Ribosomes are cytoplasmic organelles found in prokaryotes and eukaryotes. They are large complexes of proteins and three (prokaryotes) or four (eukaryotes) rRNA (ribosomal ribonucleic acid) molecules called subunits made in the nucleolus. The main function of ribosomes is to serve as the site of mRNA translation ( = protein synthesis, the assembly of amino acids into proteins); once the two (large and small) subunits are joined by the mRNA from the nucleus, the ribosome translates the mRNA into a specific sequence of amino acids, or a polypeptide chain.
Type and Location of Ribosomes
Ribosomes are found in cells in two ways: free and bound. In an electron micrograph as shown above, ribosomes appear as dark granules. Ribosomes exist at various locations within the cell; however, the location of ribosomes depends on the function of the cell:
Free ribosomes
Bound ribosomes
Ribosomes are also located in mitochondria and chloroplasts of eukaryotic cells; they are always smaller than cytoplasmic ribosomes and are comparable to prokaryotic ribosomes in both size and sensitivity to anitbiotics; however, the sedimentation values s (s = Svedberg units: a measure of the rate of sedimentation of a component in a centrifuge, related both to the molecular weight and the 3-D shape of the component) vary somewhat in different phyla. Prokaryotic and eukaryotic ribosomes perform the same functions by the same set of chemical reactions; however, eukaryotic ribosomes are much larger than prokaryotic ones and most of their proteins are different. Mitochondrial and chloroplast ribosomes resemble bacterial ribosomes.
Cells put forth considerable effort to the production of these essential organelles; for example, one E. coli cell contains about 15,000 ribosomes, each one with a molecular weight of approximately 3 x 106 daltons constituting 25% of the total mass of these bacterial cells.
Form: Small and Large Subunits
Prokaryotic and eukaryotic ribosomes are very similar
in their form. Small prokaryotic and eukaryotic ribosomal subunits
have a head and
a base with an
armlike platform
extending from one side as seen below left; however, additional
features of the small eukaryotic ribosome subunit include a bill
that extends from the head of the small subunit on the side opposite
the cleft and a set of lobes
at the end of the subunit opposite the head (below right); the
lobes are believed to contain the additional sequences that make
18s rRNA larger that its 16s bacterial counterpart.
The large subunit has a prominent central protuberance,
stalk, and ridge
extending from one side as seen pictured above right. The large
subunit has a tunnel
about 10nm long and 2.5 nm in diameter; the tunnel extends from
the region containing the A (aminoacyl) and P (peptidyl) sites
to the part of the large subunit from which the newly assembled
polypeptide chain exits the ribosome. This tunnel is thought to
be the channel that newly assembled polypeptide chains travel
on the way out of the ribosome.
Components
Ribosomes are small, but complex structures, roughly 20 to 30 nm in diameter, consisting of two unequally sized subunits, referred to as large and small which fit closely together as seen below. A subunit is composed of a complex between RNA molecules and proteins; each subunit contains at least one ribosomal RNA (rRNA) subunit and a large quantity of ribosomal proteins. The subunits together contain up to 82 specific proteins assembled in a precise sequence.
The prokaryotic ribosome in E. coli has a
size of 70s. The two subunits have distinct and recognizable 3-D
shapes. Approximately two-thirds of the E.
coli ribosome consists of rRNA with the
rest consisting of ribosomal proteins. Overall, the 50s and 30s
subunits combined are 70s due to the 3-D shape of the ribosomes.
The Components of the Prokaryotic Ribsome
In general, eukaryotic ribosomes are larger and more complex than prokaryotic ribosomes. The size of the ribosome and the molecular weights of the rRNA molecules differ from organism to organism. Simple eukaryotes have the smallest ribosomes, although they are larger than E. coli ribosomes, while mammals have the largest ribosomes. Nonetheless, all eukaryotic ribosomes have many common structural and chemical features. For example, the mammalian ribosome has a size of 80s consisting of a large 60s subunit and a small 40s subunit. The 80s mammalian ribosome consists of about equal weights of rRNA and ribosomal proteins.
The Components of the Eukaryotic Ribosome
The nucleus is the ultimate control center for cell activities, including translation. Within the chromatin, information required for cellular protein synthesis is coded into the DNA; each DNA segment containing the information for making a protein consitiutes a gene. The information in a protein-encoding gene is copied into a messenger RNA (mRNA) molecule that moves to the cytoplasm through the pores of the nuclear envelope. In the cytoplasm, mRNA molecules are used by ribosomes as directions for protein assembly. The DNA of the entire nucleus contains the codes for many thousands of different proteins.
In protein synthesis, a ribosome moves along an mRNA
molecule, reading the codon for protein assembly as it goes. As
it moves, the ribosome assembles amino acids into a guradually
lengthening protein chain. At the end of the coded message, translation
stops, the ribosomal subunits separate and detach from the mRNA,
and the completed protein is realased. Transfer RNA (tRNA) molecules
function as the "dictionary" in the translation mechanism.
Each of the 20 amino acids used in protein synthesis is linked
to a specific kind of tRNA. The tRNA is capable is recognizing
and binding the nucleid acid code word (called a codon) specifiying
its attached amino acid in an mRNA molecule.
Overview of Translation
1. Formation of the initiation complex
2. Elongation of the polypeptide chain (one repetition of the steps a, b and c for every amino acid incoporated into the protein being synthesized:
a. binding of the aminoacyl-tRNA
b. peptide bond formation
c. tranlocation
3. Termination
Steps in Translation
Initiation
Elongation
Peptide Bond Formation
Peptide bond formation is catalyzed on the ribosome
by peptidyl transferase. (a) Adjacent aminoacyl-tRNAs bound to
the mRNA at the ribosome (b) folowing peptide bond formation,
an uncharged tRNA is in the P site and a dipeptidyl-tRNA in the
A site.
Termination of Translation
The ribosome recognizes a chain termination codon (here, UAG) with the aid of release factors. The release factor reads the stop codon, and this initiatiartes a series of specific termination events leading to the release of the completed polypeptide
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