly to create an active site that can catalyze biochemical reactions. If a protein misfolds, it can lose its functionality, potentially leading to dysfunctional proteins or contributing to various diseases.