intracellular organelle, site of protein biosynthesis resulting from translation of messenger RNA (mRNA), consisting of two subunits

Ribosomes are important cell organelles. They are machines: tiny molecular robots which make proteins. They do RNA translation, building proteins from amino acids using messenger RNA as a template. Ribosomes are found in all living cells, prokaryotes as well as eukaryotes.

A ribosome is a mixture of protein and RNA that starts being made in the nucleolus of a cell. The nucleolus is in the nucleus. The nucleus is protected by the nuclear envelope, and lets things out through the nuclear pores. Ribosomes move from the nucleolus into the cytoplasm. Most ribosomes sit on the endoplasmic reticulum, but are also found throughout the cytoplasm.

The job of the ribosome is to make new proteins. It does this by moving along a strand of messenger RNA and building a protein based on the code it reads. Making a protein this way is called translation.

We need up to 10 million robots (ribosomes) in every cell. To get this number, cells have many copies of rRNA genes. We inherit about 400 rRNA genes across five different chromosomes.[1][2]

Ribosomes read the sequence of messenger RNAs and assemble proteins out of amino acids bound to transfer RNAs.
Translation of mRNA (1) by a ribosome (2)(shown as small and large subunits) into a polypeptide chain (3). The ribosome begins at the start codon of mRNA (AUG) and ends at the stop codon (UAG).

Structure change

Figure 2 : Large (red) and small (blue) subunit fit together

Ribosomes are made out of two things: a small ribosomal subunit that reads the mRNA, while the large subunit joins amino acids to form a polypeptide chain. Each subunit is composed of one or more ribosomal RNA (rRNA) molecules and a variety of proteins.

Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit.[3][4][5] Their small subunit has a 16S RNA sub-unit (consisting of 1540 nucleotides) bound to 21 proteins. The large subunit has a 5S RNA (120 nucleotides), a 28S RNA (4700 nucleotides), a 5.8S RNA (160 nucleotides) subunits and 46 proteins.[4][6][7]

Types of ribosome change

Ribosomes evolved as cells evolved. Prokaryote (bacterial) ribosomes have just a single RNA chain. Archaeal ribosomes are similar.

Only Eukaryotic ribosomes have the full gear, which is a total 80S ribosome with a small 40S unit plus a large 60S subunit.

References change

  1. Carey, Nessa 2015. Junk DNA: A journey through the dark matter of the genome, p149. Icon Books ISBN 978-1-84831-826-7
  2. Zentner G.E. et al July 2011. Nucleic Acid Research 39(12) 4949–4960.
  3. The unit of measurement is the Svedberg unit, a measure of the rate of sedimentation in centrifugation rather than size. This accounts for why fragment numbers do not add up (80S is made of 40S and 60S).
  4. 4.0 4.1 Ben-Shem A. et al 2011 (2011). "The structure of the eukaryotic ribosome at 3.0 Å resolution". Science. 334 (6062): 1524–1529. Bibcode:2011Sci...334.1524B. doi:10.1126/science.1212642. PMID 22096102. S2CID 9099683.{{cite journal}}: CS1 maint: numeric names: authors list (link)
  5. Rabl et al 2010 (2011). "Crystal structure of the eukaryotic 40S ribosomal subunit in complex with initiation factor 1". Science. 331 (6018): 730–736. Bibcode:2011Sci...331..730R. doi:10.1126/science.1198308. hdl:20.500.11850/153130. PMID 21205638. S2CID 24771575.{{cite journal}}: CS1 maint: numeric names: authors list (link)
  6. Alberts, Bruce et al 2002. The molecular biology of the cell. 4th ed, Garland Science, 342. ISBN 0-8153-3218-1
  7. Klinge et al 2011 (2011). "Crystal structure of the eukaryotic 60S ribosomal subunit in complex with initiation factor 6". Science. 334 (6058): 941–948. Bibcode:2011Sci...334..941K. doi:10.1126/science.1211204. PMID 22052974. S2CID 206536444.{{cite journal}}: CS1 maint: numeric names: authors list (link)