_"Ribosomes are the key piece of the puzzle: they take the genetic blueprint and make the main actors of the biological world, the proteins"_ **-Venki Ramakrishnan**
Ribosomes are cell organelles, which means they are specialised structures that perform specific functions within the cell. Ribosomes are found in every cell – both prokaryotic (cells without a nucleus) and eukaryotic (cells with a true, membrane bound nucleus). Escherichia coli (E. coli), the bacteria, has about 15,000 ribosomes, and the average mammalian cell has around 10 million!
In eukaryotic cells, ribosomes are synthesised in the nucleolus, a dense structure present inside the nucleus. They are either present as free-floating bodies in the cytoplasm or attached to the endoplasmic reticulum (ER, the skeleton of the cell). The ER associated with ribosomes is called rough endoplasmic reticulum (RER). Ribosomes are also found in chloroplasts (the sites of photosynthesis in plant cells) and mitochondria (the powerhouses of the cell). They are composed of 50% ribosomal RNA (rRNA) – a special type of RNA which plays a structural role – and 50% protein. These ribosomes are called 80S ribosomes. The S stands for Svedberg unit, and is used to measure sedimentation rate – that is, how fast a particle can settle down under the gravitational field induced by a centrifuge. The eukaryotic ribosomes consist of 2 subunits, a larger subunit (60S) and a smaller subunit (40S).
In prokaryotic cells, ribosomes are free-floating in the cytoplasm. Prokaryotic cells do not have ER. They are composed of 65% rRNA and 35% proteins. They are 70S ribosomes, also consisting of 2 subunits – a larger subunit (50S) and a smaller subunit (30S).
The primary function of ribosomes is protein synthesis for the cell. Protein synthesis involves two steps – transcription and translation – following the Central Dogma. Transcription occurs in the nucleus and involves the production of a messenger RNA (mRNA) molecule based on the DNA sequence in a given gene coding for a protein. This mRNA strand is picked up by the smaller subunit of the ribosome for the next step – translation.
Proteins are made of sequences of amino acids. The mRNA contains information about the sequence of amino acids required for making a specific protein. In the cell, amino acids are carried by transfer RNA (tRNA). tRNA acts as an adapter molecule – it reads the sequence of nucleotide bases on the mRNA and brings corresponding amino acids. Once the smaller subunit binds to mRNA, the larger subunit also arrives and catalyses the creation of peptide bonds between adjacent amino acids brought by tRNA. Ribosomes also play a part in protein folding – that is, the change of the protein into a 3-D structure that is biologically functional. This makes them a fascinating organelle to study.
Venkataraman Ramakrishnan, along with two others, won the Nobel Prize in Chemistry in 2009 for their research on the structure and function of the ribosome. In his words, ribosomes stand at the crossroads of biology – between the gene and what comes out of the gene.