Bacterial cell structure & function: Ribosomes

Written By: Doraline
MB0902
092472X

Ribosomes are small organelles made of rRNA and protein in the cytoplasm of prokaryotic and eukaryotic cells which aid in the production of proteins on the rough endoplasmic reticulum and ribosome complexes. The site of protein synthesis. Ribosomes that are from bacteria, archaea and eukaryotes have significantly different in its structure and it’s RNA.
The ribosome is composed of two subunits that attach to the mRNA at the beginning of protein synthesis in the cytoplasm and detach when the polypeptide has been translated. Ribosomal subunits are synthesized by the nucleolus.

(Blue-large subunit. Red- Small subunit)

The ribosome functions in the expression of the genetic code from nucleic acid into protein, in a process called translation. Ribosomes do this by catalyzing the assembly of individual amino acids into polypeptide chains. This involves the binding of a mRNA and then using this as a template to join together the correct sequence of amino acids. This reaction uses adapters called transfer RNA molecules, which reads the sequence of the mRNA and are attached to the amino acids.

There are two places that ribosomes usually exist in the cell:
1. suspended in the cytosol
2. bounded to the endoplasmin reticulum.
These ribosomes are called free ribosomes and bound ribosomes respectively. In both cases, the ribosomes usually form aggregates called polysomes.Free ribosomes usually make proteins that will function in the cytosol, while bound ribosomes usually make proteins that are exported or included in the cell's membranes. Interestingly enough, free ribosomes and bound ribosomes are interchangeable and the cell can change their numbers according to metabolic needs!!

Prokaryotes have 70S ribosomes, each consisting of a small (30S) and a large (50S) subunit. Their large subunit is composed of a 5S RNA subunit (consisting of 120 nucleotides), a 23S RNA subunit (2900 nucleotides) and 34 proteins. The 30S subunit has a 1540 nucleotide RNA subunit (16S) bound to 21 proteins.
Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit. Their large subunit is composed of a 5S RNA (120 nucleotides), a 28S RNA (4700 nucleotides), a 5.8S subunit (160 nucleotides) and ~49 proteins. The 40S subunit has a 1900 nucleotide (18S) RNA and ~33 proteins.

Source :http://en.wikipedia.org/wiki/Ribosome
(The image shows an isosurface representation of the 70S ribosome (50S blue; 30S yellow), derived from a crystal structure of E. coli ribosome - Schuwirth et al (2005).)