Ribosome
Overview
 
A ribosome is a component of cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 that assembles the twenty specific amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 molecules to form the particular protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 molecule determined by the nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...

 sequence of an RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 molecule.

One of the central tenets of biology, often referred to as the central dogma of molecular biology
Central dogma of molecular biology
The central dogma of molecular biology was first articulated by Francis Crick in 1958 and re-stated in a Nature paper published in 1970:In other words, the process of producing proteins is irreversible: a protein cannot be used to create DNA....

, is that DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 is used to make RNA, which is used to make proteins. The DNA sequence in gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s is copied into a messenger RNA
Messenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...

 (mRNA). Ribosomes then read the information in this mRNA and use it to create proteins.
Encyclopedia
A ribosome is a component of cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 that assembles the twenty specific amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 molecules to form the particular protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 molecule determined by the nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...

 sequence of an RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 molecule.

One of the central tenets of biology, often referred to as the central dogma of molecular biology
Central dogma of molecular biology
The central dogma of molecular biology was first articulated by Francis Crick in 1958 and re-stated in a Nature paper published in 1970:In other words, the process of producing proteins is irreversible: a protein cannot be used to create DNA....

, is that DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 is used to make RNA, which is used to make proteins. The DNA sequence in gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s is copied into a messenger RNA
Messenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...

 (mRNA). Ribosomes then read the information in this mRNA and use it to create proteins. This process is known as translation
Translation (genetics)
In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...

; the ribosome translates the genetic information from the RNA into proteins. Ribosomes do this by binding to an mRNA and using it as a template for determining the correct sequence of amino acids in a particular protein. The amino acids are attached to transfer RNA
Transfer RNA
Transfer RNA is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the three-letter genetic code in messenger RNA with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by...

 (tRNA) molecules, which enter one part of the ribosome and bind to the messenger RNA sequence. The attached amino acids are then joined together by another part of the ribosome. The ribosome moves along the mRNA, "reading" its sequence and producing a corresponding chain of amino acids.

Ribosomes are made from complexes of RNAs and proteins called ribonucleoprotein
Ribonucleoprotein
Ribonucleoprotein is a nucleoprotein that contains RNA, i.e. it is an association that combines ribonucleic acid and protein together. A few known examples include the ribosome, the enzyme telomerase, vault ribonucleoproteins, and small nuclear RNPs , which are implicated in pre-mRNA splicing and...

s. Ribosomes are divided into two subunits. The smaller subunit binds to the mRNA, while the larger subunit binds to the tRNA and the amino acids. When a ribosome finishes reading a mRNA, these two subunits split apart. Ribosomes have been classified as ribozyme
Ribozyme
A ribozyme is an RNA molecule with a well defined tertiary structure that enables it to catalyze a chemical reaction. Ribozyme means ribonucleic acid enzyme. It may also be called an RNA enzyme or catalytic RNA. Many natural ribozymes catalyze either the hydrolysis of one of their own...

s, because the ribosomal RNA
Ribosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...

 seems to be most important for the peptidyl transferase
Peptidyl transferase
The Peptidyl transferase is an aminoacyltransferase as well as the primary enzymatic function of the ribosome, which forms peptide links between adjacent amino acids using tRNAs during the translation process of protein biosynthesis....

 activity that links amino acids together.

Ribosomes from bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

 and eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s (the three domains of life on Earth
Life on Earth
Life on Earth: A Natural History by David Attenborough is a television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...

), have significantly different structures and RNA sequences. These differences in structure allow some antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s to kill bacteria by inhibiting their ribosomes, while leaving human ribosomes unaffected. The ribosomes in the mitochondria of eukaryotic cells functionally resemble in many features those in bacteria, reflecting the likely evolutionary origin of mitochondria. The word ribosome comes from ribo nucleic acid and the Greek: soma (meaning body).

Description

Archaeal, eubacterial and eukaryotic ribosomes differ in their size, composition and the ratio of protein to RNA. Because they are formed from two subunits of non-equal size, they are slightly longer in the axis than in diameter.
Prokaryotic ribosomes are around 20 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

 (200 ångström
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

s) in diameter and are composed of 65% ribosomal RNA
Ribosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...

 and 35% ribosomal protein
Ribosomal protein
A ribosomal protein is any of the proteins that, in conjunction with rRNA, make up the ribosomal subunits involved in the cellular process of translation. A large part of the knowledge about these organic molecules has come from the study of E. coli ribosomes. Most ribosomic proteins have been...

s. Eukaryotic ribosomes are between 25 and 30 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

 (250-300 angstrom
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

s) in diameter and the ratio of rRNA to protein is close to 1.
Ribosomes translate
Translation (genetics)
In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...

 messenger RNA (mRNA) and build polypeptide chains (e.g., protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s) using amino acids delivered by transfer RNA (tRNA). Their active sites are made of RNA, so ribosomes are now classified as "ribozyme
Ribozyme
A ribozyme is an RNA molecule with a well defined tertiary structure that enables it to catalyze a chemical reaction. Ribozyme means ribonucleic acid enzyme. It may also be called an RNA enzyme or catalytic RNA. Many natural ribozymes catalyze either the hydrolysis of one of their own...

s".

