3.6.1: Ribosomes - Biology

3.6.1: Ribosomes - Biology

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  • Compare and contrast ribosome structure and function in prokaryotes and eukaryotes

Ribosomes are tiny spherical organelles that make proteins by joining amino acids together. Many ribosomes are found free in the cytosol, while others are attached to the rough endoplasmic reticulum. The purpose of the ribosome is to translate messenger RNA (mRNA) to proteins with the aid of tRNA. In eukaryotes, ribosomes can commonly be found in the cytosol of a cell, the endoplasmic reticulum or mRNA, as well as the matrix of the mitochondria. Proteins synthesized in each of these locations serve a different role in the cell. In prokaryotes, ribosomes can be found in the cytosol as well. This protein-synthesizing organelle is the only organelle found in both prokaryotes and eukaryotes, asserting the fact that the ribosome is a trait that evolved early on, most likely present in the common ancestor of eukaryotes and prokaryotes. Ribosomes are not membrane bound.

Ribosomes are composed of two subunits, one large and one small, that only bind together during protein synthesis. The purpose of the ribosome is to take the actual message and the charged aminoacyl-tRNA complex to generate the protein. To do so, they have three binding sites. One is for the mRNA; the other two are for the tRNA. The binding sites for tRNA are the A site, which holds the aminoacyl-tRNA complex, and the P site, which binds to the tRNA attached to the growing polypeptide chain.

In most bacteria, the most numerous intracellular structure is the ribosome which is the site of protein synthesis in all living organisms. All prokaryotes have 70S (where S=Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol. The 70S ribosome is made up of a 50S and 30S subunits. The 50S subunit contains the 23S and 5S rRNA while the 30S subunit contains the 16S rRNA. These rRNA molecules differ in size in eukaryotes and are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms. The ribosome is the most commonly observed intracellular multiprotein complex in bacteria.

Ribosome assembly consists of transcription, translation, the folding of rRNA and ribosomal proteins, the binding of ribosomal proteins, and the binding and release of the assembly components to make the ribosome. In vivo assembly of the 30S subunit has two intermediates (p130S and p230S) and the 50S subunit has three intermediates (p150S, p250S, and p350S). However, the reconstitution intermediates are not the same as in vitro. The intermediates of the 30S subunit yield 21S and 30S particles while the intermediates of the 50S subunit yield 32S, 43S, and 50S particles. The intermediates in the in vivo assembly are precursor rRNA which is different from in vitro which uses matured rRNA. To complete the mechanism of ribosome assembly, these precursor rRNA gets transformed in the polysomes.

Key Points

  • All prokaryotes have 70S (where S=Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol. The 70S ribosome is made up of a 50S and 30S subunits.
  • Ribosomes play a key role in the catalysis of two important and crucial biological processes. peptidyl transfer and peptidyl hydrolysis.
  • Ribosomes are tiny spherical organelles that make proteins by joining amino acids together. Many ribosomes are found free in the cytosol, while others are attached to the rough endoplasmic reticulum.

Key Terms

  • ribosome: Small organelles found in all cells; involved in the production of proteins by translating messenger RNA.
  • translation: A process occurring in the ribosome, in which a strand of messenger RNA (mRNA) guides assembly of a sequence of amino acids to make a protein.
  • Svedberg: The Svedberg unit (S) offers a measure of particle size based on its rate of travel in a tube subjected to high g-force.

11 Molecular Biology of The Gene

Molecular biology is the branch of biology that concerns the molecular basis of biological activity in and between cells, including molecular synthesis, modification, mechanisms and interactions. Molecular biology arose as an attempt to answer the questions regarding the mechanisms of genetic inheritance and the structure of a gene. In 1953, James Watson and Francis Crick published the double helical structure of DNA courtesy of the X-ray crystallography work done by Rosalind Franklin and Maurice Wilkins. Watson and Crick described the structure of DNA and the interactions within the molecule. This publication jump-started research into molecular biology and increased interest in the subject.

Nucleic acids are biopolymers that are essential to all known forms of life. The term nucleic acid is the overall name for DNA and RNA. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. If the sugar is a compound ribose, the polymer is RNA (ribonucleic acid) if the sugar is derived from ribose as deoxyribose, the polymer is DNA (deoxyribonucleic acid).

