Ribosomes are tiny featureless particles at low magnification in electron micrographs but they are made up of an equal amount of RNA called ribosomal RNA (rRNA) and protein. Ribosomes are the site of protein synthesis.

Subunits of Ribosomes

Two subunits construct the ribosomes, one large and one small, that only bind together during protein synthesis. A large subunit binds with the tRNA, amino acids, and small subunit. While a small subunit binds with mRNA and a large subunit.

Ribosomes are not membrane-bounded and present both in eukaryotic and prokaryotic cells. The ribosomes of eukaryotic cells are the 80S (where S=Svedberg Unit) and about 22 nm in diameter while prokaryotic ribosomes are smaller than eukaryotic ribosomes.

The large subunit of eukaryotic cells is the 60S containing three molecules of rRNA and the small subunit is 40S with one molecule of rRNA; these subunits together contain about 80 proteins articulated around highly modified ribosomal RNAs (rRNAs).  Prokaryotic cells commonly are called 70S ribosomes, have dimensions of about 14 to 15 nm by 20 nm, a molecular weight of approximately 2.7 million.

Prokaryotic ribosomes constructed of the 50S  composed of a 5S RNA subunit (120 nucleotides), a 23S RNA subunit (2900 nucleotides), and 31 proteins, and a 30S subunit has a 16S RNA subunit (consisting of 1540 nucleotides) that bound to 21 proteins. A rapidly growing E. coli cell may have as many as 15,000 to 20,000 ribosomes, about 15% of the total cell mass.

Ribosomes as proteins synthesizing factory

Ribosomes are present in two different forms in the cells either they are free-floating in the cytoplasm called free ribosomes or attached with the endoplasmic reticulum and nuclear membrane called membrane-bound ribosomes. 

But the function of both forms of the ribosomes in protein synthesis; the location of the ribosomes in a cell determines what kind of protein it makes. Primarily, free ribosomes synthesis proteins used inside the cell. The protein synthesizes from the membrane-bound ribosomes destined for the insertion in the plasma membrane or for export from the cell. Cells that have high rates of protein synthesis, such as those actively growing, have a large number of ribosomes that make the dense appearance of the cell cytoplasm.

Protein Synthesis: Translation

As we know the main function of ribosomes in protein synthesis or you can say it helps in the conversion of our genetic information into proteins as we move from DNA to RNA and finally to protein. Information in the DNA is used to make proteins that control the cell’s activities. In the process of transcription, the genetic information in the DNA is copied into a complementary base sequence of RNA. Then the information encoded in the RNA is used by the ribosomes to synthesis the protein through the specific process called the translation.

Components Of Translation

The key components required for translation are:

  • mRNA
  • ribosomes
  • tRNA
  • aminoacyl-tRNA synthetases

Types of RNA

The types of RNA help the ribosomes to synthesis the protein by encoded the information from the DNA. There are following types of RNA

  1. Messenger RNA (mRNA)
  2. Transfer RNA (tRNA)
  3. Ribosomal RNA (rRNA)

Messenger RNA (mRNA)

Carries the coded information for making the specific proteins from DNA to ribosomes where proteins are synthesized. Genetic Information on the mRNA to synthesis protein present in the form of Codons (group of three nucleotide) such as AUG, GGC etc. During translation mRNA nucleotide bases are read as three base codons, each of which codes for a particular amino acid in the polypeptide.

Transfer RNA (tRNA)

Transfer RNA carries the amnio acid to the ribosomes. tRNAs function at the specific site in the ribosomes during the translation and read the codons from the messenger RNA or mRNA to make sure that the amino acids are added to the protein in a specific pattern. One tRNA for each of the 20 amino acids is required that incorporated into proteins, to make polypeptide chain thus at least 20 different tRNA molecules are needed

Ribosomal RNA (rRNA)

Ribosomal RNA forms an integral part of the ribosomes, the cellular machinery for protein synthesis. rRNA molecules direct the catalytic steps of protein synthesis and help in the stitching together of amino acids to make a protein molecule

The steps involved in protein synthesis are initiation, elongation and termination

Initiation

In first stage of protein synthesis the amino acids are attached to transfer RNA molecules called the amino acid activation that is catalyzed by the aminoacyl-tRNA synthetases. The amino acid is attached to the 3′-hydroxyl of the terminal adenylic acid on the tRNA by a high-energy bond to initiate the translation process.  The ribosome has two tRNA binding sites; the P site which holds the peptide chain and the A site which accepts the tRNA for the amino acid activation.

Elongation

While tRNA occupies the P site, another aminoacyl-tRNA with an anticodon complementary to the next codon comes to occupy the A site in the presence of GTP. Bond id formed between the amino acid and the next aminoacyl tRNA by peptidyl-transferase enzyme. Then tRNA molecule in the P site becomes uncharged and leaves the ribosome. The ribosome then trans-locates along the mRNA molecule to the next codon; open up the A site for the next aminoacyl-tRNA. The polypeptide chain is built up from the N terminal to the C terminal direction.

Termination

 Stop codons are used to terminate the synthesis of the protein. As one of the three stop codons enter in the A site no tRNA molecules bind to these codons. Then peptide and tRNA in the P site become hydrolyzed releasing the polypeptide into the cytoplasm and the small and large subunits of the ribosome dissociate ready for the next round of translation.

Sana Riaz
Author

Sana has just completed her MPhil in Microbiology. She loves reading books and the latest discoveries in sciences.

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