Ribosomes build proteins from the genetic instructions held within messenger RNA
Messenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...

. Free ribosomes are suspended in the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

 (the semi-fluid portion of the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

); others are bound to the rough endoplasmic reticulum, giving it the appearance of roughness and thus its name, or to the nuclear envelope
Nuclear envelope
A nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...

. As ribozymes are partly constituted from RNA, it is thought that they might be remnants of the RNA world. Although catalysis of the peptide bond
Peptide bond
This article is about the peptide link found within biological molecules, such as proteins. A similar article for synthetic molecules is being created...

 involves the C2 hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 of RNA's P-site (see Function section below) adenosine in a protein shuttle mechanism, other steps in protein synthesis (such as translocation) are caused by changes in protein conformations.

Ribosomes are sometimes referred to as organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

s, but the use of the term organelle is often restricted to describing sub-cellular components that include a phospholipid membrane, which ribosomes, being entirely particulate, do not. For this reason, ribosomes may sometimes be described as "non-membranous organelles".

Ribosomes were first observed in the mid-1950s by Romania
Romania
Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...

n cell biologist George Palade using an electron microscope
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

 as dense particles or granules for which he would win the Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

. The term "ribosome" was proposed by scientist Richard B. Roberts in 1958:
The structure and function of the ribosomes and associated molecules, known as the translational apparatus, has been of research interest since the mid-twentieth century and is a very active field of study today.

Ribosomes consist of two subunits (Figure 1) that fit together (Figure 2) and work as one to translate the mRNA into a polypeptide chain during protein synthesis (Figure 3). Bacterial subunits consist of one or two and eukaryotic of one or three very large RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 molecules (known as ribosomal RNA
Ribosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...

 or rRNA) and multiple smaller protein molecules. Crystallographic
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...

 work has shown that there are no ribosomal proteins close to the reaction site for polypeptide synthesis. This suggests that the protein components of ribosomes act as a scaffold that may enhance the ability of rRNA to synthesize protein rather than directly participating in catalysis (See: Ribozyme
Ribozyme
A ribozyme is an RNA molecule with a well defined tertiary structure that enables it to catalyze a chemical reaction. Ribozyme means ribonucleic acid enzyme. It may also be called an RNA enzyme or catalytic RNA. Many natural ribozymes catalyze either the hydrolysis of one of their own...

).

Biogenesis

In bacterial cells, ribosomes are synthesized in the cytoplasm through the transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

 of multiple ribosome gene operon
Operon
In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...

s. In eukaryotes, the process takes place both in the cell cytoplasm and in the nucleolus
Nucleolus
The nucleolus is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA is transcribed and assembled within the nucleolus...

, which is a region within the cell nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

. The assembly process involves the coordinated function of over 200 proteins in the synthesis and processing of the four rRNAs, as well as assembly of those rRNAs with the ribosomal proteins.

Ribosome locations

Ribosomes are classified as being either "free" or "membrane-bound".

Free and membrane-bound ribosomes differ only in their spatial distribution; they are identical in structure. Whether the ribosome exists in a free or membrane-bound state depends on the presence of an ER-targeting signal sequence
Signal peptide
A signal peptide is a short peptide chain that directs the transport of a protein.Signal peptides may also be called targeting signals, signal sequences, transit peptides, or localization signals....

 on the protein being synthesized, so an individual ribosome might be membrane-bound when it is making one protein, but free in the cytosol when it makes another protein.

Free ribosomes

Free ribosomes can move about anywhere in the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

, but are excluded from the cell nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 and other organelles. Proteins that are formed from free ribosomes are released into the cytosol and used within the cell. Since the cytosol contains high concentrations of glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...

 and is, therefore, a reducing environment
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

, proteins containing disulfide bonds, which are formed from oxidized cysteine residues, cannot be produced in this compartment.