In an influential published in 1941 paper, George Beadle and Edward Tatum proposed the idea that genes act through the production of enzymes, with each gene responsible for producing a single enzyme that in turn affects a single step in a metabolic pathway. The concept arose from work on genetic mutations in the mold Neurospora crassa, and subsequently was dubbed the “one gene–one enzyme hypothesis” by their collaborator Norman Horowitz. In 2004 Norman Horowitz reminisced that “these experiments founded the science of what Beadle and Tatum called ‘biochemical genetics.’ These experiments are by some considered to constitute the begining of what became molecular genetics and the development of the one gene–one enzyme hypothesis is often considered the first significant result in what came to be called molecular biology. Although it has been extremely influential, the hypothesis was recognized soon after its proposal to be an oversimplification. Even the subsequent reformulation of the”one gene–one polypeptide" hypothesis is now considered too simple to describe the relationship between genes and proteins. In attributing an instructional role to genes, Beadle and Tatum implicitly accorded genes an informational capability. This insight provided the foundation for the concept of a genetic code. However, it was not until the experiments were performed showing that DNA was the genetic material, that proteins consist of a defined linear sequence of amino acids, and that DNA structure contained a linear sequence of base pairs, was there a clear basis for solving the genetic code.

In attributing an instructional role to genes, Beadle and Tatum implicitly accorded genes an informational capability. This insight provided the foundation for the concept of a genetic code. However, it was not until the experiments were performed showing that DNA was the genetic material, that proteins consist of a defined linear sequence of amino acids, and that DNA structure contained a linear sequence of base pairs, was there a clear basis for solving the genetic code.

Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNA, and scientists did not know which of the two is responsible for inheritance. In 1928, Frederick Griffith discovered the phenomenon of transformation: dead bacteria could transfer genetic material to “transform” other still-living bacteria. Sixteen years later, in 1944, the Avery–MacLeod–McCarty experiment identified DNA as the molecule responsible for transformation. The role of the nucleus as the repository of genetic information in eukaryotes had been established by Hämmerling in 1943 in his work on the single celled alga Acetabularia. The Hershey–Chase experiment in 1952 confirmed that DNA (rather than protein) is the genetic material of the viruses that infect bacteria, providing further evidence that DNA is the molecule responsible for inheritance.

James Watson and Francis Crick determined the structure of DNA in 1953, using the X-ray crystallography work of Rosalind Franklin and Maurice Wilkins that indicated DNA has a helical structure (i.e., shaped like a corkscrew). Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what look like rungs on a twisted ladder. This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for replication: if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. This property is what gives DNA its semi-conservative nature where one strand of new DNA is from an original parent strand.

Figure 11.1: A cartoon representation of DNA based on atomic coordinates of PDB 1BNA, rendered with open source molecular visualization tool PyMol.

Although the structure of DNA showed how inheritance works, it was still not known how DNA influences the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of protein production. It was discovered that the cell uses DNA as a template to create matching messenger RNA, molecules with nucleotides very similar to DNA. The nucleotide sequence of a messenger RNA is used as a template by ribosomes to create an amino acid sequence in protein this correspondence between nucleotide sequences and amino acid sequences is known as the genetic code.

With the newfound molecular understanding of inheritance came an explosion of research. One important development was chain-termination DNA sequencing in 1977 by Frederick Sanger. This technology allows scientists to read the nucleotide sequence of a DNA molecule. In 1983, Kary Banks Mullis developed the polymerase chain reaction, providing a quick way to isolate and amplify a specific section of DNA from a mixture. The efforts of the Human Genome Project, Department of Energy, NIH, and parallel private efforts by Celera Genomics led to the sequencing of the human genome in 2003.


Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Ribosomes are responsible for synthesizing proteins and forming amino acids. This process is also known as translation and occurs after DNA replication and transcription. Ribosomes read as they move along the messenger RNA template that is used to copy a particular DNA sequence and produce an amino acid chain.

One of the most basic processes of a cell is protein synthesis and the key organelle for that is the ribosome. It's what physically actually puts amino acids together to form the long chains of a protein. Now, where is it located? It's floating around inside of the cell. If we take a look at this cell here, you can see there is a bunch of little ribosomes floating around the cytoplasm there's also a whole bunch of them studded up against the membrane of the rough endoplasmic reticulum in fact it's the ribosomes that give the rough texture to the rough ER, so what does it do? Well if we take a look here we can se that it's following the instructions here of a messenger RNA molecule that came out of the nucleus, so the ribosomes are the puppet of DNA and the RNA carries that message to the ribosomes for it to follow.

