Protein Synthesis

Proteins accomplish tasks necessary for life in all organisms. They are some of the most important structural and energizing components of a cell. Some of them, for instance collagen in connective and supporting tissue, take on important structural tasks and provide the organism’s architecture. Others, such as the myosin and actin of muscle cells, enable the shortening (contraction) of muscles, and hence movement. Yet other proteins transport oxygen (the hemoglobin of the red blood cells) or serve as protective and defensive agents in the immune system (antibodies). Of special importance are the proteins that are the catalysts for the metabolism of the organism (enzymes). Enzyme proteins synthesize everything the cell needs to survive (proteins, fats, and carbohydrates).
If genetic information is seen as biological data storage, then that information must be available at any time. When needed, it must be transported within the cell from the nucleus to the site of protein synthesis (the ribosomes) by a biochemical mechanism. For this purpose, the genetic code is copied within the nucleus to ribonucleic acid (RNA), which has a structure similar to that of DNA but contains only a single strand. This process is known as transcription. Protein synthesis takes place during the interphase of cell division. Chromatin must be uncoiled to allowtranscription to take place. Hence only euchromatin is active in transcription. RNA is synthesized from free elements in the nucleus and is linked together into an RNA chain with the help of the enzyme RNA polymerase. RNA brings this message to the ribosomes of the endoplasmic reticulum, and is therefore also known as messenger RNA or mRNA. Like DNA it is composed of nucleotides, but instead of the base thymine it contains the base uracil, and contains the sugar ribose instead of the sugar deoxyribose. The mRNA bonds to the ribosome by base coupling with transfer RNA molecules.

a Inside the largely uncoiled chromosomes, amorphous DNA segments (euchromatin) undergoing transcription alternate with genetically inactive, not uncoiled DNA segments (heterochromatin)

b Section from a: DNA loop undergoing transcription

Other relatively short RNA molecules, similarly synthesized fromfree elements in the nucleus, bond to the amino acids present in the cytoplasm one-on-one and transport them to the ribosomes, where the mRNA is attached with its copies of the base triplets. These short RNA molecules are therefore also known as tRNA (transport or transfer RNA). Each tRNA is specific for one amino acid and the corresponding triplet on the mRNA. In this way, with the aid of ribosomal enzymes, the various amino acids are linked into a protein chain, corresponding to the sequence of triplets on the mRNA. The rRNA produced in the nucleus provides the information needed to manufacture these enzymes. The tRNA molecules liberated in this reaction can then be recharged with the same amino acid in the cytoplasm. This process of protein building, also known as translation, continues until the complete protein molecule has been synthesized. The protein chain varies in length according to the type of protein (from a few up to several hundred amino acids), and by chemical reactions it can be folded into a three-dimensional functional protein molecule.