Protein biosynthesis is the process that produces proteins within cells. It begins in the nucleus and finishes in the cytoplasm with a new protein being created. There are two steps to this process, which are transcription and translation. Some people use the terms protein biosynthesis and translation interchangeably, but the correct usage of protein biosynthesis refers to the entire process, not just translation.
Transcription takes place within the nucleus and is the process where strands of RNA are created based on DNA templates. Genes are lengths of nucleotides found along the strands of DNA that provide the information for which protein is to be produced. Amino acids and proteins are large molecules so they are not formed within the nucleus. Instead, genetic information is transcribed into RNA, which then passes through pores in the nuclear membrane into the cytoplasm of the cell.
There are three different kinds of RNA molecules that are involved in protein biosynthesis. Messenger RNA (mRNA) is the template for the protein that is to be created. It carries the transcribed genetic information that instructs the cell regarding which amino acids must be attached and in what order. Transfer RNA (tRNA) brings amino acids found within the cytoplasm to the growing protein chain. Ribosomal RNA (rRNA) comprises the two lobes of the ribosome, which is where the translation stage of protein biosynthesis takes place.
DNA is made up of long chains of only four nucleotides, but there are 20 different amino acids available to create proteins. This means that there is not a one-to-one relationship between nucleotides and amino acids. Instead, three nucleotides, or a codon, provide the code for which amino acid should be joined to the chain. As a result, there are 64 possible codons, so some amino acids correspond to several different codons.
Once transcription has been completed, the RNA molecules prepare to carry out the next stage of protein biosynthesis. mRNA sits in the cleft between the large and small lobe of the ribosome, and three nucleotides are passed through the ribosome at a time, so that the codon for the amino acid is lined up with the actual amino acid that tRNA carries to the ribosome.
tRNA has a distinct shape and is also specific to the amino acid that it carries to the lengthening protein chain. The amino acid binds to one end of the tRNA molecule, while the other end has three nucleotides that correspond to the codon found on the mRNA molecule. This allows the tRNA molecule to bind to the mRNA chain and be held in place while the amino acid it carries is joined to another amino acid by an enzyme.
To form a protein, amino acids continue to be joined to the peptide chain until a stop codon is reached. The stop codon does not correspond to an amino acid, but instead it signals that the protein chain is finished. At this point, mRNA is released from the ribosome and is broken down. While a protein has been made, it usually isn't functional yet. Protein biosynthesis is complete, but most proteins undergo further modifications before they are used.