Now when we look at this structure here, we see this red thing here which I've already identified it as the messenger RNA. This blue thing with its little amino acid and the thiamine attached to it, that's the transfer RNA. The two orange substances are the two parts or subunits of a ribosome. Now you may be wondering, what do we call these two subunits? Well scientists looked at it and said, "well this one is bigger let's call it the large subunit, this wee little one let's call the small subunit" cause I guess they weren't Scottish. So the most ribosomes have two subunits the large subunit and small subunit now I discussed and I hesitated there when I was talking about the ribosomes cause actually there's two different kinds of ribosomes.

There are the ribosomes found in prokaryotes like bacteria and their large subunits is just little bit smaller than the large subunit of eukaryotic cells like myself. We have a nucleus and our large subunit is larger than the large subunit of prokaryotes. Similarly, the subunit, the small subunit of prokaryotes is little bit smaller than the small subunit of eukaryotic cells.

Now messenger RNA you might be able to guess is made out of RNA, TRNA is transfer RNA it too is made out of RNA can you guess what kind of molecule makes up the bulk of ribosomes? Yes, you've got it! its RRNA which stands for ribosomal RNA. There's a few other proteins that working together with the ribosomal or RNA help catalyze the addition of amino acids to each other giving us the structure and function of a ribosome.

Biogenesis of Ribosomes (282 Word) | Biology

Synthesis of ribosome in eukaryotes is complicated and takes place inside the nucleolus. Nucleoli disappear during mitosis, but at telophase new nucleoli are formed at specific chromosomal sites called nucleolar organizers located in secondary constrictions on the chromosomes.

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These nucleolar organizers are known to contain genes for 18S, 28S and 5.8S rRNAs. In Xenopus, each nucleolar organizer contains 450 rRNA genes. These genes are tandemly repeated along DNA molecule (i.e., head to tail) and are separated from each other by stretches of spacer DNA which is not transcribed.

These rRNA genes are being actively transcribed, and nascent RNA chains are spread perpendicularly to the DNA axis. Each gene is transcribed into a long RNA molecule (varying in size from 40S to 45S according to the species) which will eventually be processed giving rise to 18S, 28S and 5.8S RNA. Nucleolar rRNA genes are transcribed by RNA polymerase I and these polymerase molecules (about 100 per gene) can be seen at the origin of each nascent RNA chain.

Genes coding for 5S RNA are not located in nucleolus. The 5S genes are also tandemly repeated along DNA molecule and separated from each other by spacer DNA. The 5S RNA is transcribed by RNA polymerase III on the chromosomes and is then transported to the nucleolus, where it is incorporated into the immature large ribosomal subunits.

The 70 ribosomal proteins are synthesized in the cytoplasm. All these components (18S, 5.8S, 28S, 5S and 70S) collect in the nucleolus where they are assembled into ribosomes and transported to the cytoplasm. Ribosome biogenesis is thus a striking example of coordination at the cellular and molecular levels.

Ribosomes: Discovery, Occurrence and Functions

In this article we will discuss about:- 1. Discovery of Ribosomes 2. Occurrence of Ribosomes 3. Functions.

Discovery of Ribosomes:

Ribosomes were discovered by Robinson and Brown (1953) in plant cells and by Palade (1955) in animal cells. Palade (1955) also coined the term of ribosome. A large number of ribosomes occur in a cell. For example, a single cell of bacterium Escherichia coli contains 20000-30000 ribosomes. Their number in eukaryote cells is several times more.

Ribosomes are naked ribonucleoprotein protoplasmic particles (RNP) with a length of 200-340 A and diameter of 170-240A which function as the sites for protein or polypeptide synthesis. Ribosomes are popularly known as protein factories. They are sub-spherical in outline. A covering membrane is absent. Each ribosome consists of two unequal subunits, larger dome shaped and smaller oblate-ellipsoid.

The large subunit has a protuberance, a ridge and a stalk. The smaller subunit possesses a platform, cleft, head and base. It is about half the size of larger subunit.

The smaller subunit fits over the larger one at one end like a cap (Fig. 8.40). Mg2+ is required for binding the two subunits (Below 0.0001 M Mg2+ the two subunits dissociate while above this strength the ribosomes can come together to form dimers Fig. 8.39).

Occurrence of Ribosomes:

Ribosomes may occur singly as monosomes or in rosettes and helical groups called polyribosomes or polysomes (Gk. poly- many, soma- body).