Membrane-bound ribosomes

When a ribosome begins to synthesize proteins that are needed in some organelles, the ribosome making this protein can become "membrane-bound". In eukaryotic cells this happens in a region of the endoplasmic reticulum (ER) called the "rough ER". The newly produced polypeptide chains are inserted directly into the ER by the ribosome undertaking vectorial synthesis
Vectorial synthesis
Vectorial synthesis is the term used to describe synthesis of exported proteins by ribosomes in which the ribosome-nascent chain complex is bound directly to the endoplasmic reticulum and the nascent peptide chain moves through the ER membrane as it emerges from the ribosome.-Further...

 and are then transported to their destinations, through the secretory pathway
Secretory pathway
The secretory pathway is a series of steps a cell uses to move proteins out of the cell; a process known as secretion. The path of a protein destined for secretion has its origins in the rough endoplasmic reticulum, a membrane-bound compartment in the cell...

. Bound ribosomes usually produce proteins that are used within the plasma membrane or are expelled from the cell via exocytosis
Exocytosis
Exocytosis , also known as 'The peni-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane...

.

Structure

The ribosomal subunits of prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

s and eukaryotes are quite similar.

The unit of measurement is the Svedberg
Svedberg
A svedberg is a non-SI physical unit used for sedimentation coefficients.  It characterizes the behaviour of a particle type in sedimentation processes, notably centrifugation.  The svedberg is technically a measure of time, and is defined as exactly 10-13 seconds A svedberg (symbol S, sometimes...

 unit, a measure of the rate of sedimentation
Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...

 in centrifugation
Centrifugation
Centrifugation is a process that involves the use of the centrifugal force for the sedimentation of mixtures with a centrifuge, used in industry and in laboratory settings. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture...

 rather than size and accounts for why fragment names do not add up (70S is made of 50S and 30S).

Prokaryotes have 70S
Svedberg
A svedberg is a non-SI physical unit used for sedimentation coefficients.  It characterizes the behaviour of a particle type in sedimentation processes, notably centrifugation.  The svedberg is technically a measure of time, and is defined as exactly 10-13 seconds A svedberg (symbol S, sometimes...

 ribosomes, each consisting of a small (30S
30S
30S is the smaller subunit of the 70S ribosome of prokaryotes. It is a complex of ribosomal RNA and ribonucleoproteins that functions in mRNA translation...

) and a large (50S
50S
50S is the larger subunit of the 70S ribosome of prokaryotes. It is the site of inhibition for antibiotics such as macrolides, chloramphenicol, clindamycin, and the pleuromutilins. It includes the subunits 5S and 23S.-Structure:...

) subunit. Their large subunit is composed of a 5S
5S ribosomal RNA
5S ribosomal RNA is a component of the large ribosomal subunit in both prokaryotes and eukaryotes .Eukaryotic 5S rRNA is synthesised by RNA polymerase III, whereas most other eukaroytic rRNAs are cleaved from a 45S precursor transcribed by RNA polymerase I...

 RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 subunit (consisting of 120 nucleotides), a 23S RNA subunit (2900 nucleotides) and 31 protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s. The 30S subunit has a 1540 nucleotide RNA subunit (16S
16S ribosomal RNA
16S ribosomal RNA is a component of the 30S subunit of prokaryotic ribosomes. It is approximately 1.5kb in length...

) bound to 21 proteins. Affinity label for the tRNA binding sites on the E. coli ribosme allowed the identification of A and P site proteins most likely associated with the peptidyltransferase activity; labelled proteins are L27, L14, L15, L16, L2; at least L27 is located at the donor site as shown by E. Collatz and A.P. Czernilofsky et al. (FEBS Letters, 63 pp 283–286, 1976, Proc. Natl. Sci., USA, 71, pp230–234, 1974. Proteins involved in the initiation of translation, S1 and S21 are associated with the 3'-end of 16S ribosomal RNA, as determined by A.P. Czernilofsky et al. by RNA-protein crosslinking (FEBS Leter, 58, pp281–284, 1975) Shine-Dalgarno sequence
Shine-Dalgarno sequence
The Shine-Dalgarno sequence , proposed by Australian scientists John Shine and Lynn Dalgarno , is a ribosomal binding site in the mRNA, generally located 8 basepairs upstream of the start codon AUG. The Shine-Dalgarno sequence exists only in prokaryotes. The six-base consensus sequence is AGGAGG;...

.

Eukaryotes have 80S ribosomes, each consisting of a small (40S
40S
40S is the small subunit of eukaryotic ribosomes.It interacts with the internal ribosome entry site of the hepatitis C virus.The following is a list of proteins contained in the 40S ribosome:...

