Introduction
Ribosomes are cell organelles in the cytosol. They serve the construction of proteins. The construction of proteins takes place in different stages within the framework of protein biosynthesis.
One part of the protein biosynthesis is the translation, the translation takes place at the ribosomes. Here, the mRNA is translated into amino acid chains from which proteins are ultimately built. Ribosomes either occur as free ribosomes in the cytosol or form on the cytosolic side of the membrane of the rough endoplasmic reticulum (rER).
Structure
Ribosomes are composed of a small and a larger subunit, each consisting of single-stranded rRNA and various proteins. According to molecular weight, eukaryotic ribosomes are divided into the 60 S and the 40 S subunits, which together have a mass of 80 S. Prokaryotic ribosomes have a mass of 70 S and consist of a 50 S and a 30 S subunit. The subunits are synthesized in the nucleolus of the cell nucleus and passed through the pores of the cell nucleus membrane into the cytoplasm, where they form the finished ribosomes.
Function of ribosomes
Ribosomes are complexes of ribosomal RNA (rRNA) and proteins. The function of ribosomes is the production of soluble proteins that cover the cell’s own requirements. All proteins occurring in a cell are synthesized on ribosomes.
The exact amino acid sequence is genetically determined in the DNA and is translated into a messenger RNA (mRNA) during transcription. The mRNA released from the cell nucleus binds to the small subunit of a ribosome, whereupon the large subunit also binds and protein synthesis begins. The small subunit recognizes certain sequences in the mRNA, while the large subunit links the individual amino acids to form a protein chain.
The transcription of the mRNA into a chain of amino acids is called translation. The complete ribosome travels along the mRNA, while a transfer RNA (tRNA) retrieves and connects the matching amino acid building blocks. The translation of proteins whose destination is outside the cell or which are intended for incorporation into the membrane takes place at the rough endoplasmic reticulum (rER), whereas proteins which are required in the cell itself are synthesized at free ribosomes.