The different ribosomes of a polyribosome are connected with a 10-20 A thick strand of messenger or mRNA (Fig. 8.41). The maintenance of polyribosome requires energy. Polyribosomes are formed during periods of active protein synthesis when a number of copies of the same polypeptide are required.

Ribosomes occur in all living cells with the exception of mammalian erythrocytes or red blood corpuscles. Depending upon the place of their occurrence, ribosomes are of two types, cytoplasmic and organelle. The organelle ribosomes are found in plastids (plastid ribosomes) and mitochondria (mitoribosomes).

The cytoplasmic ribosomes (cytoribosomes) may re­main free in the cytoplasmic matrix or attached to the cytosolic surface of endoplasmic reticulum with the help of a special ribophorin or SRP protein.

Attachment occurs through larger or 60 S subunits. Different types of ribosomes may produce different types of proteins, e.g., structural proteins from free cytoplasmic ribosomes and globular proteins from ribosomes bound to ER.

The bound ribosomes generally transfer their proteins to cisternae of the endoplasmic reticulum for transport to other parts both inside and outside the cell. They are also sent to intracellular organelles like nucleus, mitochondria and chloroplasts. Newly synthesised proteins are assisted in their folding and transport by spe­cific proteins called chaperones.

The size of the ribosomes is determined by sedimentation coefficient in the centrifuge. It is measured as Svedberg unit called S (S =1 x 10 -13 sec). The cytoplasmic ribosomes of eukaryotes are 80 S.

They have a size of 300—340 Ax 200-240 A and mass of 4.0—4.5 million daltons. The cytoplasmic ribosomes of prokaryotes (PPLO, bacteria, and blue – green algae) are 70 S. The size is 200-290 A x 170-210 A and mass is 2.7-3.0 million daltons (Fig. 8.42).

The organelle ribosomes are also 70 S but in mammalian mitochondria they have sedimentation coefficient of 55 S. The two subunits of 80 S ribosomes are 60S and 40S while 70S ribosomes have 50S and 30 S subunits. A tunnel occurs between the two subunits for passage of mRNA. The larger subunit has a groove for pushing out the newly synthesized polypeptide.

A ribosome has four sites for specific attachments:

(ii) A or amminoacyl site for binding to newly arrived amino acid carrying tRNA.

(iii) P or peptidyl site with tRNA carrying growing polypeptide,

(iv) E or exit site for freed tRNA before it leaves the ribosome.

80S ribosomes are synthesized inside the nucleolus. Proteins come from cytoplasm. 5S RNA is synthesized separately while others are formed by the nucleolus. 80S ribosomes do not become functional inside the nucleolus.

Their subunits come out of the nucleus and become operational in cyto­plasm. 70S ribosomes of prokaryotes are formed in the cytoplasm while those of semi-autonomous cell organelles are formed in their matrix.

Chemically a ribosome is made of two parts, proteins and rRNA. The ribosomes of liver cells may also contain lipids to the extent of 5-10%. Usually more rRNA is present in 70S ribosomes as compared to protein (60-65: 35-40) while the reverse is true for 80S ribo­somes (40-44: 56-60). 40S subunit of 80S ribosome contains 33 protein molecules and a single 18S rRNA.

30S subunit of 70S ribosome possesses 21 protein molecules and 16S rRNA. 60S subunit of 80S ribosome has 40 protein molecules and three types of rRNAs- 28S, 5.8S and 5S. 50S subunit of 70S ribosome contains 34 protein molecules and two types of rRNAs— 23S and 5S. Proteins are both structural and enzymatic.

Functions of Ribosomes:

(i) Protein Factories:

Ribosomes are sites for polypeptide or protein synthe­sis.

(ii) Free and Attached Ribosomes:

Free ribosomes synthesise structural and enzymatic proteins for use inside the cell. The attached ribosomes synthesise proteins for transport,

(iii) Enzymes and Factors:

Ribosomes provide enzymes (e.g., Peptidyl transferees) and factors for condensation of amino acids to form polypeptide,

Ribosome contains rRNAs for providing attaching points to mRNA and tRNAs.

Ribosome has a tunnel for mRNA so that it can be translated properly,

(vi) Protection:

Newly synthesized polypeptide is provided protection from cytoplasmic enzymes by enclosing it in the groove of larger subunit of ribosome till it attains secondary structure.


