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A ribosome is the organelle upon which mRNA from DNA transcription is translated to a polypeptide with a specific amino acid sequence defined by the genetic code. The size of the ribosome in eukaryotes and prokaryotes are slightly different. Prokaryotes have a 70S ribosome that is comprised of 2 subunits:

30S unit: This is the smaller unit which consists of 21 proteins and a 16S rRNA molecule. It is involve in initiating translation by selecting an AUG or alternative start codon via the 30S ribosomal subunit. This acts as the starting point for the rest of the translation process. The 16S rRNA molecule is responsible for the correct positioning of the ribosome unit.

50S unit: This is the larger unit which consists of 31 proteins and 2 RNA molecules, 23S and 5S[1].

The 2 units together form the complete ribosome, known as the 70S unit. S refers to the Svedberg unit, which is a measure of rate at which a compound moves when centrifuged. It is used as a measure of size of a molecule but is not directly proportional to molecular weigh [2].

As ribosomes contain RNA (also referred to as ribosomal RNA-rRNA) and proteins, they are also referred to as ribonucleoproteins. Ribosomes translate mRNA in triplets (codons) by aligning complementary triplets found in tRNA molecules (anticodons). Each tRNA is assigned a specific anticodon and amino acid and therefore translation leads to the formation of a protein by forming peptide bonds between adjacently aligned amino acids[3].

Ribosomes have three tRNA binding sites in the 30S subunit - the A-site (aminoacyl site), the P-site (peptidyl site) and the E-site (empty site) - which allow peptide bonds to form between adjacent amino acids. They are in order A, P, E from the 3' to 5' end or the mRNA strand and are involved in the elongation process of translation. The first charged tRNA attaches to the start codon in the P-site and is joined by a second, charged tRNA molecule adjacent in the A-site. A peptide bond forms between the two amino acids and the first amino acid is released from its tRNA. The uncharged tRNA moves along into the E-site, whilst the second charged tRNA moves from the A-site and into the P-site. The A-site becomes occupied by the next charged tRNA molecule. This process continues and the polypeptide continues to grow until a stop codon and termination occurs[4].

Ribosomes are small structures found in all living cells. They can be free in the cytoplasm or attached to endoplasmic reticulum (ER), making Rough ER. They can differ in size and number, according to whether they are found in bacteria, archaea or in eukaryotes. There are a large number of ribosomes in cells. In eukaryotes, there can be millions in one cell alone. As ribosomes are so small, (it has a diameter of 25-30 nm approximately)[5]. The World of the Cell, 7th Edition, San Francisco: Pearson Benjamin Cummings. P95., an electron microscope is needed to see it. Ribosomes are made up of two subunits, one larger than the other. The two subunits join together when attached to mRNA to make a protein in protein synthesis. Ribosomes are also found in mitochondria and chloroplasts and carry out protein synthesis, specifically for these organelles.


  1. Berg JM, Tymoczko JL, Stryer L: Biochemistry 6th (2007), WH Freeman and Company, New York. Pg 866
  2. Berg JM, Tymoczko JL, Stryer L: Biochemistry 6th (2007), WH Freeman and Company, New York. Pg 76
  3. Berg JM, Tymoczko JL, Stryer L: Biochemistry 6th (2007), WH Freeman and Company, New York. Pg 872
  4. Berg JM, Tymoczko JL, Stryer L. Biochemistry 7th ed.(2012), WH Freeman and Company, New York. Pg 934.
  5. Becker, Wayne M., Kleinsmith, Lewis J., Hardin, Jeff., Bertoni, Gregory Paul. (2009)
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