) and large (60S
60S
60S is the large ribosomal subunit in eukaryotes. It corresponds to 50S in prokaryotes.It consists of the following:* 5S* 28S* 5.8SThe following is a list of proteins contained within the 60S ribosome:...

) 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.The identification of the tRNA-Binding Sites on Rat Liver Ribosomes by Affinity Labeling was performed by A.P. Czernilofsky et al. 1977, several proteins including L32/33, L36, L21, L23, L28/29 and L13 (Molec. Gen. Genet. 153, pp231–235, 1977).

The ribosomes found in chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s and mitochondria of eukaryotes also consist of large and small subunits bound together with protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s into one 70S particle. These organelles are believed to be descendants of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 (see Endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...

) and as such their ribosomes are similar to those of bacteria.

The various ribosomes share a core structure, which is quite similar despite the large differences in size. Much of the RNA is highly organized into various tertiary structural motifs, for example pseudoknots that exhibit coaxial stacking. The extra RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 in the larger ribosomes is in several long continuous insertions, such that they form loops out of the core structure without disrupting or changing it. All of the catalytic activity of the ribosome is carried out by the RNA
Ribozyme
A ribozyme is an RNA molecule with a well defined tertiary structure that enables it to catalyze a chemical reaction. Ribozyme means ribonucleic acid enzyme. It may also be called an RNA enzyme or catalytic RNA. Many natural ribozymes catalyze either the hydrolysis of one of their own...

; the proteins reside on the surface and seem to stabilize the structure.

The differences between the bacterial and eukaryotic ribosomes are exploited by pharmaceutical chemists
Medicinal chemistry
Medicinal chemistry and pharmaceutical chemistry are disciplines at the intersection of chemistry, especially synthetic organic chemistry, and pharmacology and various other biological specialties, where it is involved with design, chemical synthesis and development for market of pharmaceutical...

 to create antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s that can destroy a bacterial infection without harming the cells of the infected person. Due to the differences in their structures, the bacterial 70S ribosomes are vulnerable to these antibiotics while the eukaryotic 80S ribosomes are not. Even though mitochondria possess ribosomes similar to the bacterial ones, mitochondria are not affected by these antibiotics because they are surrounded by a double membrane that does not easily admit these antibiotics into the organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

.

High-resolution structure

The general molecular structure of the ribosome has been known since the early 1970s. In the early 2000s the structure has been achieved at high resolutions, on the order of a few ångströms.

The first papers giving the structure of the ribosome at atomic resolution were published in rapid succession in late 2000. First, the 50S (large prokaryotic) subunit from the archaeon
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

 Haloarcula marismortui was published. Soon after, the structure of the 30S subunit from Thermus thermophilus
Thermus thermophilus
Thermus thermophilus is a Gram negative eubacterium used in a range of biotechnological applications, including as a model organism for genetic manipulation, structural genomics, and systems biology. The bacterium is extremely thermophilic, with an optimal growth temperature of about...

was published. Shortly thereafter, a more detailed structure was published. These structural studies were awarded the Nobel Prize in Chemistry in 2009. Early the next year (May 2001) these coordinates were used to reconstruct the entire T. thermophilus
Thermus thermophilus
Thermus thermophilus is a Gram negative eubacterium used in a range of biotechnological applications, including as a model organism for genetic manipulation, structural genomics, and systems biology. The bacterium is extremely thermophilic, with an optimal growth temperature of about...

70S particle at 5.5 ångström resolution.

Two papers were published in November 2005 with structures of the Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

70S ribosome. The structures of a vacant ribosome were determined at 3.5-ångström resolution using x-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

. Then, two weeks later, a structure based on cryo-electron microscopy
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

 was published, which depicts the ribosome at 11-15 ångström resolution in the act of passing a newly synthesized protein strand into the protein-conducting channel.

First atomic structures of the ribosome complexed with tRNA and mRNA molecules were solved by using X-ray crystallography by two groups independently, at 2.8 ångström and at 3.7 ångström. These structures allow one to see the details of interactions of the Thermus thermophilus
Thermus thermophilus
Thermus thermophilus is a Gram negative eubacterium used in a range of biotechnological applications, including as a model organism for genetic manipulation, structural genomics, and systems biology. The bacterium is extremely thermophilic, with an optimal growth temperature of about...

ribosome with mRNA and with tRNAs bound at classical ribosomal sites. Interactions of the ribosome with long mRNAs containing Shine-Dalgarno sequence
Shine-Dalgarno sequence
The Shine-Dalgarno sequence , proposed by Australian scientists John Shine and Lynn Dalgarno , is a ribosomal binding site in the mRNA, generally located 8 basepairs upstream of the start codon AUG. The Shine-Dalgarno sequence exists only in prokaryotes. The six-base consensus sequence is AGGAGG;...

s were visualized soon after that at 4.5- to 5.5-ångström resolution.

