Ribosomes are the cellular organelles that carry out protein synthesis, through a process called translation. They are found in both prokaryotes and eukaryotes,
these molecular machines are responsible for accurately translating the
linear genetic code, via the messenger RNA, into a linear sequence of
amino acids to produce a protein. All cells contain ribosomes because
growth requires the continued synthesis of new proteins. Ribosomes can
exist in great numbers, ranging from thousands in a bacterial cell to
hundreds of thousands in some human cells and hundreds of millions in a
frog ovum. Ribosomes are also found in mitochondria and chloroplasts.
Structure
The ribosome is a large ribonucleoprotein (RNA-protein) complex,
roughly 20 to 30 nanometers in diameter. It is formed from two unequally
sized subunits, referred to as the small subunit and the large subunit.
The two subunits of the ribosome must join together to become active in
protein synthesis. However, they have distinguishable functions. The
small subunit is involved in decoding the genetic information, while the
large subunit has the catalytic activity responsible for peptide bond
formation (that is, the joining of new amino acids to the growing
protein chain).
In prokaryotes, the small subunit contains one RNA molecule and about
twenty different proteins, while the large subunit contains two
different RNAs and about thirty different proteins. Eukaryotic ribosomes
are even more complex: the small subunit contains one RNA and over
thirty proteins, while the large subunit is formed from three RNAs and
about fifty proteins. Mitochondrial and chloroplast ribosomes are
similar to prokaryotic ribosomes.
In spite of its complex composition, the architecture of the ribosome
is very precise. Even more remarkable, ribosomes from all organisms,
ranging from bacteria to humans, are very similar in their form and
function. Recent breakthroughs in studies of ribosome structure, using
techniques such as scanning, cryo-electron microscopy, and X-ray
crystallography, have provided scientists with highly refined structures
of this complex organelle. One particularly exciting conclusion from
studies of the large subunit is that it is ribosomal RNA (rRNA), and not
protein, that provides the catalytic activity for peptide bond
formation. That is, it forms the chemical linkage between the amino
acids of the growing protein molecule.
Synthesis
The synthesis of ribosomes is itself a very complex process,
requiring the coordinated output from dozens of genes encoding ribosomal
proteins and rRNAs. Ribosomes are assembled from their many component
parts in an orderly pathway. In eukaryotes, rRNA synthesis and most of
the assemblysteps occur in a structure within the nucleus called the
nucleolus. Eukaryotic ribosome synthesis is especially complicated,
because the ribosomal proteins themselves are made by ribosomes in the
cytoplasm (that is, outside of the nucleus), so they then must be
imported into the nucleolus for assembly onto the nucleolus-derived
rRNA. Once assembled, the nearly complete ribosomal subunits are then
exported out of the nucleus and back into the cytoplasm for the final
steps of assembly.
Ribosomes
from a liver cell are represented by the darkened ares of the magnified
image. These organelles contain RNA and use it for protein synthesis.
The exact details of the in vivo ribosome assembly pathway
(the process of ribosome assembly within the living cell) are still
under investigation. Assembly in eukaryotic cells involves not only the
components of the mature particles, but also dozens of auxiliary factors
that promote the efficient and accurate construction of the ribosome
during its assembly. However, bacterial ribosomes can be constructed in vitro using purified ribosomal proteins and rRNAs. These ribosomes appear to function normally in in vitro translation reactions.
Ribosome Function
Translation of messenger RNA (mRNA) by ribosomes occurs in the
cytoplasm. In bacterial cells, ribosomes are scattered throughout the
cytoplasm. In eukaryotic cells, they can be found both as free ribosomes
and as bound ribosomes, their location depending on the function of the
cell. Free ribosomes are found in the cytosol, which is the fluid
portion of the cytoplasm, and are responsible for manufacturing proteins
that will function as soluble proteins within the cytoplasm or form structural elements, including the cytoskeleton, that are found within the cytosol.
Bound ribosomes are attached to the outside of a membranous network called the endoplasmic reticulum
to form what is termed the "rough" endoplasmic reticulum. Proteins made
by bound ribosomes are intended to beincorporated into membranes, or
packaged for storage, or exported outside of the cell. Ribosomes exist
either as a single ribosome (that is, one ribosome translating an mRNA)
or as polysomes (two or more ribosomes sequentially translating the same
mRNA in order to make multiple copies of the same protein).
Ribosomes have the critical role of mediating the transfer of genetic
information from DNA to protein. Ribosomes translate this code using an
intermediary, the messenger RNA, which is a copy of the DNA that can be
interpreted by ribosomes. To begin translation, the small subunit first
identifies, with the help of other protein factors, the precise point
in the RNA sequence where it should begin linking amino acids, the
building blocks of protein. The small subunit, once bound to the mRNA,
is then joined by the large subunit and translation begins. The amino
acid chain continues to grow until the ribosome reaches a signal that
instructs it to stop.
Many of the antibiotics used in humans and other animals to treat
bacterial infections specifically inhibit ribosome activity in the
disease-causing bacteria, without affecting ribosome function in the
host-animal's cells. These antibiotics work by binding to a protein or
RNA target in the bacterial ribosome and inhibiting translation. In
recent years, the misuse of antibiotics has resulted in the natural
selection of bacteria that are resistant to many of these antibiotics,
either because they have mutations in the antibiotic's target in the
ribosome or because they have acquired a mechanism for excluding or
inactivating the antibiotic.
Bibliography
Frank, Joachim. "How the Ribosome Works." American Scientist 86 (1998): 428-439
Garrett, Robert A., et al, eds. The Ribosome: Structure, Function, Antibiotics, and Cellular Interactions. Washington, DC: ASM Press, 2000
Karp, Gerald. Cell and Molecular Biology: Concepts and Experiments, 3rd ed. New York: John Wiley & Sons, 2002.
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