Ribosome collisions are widespread in fast proliferating yeast cells, especially on mRNA with high ribosomal flux. Ribosome collisions tend to occur at stop codons and are often related to the translation completion of α-helices. A large number of collided ribosomes are structurally incompetent to trigger the RQC pathway instead, they are often associated with chaperones, which likely aid in cotranslational protein folding. Taken together, we offer a mechanism that chaperones sense translation elongation rate through ribosome collisions to determine which proteins/peptide regions require cotranslational folding.

Section Summary

Living things are carbon-based because carbon plays such a prominent role in the chemistry of living things. The four covalent bonding positions of the carbon atom can give rise to a wide diversity of compounds with many functions, accounting for the importance of carbon in living things. Carbohydrates are a group of macromolecules that are a vital energy source for the cell, provide structural support to many organisms, and can be found on the surface of the cell as receptors or for cell recognition. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides, depending on the number of monomers in the molecule.

Lipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and oils, waxes, phospholipids, and steroids. Fats and oils are a stored form of energy and can include triglycerides. Fats and oils are usually made up of fatty acids and glycerol.

Proteins are a class of macromolecules that can perform a diverse range of functions for the cell. They help in metabolism by providing structural support and by acting as enzymes, carriers or as hormones. The building blocks of proteins are amino acids. Proteins are organized at four levels: primary, secondary, tertiary, and quaternary. Protein shape and function are intricately linked any change in shape caused by changes in temperature, pH, or chemical exposure may lead to protein denaturation and a loss of function.

Nucleic acids are molecules made up of repeating units of nucleotides that direct cellular activities such as cell division and protein synthesis. Each nucleotide is made up of a pentose sugar, a nitrogenous base, and a phosphate group. There are two types of nucleic acids: DNA and RNA.



amino acid: a monomer of a protein

carbohydrate: a biological macromolecule in which the ratio of carbon to hydrogen to oxygen is 1:2:1 carbohydrates serve as energy sources and structural support in cells

cellulose: a polysaccharide that makes up the cell walls of plants and provides structural support to the cell

chitin: a type of carbohydrate that forms the outer skeleton of arthropods, such as insects and crustaceans, and the cell walls of fungi

denaturation: the loss of shape in a protein as a result of changes in temperature, pH, or exposure to chemicals

deoxyribonucleic acid (DNA): a double-stranded polymer of nucleotides that carries the hereditary information of the cell

disaccharide: two sugar monomers that are linked together by a peptide bond

enzyme: a catalyst in a biochemical reaction that is usually a complex or conjugated protein

fat: a lipid molecule composed of three fatty acids and a glycerol (triglyceride) that typically exists in a solid form at room temperature

glycogen: a storage carbohydrate in animals

hormone: a chemical signaling molecule, usually a protein or steroid, secreted by an endocrine gland or group of endocrine cells acts to control or regulate specific physiological processes

lipids: a class of macromolecules that are nonpolar and insoluble in water

macromolecule: a large molecule, often formed by polymerization of smaller monomers

monosaccharide: a single unit or monomer of carbohydrates

nucleic acid: a biological macromolecule that carries the genetic information of a cell and carries instructions for the functioning of the cell

nucleotide: a monomer of nucleic acids contains a pentose sugar, a phosphate group, and a nitrogenous base

oil: an unsaturated fat that is a liquid at room temperature

phospholipid: a major constituent of the membranes of cells composed of two fatty acids and a phosphate group attached to the glycerol backbone

polypeptide: a long chain of amino acids linked by peptide bonds

polysaccharide: a long chain of monosaccharides may be branched or unbranched

protein: a biological macromolecule composed of one or more chains of amino acids

ribonucleic acid (RNA): a single-stranded polymer of nucleotides that is involved in protein synthesis

saturated fatty acid: a long-chain hydrocarbon with single covalent bonds in the carbon chain the number of hydrogen atoms attached to the carbon skeleton is maximized

starch: a storage carbohydrate in plants

steroid: a type of lipid composed of four fused hydrocarbon rings

trans-fat: a form of unsaturated fat with the hydrogen atoms neighboring the double bond across from each other rather than on the same side of the double bond

triglyceride: a fat molecule consists of three fatty acids linked to a glycerol molecule

unsaturated fatty acid: a long-chain hydrocarbon that has one or more than one double bonds in the hydrocarbon chain

Watch the video: What are Ribosomes? Ribosome Function and Structure (August 2022).