Function

Ribosomes are the workhorses of protein biosynthesis
Protein biosynthesis
Protein biosynthesis is the process in which cells build or manufacture proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA, which is then...

, the process of translating mRNA into protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

. The mRNA comprises a series of codons that dictate to the ribosome the sequence of the amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s needed to make the protein. Using the mRNA as a template, the ribosome traverses each codon (3 nucleotides) of the mRNA, pairing it with the appropriate amino acid provided by a tRNA. Molecules of transfer RNA
Transfer RNA
Transfer RNA is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the three-letter genetic code in messenger RNA with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by...

 (tRNA) contain a complementary anticodon on one end and the appropriate amino acid on the other. The small ribosomal subunit, typically bound to a tRNA containing the amino acid methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...

, binds to an AUG codon on the mRNA and recruits the large ribosomal subunit. The ribosome then contains three RNA binding sites, designated A, P and E. The A site binds an aminoacyl-tRNA (a tRNA bound to an amino acid); the P site binds a peptidyl-tRNA (a tRNA bound to the peptide being synthesized); and the E site binds a free tRNA before it exits the ribosome. Protein synthesis begins at a start codon
Start codon
The start codon is generally defined as the point, sequence, at which a ribosome begins to translate a sequence of RNA into amino acids.When an RNA transcript is "read" from the 5' carbon to the 3' carbon by the ribosome the start codon is the first codon on which the tRNA bound to Met,...

 AUG near the 5' end of the mRNA. mRNA binds to the P site of the ribosome first. The ribosome is able to identify the start codon by use of the Shine-Dalgarno sequence
Shine-Dalgarno sequence
The Shine-Dalgarno sequence , proposed by Australian scientists John Shine and Lynn Dalgarno , is a ribosomal binding site in the mRNA, generally located 8 basepairs upstream of the start codon AUG. The Shine-Dalgarno sequence exists only in prokaryotes. The six-base consensus sequence is AGGAGG;...

 of the mRNA in prokaryotes and Kozak box
Kozak consensus sequence
The Kozak consensus sequence, Kozak consensus or Kozak sequence, is a sequence which occurs on eukaryotic mRNA and has the consensus gccRccAUGG, where R is a purine three bases upstream of the start codon , which is followed by another 'G'. The Kozak consensus sequence plays a major role in the...

 in eukaryotes.

In Figure 3, both ribosomal subunits (small and large) assemble at the start codon (towards the 5' end of the mRNA). The ribosome uses tRNA that matches the current codon (triplet) on the mRNA to append an amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 to the polypeptide chain. This is done for each triplet on the mRNA, while the ribosome moves towards the 3' end of the mRNA. Usually in bacterial cells, several ribosomes are working parallel on a single mRNA, forming what is called a polyribosome or polysome
Polysome
Polyribosomes also known as ergosomes are a cluster of ribosomes, bound to a mRNA molecule, first discovered and characterized by Jonathan Warner, Paul Knopf, and Alex Rich in 1963. Many ribosomes read one mRNA simultaneously, progressing along the mRNA to synthesize the same protein...

.

Nobel Prize

Together with Albert Claude
Albert Claude
Albert Claude was a Belgian biologist who shared the Nobel Prize in Physiology or Medicine in 1974 with Christian de Duve and George Emil Palade. He studied engineering, and then medicine...

 and Christian de Duve
Christian de Duve
Christian René, viscount de Duve is a Nobel Prize-winning cytologist and biochemist. De Duve was born in Thames Ditton, Surrey, Great Britain, as a son of Belgian refugees. They returned to Belgium in 1920...

, George Emil Palade
George Emil Palade
George Emil Palade was a Romanian cell biologist. Described as "the most influential cell biologist ever", in 1974 he was awarded the Nobel Prize in Physiology and Medicine, together with Albert Claude and Christian de Duve. The prize was granted for his innovations in electron microscopy and...

 was awarded the Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...

, in 1974, for the discovery of the ribosomes.
The Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

 in Chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 2009 was awarded to Drs Venkatraman Ramakrishnan, Thomas A. Steitz
Thomas A. Steitz
-Publications:* Steitz, T. A., et al. , nsls newsletter, .* Steitz, T. A., et al. , NSLS Activity Report .-External links:* , from the Office of Scientific and Technical Information, United States Department of Energy...

 and Ada E. Yonath "for studies of the structure and function of the ribosome"

External links

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