Polypeptides are molecular chains that serve as the building blocks for proteins. Proteins perform a vast array of functions within living organisms, involved in nearly every cellular process, from catalyzing metabolic reactions to transporting molecules. Polypeptides are linear chains of amino acids, and their precise arrangement dictates the polypeptide’s eventual three-dimensional shape and biological role.
The Genetic Instruction Manual
The instructions for creating polypeptides are stored within deoxyribonucleic acid (DNA), the cell’s blueprint. Specific segments called genes contain the unique codes for particular polypeptides. For humans, approximately 20,000 protein-coding genes exist, though they constitute only a small fraction of the entire genome.
Ribonucleic acid (RNA) acts as an intermediary in accessing and interpreting this genetic information. RNA molecules carry instructions from the DNA and help assemble amino acids. Three main types of RNA play distinct roles: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
Copying the Message: Transcription
The first step in forming a polypeptide is transcription, where genetic information from a DNA gene is copied into an mRNA molecule. This occurs primarily within the nucleus of eukaryotic cells. RNA polymerase, a specialized enzyme, unwinds a segment of the DNA double helix to expose the nucleotide bases.
RNA polymerase synthesizes a complementary mRNA strand using one of the DNA strands as a template. RNA nucleotides pair with their DNA counterparts: uracil (U) with adenine (A), and guanine (G) with cytosine (C). The new mRNA molecule carries the genetic message out of the nucleus.
Building the Chain: Translation
Once the mRNA molecule leaves the nucleus, it travels to the cytoplasm where polypeptide chain construction occurs through translation. This process takes place on ribosomes, cellular machines composed of ribosomal RNA (rRNA) and proteins. Ribosomes bind to the mRNA molecule and read the genetic code.
The mRNA sequence is read in groups of three nucleotides, known as codons. Each codon specifies a particular amino acid to be added to the growing polypeptide chain. Transfer RNA (tRNA) molecules bring the correct amino acids to the ribosome. Each tRNA molecule has a specific anticodon, complementary to an mRNA codon, and carries the corresponding amino acid.
Translation proceeds in three main stages: initiation, elongation, and termination. During initiation, the ribosome, mRNA, and the first tRNA (carrying methionine) assemble. In elongation, tRNAs deliver amino acids to the ribosome’s A site, where their anticodons match the mRNA codons, and a peptide bond forms. The ribosome then shifts, moving the tRNAs and mRNA, repeating the cycle. Termination occurs when the ribosome encounters a stop codon, signaling the release of the completed polypeptide chain.
Shaping the Protein: Folding and Function
After translation, the newly synthesized polypeptide chain is a linear chain of amino acids, but not yet a functional protein. To become biologically active, this linear chain must fold into a precise three-dimensional structure. This folding process is guided by the amino acid sequence itself, as interactions between different amino acids within the chain cause it to form a specific shape.
Protein folding involves various levels of structural organization. The primary structure is the linear sequence of amino acids. Secondary structures, like alpha-helices and beta-sheets, form through hydrogen bonds within the polypeptide backbone. These then fold further to create the tertiary structure, the overall three-dimensional shape of a single polypeptide chain, driven by interactions between amino acid side chains. Some proteins, like hemoglobin, consist of multiple polypeptide chains that assemble, forming a quaternary structure, which is essential for a protein’s function.