Ribosomes: Definition, Structure & Function

Proteins perform a variety of functional and structural functions in all organisms. In the course of protein biosynthesis, the information in the genetic material (DNA) is translated into proteins. The ribosomes play a crucial role here.

Ribosome Definition

Before you get to know the definition of ribosomes, you should know the protein biosynthesis be a term.

the protein biosynthesis is the new synthesis of proteins in living cells. It is a central and essential intracellular process for all living beings. During protein biosynthesis, new proteins are built from amino acids according to the specifications of genetic information (DNA).

Here, protein biosynthesis is divided into transcription and translation. During transcription, the genetic information is written onto a messenger molecule (mRNA). Then, in the course of translation, the mRNA is translated into a polypeptide (protein).

You can find more detailed information on the process of protein biosynthesis in a separate article. Come around!

Figure 1: Schematic representation of protein biosynthesis.

the ribosomes play the crucial role in translations. They enable the mRNA to be translated into a polypeptide.

ribosomes are molecular complexes made up of proteins and RNA. They catalyze the translation of mRNAs into polypeptides.

Ribosomes are found in the cytoplasm of eukaryotic and prokaryotic cells. In eukaryotic cells, they can also be found on the membrane of the endoplasmic reticulum (ER). Endoplasmic reticulum to which ribosomes are attached is called rough endoplasmic reticulum (rough ER).

Figure 2: Representation of an animal cell with ribosomes.

Ribosomes as biocatalysts

Ribosomes catalyze translation. Therefore, ribosomes are among the biocatalysts.

biocatalysts are molecules that speed up or slow down biochemical reactions by decreasing or increasing the amount of energy required for a reaction to occur (activation energy). Some biochemical reactions cannot take place without biocatalysts.

RNA molecules, which act as biocatalysts, are called ribozymes designated. Since the functional units of the ribosomes are made up of RNA, ribosomes belong to the ribozymes.

The term ribozyme is made up of the terms ribonucleic acid (RNA) and the word enzyme.

ribosome structure

The ribosomes in different organisms differ from each other in a number of ways. Nevertheless, all ribosomes have fundamental structural features in common.

  • All ribosomes are made up of two subunits. A distinction is made between the large and the small subunit.
  • The individual subunits are made up of one to three RNA molecules and various protein building blocks. These are in a weight ratio of approximately 60% RNA and 40% protein.
  • The RNA molecules that make up the ribosomes are so-called ribosomal RNA (rRNA).

Ribosomal Ribonucleic Acid (rRNA) are RNA molecules that build the ribosomes or ribosomal subunits. They belong to the non-coding RNAs (ncRNAs) because they are not translated into a peptide sequence.

The difference between different ribosomes from different organisms lies in the difference in the RNA molecules that make them up. A basic distinction can be made between ribosomes from prokaryotes (bacteria and archaea) and ribosomes from eukaryotes (unicellular organisms, plants and animals).

ribosomes prokaryotes

The so-called 70S ribosomes are found in prokaryotes. These are made up of a large 50S subunit and a small 30S subunit.The S denotes the so-called sedimentation coefficient, which is given in the unit Svedberg (S). The sedimentation coefficient describes the behavior of particles in a centrifuge and depends on the mass and shape of the particles.

The 50S subunit of the ribosome is made up of two RNA molecules (23S rRNA and 5S rRNA) and 31 proteins. The 30S subunit consists of a single rRNA (16S rRNA) and 21 proteins.

Under good living conditions, a bacterial cell has up to 20,000 ribosomes, which account for up to a quarter of its dry mass. These numbers reflect the importance of the role of ribosomes and protein biosynthesis.

Ribosomes Eukaryotes

Ribosomes in eukaryotic cells are larger and more complex than those in prokaryotes. Eukaryotic ribosomes sediment in a centrifuge at 80S and are composed of a 60S subunit and a 40S subunit. Overall, the 80S ribosome is made up of four different rRNAs and contains 80 ribosomal proteins.

The 60S subunit of the ribosome consists of three rRNAs (28S rRNA, 5.8S rRNA and 5S rRNA) and is made up of 49 proteins. The smaller 40S subunit is made up of a single rRNA (18S rRNA) and 33 proteins.

ribosome function

Ribosomes catalyze the translation of protein biosynthesis. They are responsible for translating the mRNA into an amino acid sequence (polypeptide).

In addition to the mRNA and the ribosome, so-called transfer RNAs (tRNAs) needed. These are responsible for the delivery of the individual amino acids and for the transfer to the polypeptide chain.

The functions of the two subunits of the ribosome can be distinguished during translation. The smaller subunit binds the mRNA while the large subunit provides binding sites for the tRNAs. The ribosome mediates the binding between the anticodons the tRNAs and the codons on the mRNA. The addition of the tRNA allows the amino acids to be transferred to the resulting amino acid chain.

A anticodon is a triplet of bases (nucleotides) on tRNA that is complementary to a codon on mRNA.

A codon is a sequence of three (triplet) on the mRNA that determines the position of an amino acid in a protein or encodes the translation stop.

The process of translation or protein biosynthesis is very complex and will only be discussed superficially here. For a deeper insight and understanding, check out the articles on these topics.

When ribosomes are not translating mRNA into a polypeptide, the subunits exist individually in the cytoplasm. Only with the start of translation and in complex with the mRNA do the subunits form the complete ribosome.

Rough endoplasmic reticulum

as rough endoplasmic reticulum is the part of the endoplasmic reticulum that is occupied by ribosomes. The basic The function is the same, protein biosynthesis or translation takes place on the rough ER.

The difference to translation in the cytoplasm is that further steps of modification of the polypeptide and transport take place immediately after translation. These include the folding of the proteins inside the ER (lumen) and the vesicular transport of the proteins to other cell organelles.

In particular, integral membrane proteins and proteins that are subsequently to be secreted are translated on the rough ER.

You will also find the right article on the endoplasmic reticulum at to deepen your knowledge on this topic.

Ribosomes – The Most Important

  • the ribosomes play the crucial role in translation and thus in protein biosynthesis.
  • Ribosomes are in the cytoplasm of eukaryotic and prokaryotic cells. In eukaryotic cells, they can also be found on the membrane of the endoplasmic reticulum (ER).
  • All ribosomes are made up of two subunits. A distinction is made between the large and the small subunit.
  • Ribosomes catalyze the translation of protein biosynthesis. They are responsible for translating the mRNA into an amino acid sequence (polypeptide).

  • During translation at the endoplasmic reticulum, further steps of modification of the polypeptide and the transport of the proteins take place directly afterwards.

proof

  1. north home; Knippers (2018). Molecular Genetics. Thieme.
  2. Horn (2020). human biochemistry. Thieme.