Ribosome – Definition, Structure, Size, Location, Function, Characteristics

What are Ribosomes?

• Ribosomes are intricate cellular entities responsible for the translation of genetic information into polypeptide chains, which subsequently fold to become functional proteins within cells.
• These structures are composed of numerous ribosomal proteins and several ribosomal RNAs (rRNAs), collectively forming a large complex. The term “ribosome” is derived from “ribo,” signifying ribonucleic acid, and the Greek word “soma,” meaning body.
• Present in both prokaryotic and eukaryotic cells, ribosomes can either be freely dispersed within the cytoplasm or associated with the endoplasmic reticulum. Despite their ubiquitous presence across diverse cell types, the fundamental structure and function of ribosomes remain conserved.
• Each ribosome is bifurcated into two distinct subunits: a larger subunit and a smaller one. These subunits collaboratively engage with messenger RNA (mRNA) molecules, initiating the process of protein synthesis.
• The smaller ribosomal subunit recognizes and pairs with the codons present on the mRNA, utilizing transfer RNAs (tRNAs) as intermediaries. These tRNAs carry specific amino acids, which are sequentially linked by peptide bonds to form a growing polypeptide chain.
• As the ribosome progresses along the mRNA in a 5′-to-3′ direction, it deciphers the nucleotide sequence, ensuring the accurate addition of amino acids in the correct order. This translation process is facilitated by tRNAs, which serve as adaptors, ensuring the precise alignment of amino acids.
• The culmination of this process results in the formation of a polypeptide chain, which eventually folds into a functional protein. Notably, the rate of protein synthesis varies between organisms; bacterial ribosomes, for instance, exhibit a faster synthesis rate compared to their eukaryotic counterparts.
• In summary, ribosomes are essential cellular machineries that play a pivotal role in protein synthesis. Comprising RNA and protein components, they decode genetic information from mRNA, orchestrating the assembly of amino acids into proteins. This process is fundamental to the sustenance and functionality of all living cells.

Definition of Ribosomes

Ribosomes are cellular structures responsible for synthesizing proteins by translating messenger RNA (mRNA) into amino acid chains. Composed of ribosomal RNA (rRNA) and proteins, they are present in both prokaryotic and eukaryotic cells.

Ribosomes Size

• Ribosomes are intricate molecular machines that play a pivotal role in the synthesis of proteins within cells. These structures are composed of two distinct subunits, each differing in size, which come together to facilitate the translation of mRNA into a protein sequence.
• In prokaryotic cells, ribosomes are characterized by their 70S size, derived from the combination of a smaller 30S subunit and a larger 50S subunit. The term “Svedberg” (denoted as ‘S’) is a unit that represents the sedimentation rate during ultracentrifugation, and it is crucial to note that these values are not additive due to their non-linear relationship with size. Prokaryotic ribosomes, with a molecular weight approximating 2.7×10^6 Daltons, span about 20 nm in diameter. Their composition is predominantly rRNA, accounting for 65%, while the remaining 35% is made up of ribosomal proteins.
• On the other hand, eukaryotic cells house ribosomes that are slightly larger, typically ranging between 25 to 30 nm in diameter. These ribosomes are denoted as 80S, stemming from the association of a 40S small subunit with a 60S large subunit. The combined molecular weight of eukaryotic ribosomes is around 4×10^6 Daltons.
• In summary, while ribosomes across both prokaryotic and eukaryotic cells share the fundamental role of protein synthesis, they exhibit differences in size, composition, and molecular weight, reflecting the intricacies of their respective cellular environments.

Types of Ribosomes Based on Sedimentation Coefficients

Ribosomes, essential cellular entities for protein synthesis, can be categorized based on their sedimentation coefficients, denoted as “S.” Two primary types of ribosomes have been identified:

1. 70S Ribosomes:
   • Size: These ribosomes are relatively smaller.
   • Sedimentation Coefficient: 70S
   • Molecular Weight: Approximately 2.7 × 10^6 daltons.
   • Location: Predominantly observed in prokaryotic cells, specifically in bacteria and blue-green algae. Additionally, they are present in the mitochondria and chloroplasts of eukaryotic cells.
2. 80S Ribosomes:
   • Size: These ribosomes are larger compared to the 70S type.
   • Sedimentation Coefficient: 80S
   • Molecular Weight: Roughly 40 × 10^6 daltons.
   • Location: Primarily located in eukaryotic cells, encompassing both plant and animal cells. It’s noteworthy that the ribosomes in mitochondria and chloroplasts are smaller than the cytoplasmic 80S ribosomes.
   • Composition: In the yeast’s 80S ribosome, 79 distinct ribosomal proteins are present, with 12 of them being unique to the species.

In summary, ribosomes are differentiated into 70S and 80S types based on their sedimentation coefficients and molecular weights. While 70S ribosomes are common in prokaryotic cells and certain organelles of eukaryotic cells, 80S ribosomes are characteristic of eukaryotic cells.

Types of Ribosomes Based on Locations

Ribosomes can be classified as “free” or “membrane-bound”.

The membrane-bound and free ribosomes are different just in the way they are distributed spatially. they share the same the structure. The existence of a ribosome in a membrane-bound or free condition is dependent upon the existence of an ER-targeting sequence that is present on that protein synthesized consequently, an individual ribosome may be membrane-bound when producing one protein, and at the same time, it’s free in the cytosol as it is making another protein.

Ribosomes can be described as organelles. However, the usage of the term organelle is usually limited to sub-cellular elements that have the membrane of phospholipids, something that Ribosomes, which are entirely particulate, are not. Because of this, they are often classified as “non-membranous organelles”.

Free Ribosomes

Free ribosomes are able to move throughout the cell’s cytosol however, they are omitted by within the cells nucleus and organelles. Proteins made from free ribosomes get transported into the cell cytosol, and are utilized in the cell. The cytosol is a place that contains large amounts of glutathione and is, consequently also a reduced environment and contains disulfide bonds that result from cysteine residues that have been oxidized, are not produced in it.

Membrane-bound Ribosomes

If a ribosome starts to produce proteins needed by certain organelles, the organelles creating this protein could turn into “membrane-bound”. In eukaryotic cells, this occurs in the region of the endoplasmic-reticulum (ER) is known as”the “rough ER”. The newly formed polypeptide chains are introduced directly into the ER through the ribosome’s vectorial synthesizing and then transferred to their final destinations via the secretory pathway. Bound ribosomes generally create proteins that are utilized in the plasma membrane or are removed out of the cell by Exocytosis.

Ribosome Animation Video

Location of Ribosome

There are two areas in which ribosomes are commonly found within an eukaryotic cell. They are floating in the cytosol, and attached to the endoplasmic-reticulum. These ribosomes are known as free ribosomes as well as bound the ribosomes. In both cases, ribosomes typically form aggregates known as polysomes or polyribosomes in the course of the process of protein synthesizing. Polyribosomes are ribosome clusters which attach to an MRNA molecule when the process of protein production. This allows many copies of a particular protein to be synthesized simultaneously by a single mRNA molecule.

Free ribosomes are usually responsible for making proteins that function within the cytosol (fluid part within the cell’s cytoplasm) and bound ribosomes are typically responsible for making proteins that are released out of the cell, or are incorporated within the cell’s membranes. Incredibly, bound ribosomes and free ribosomes can work together and cells are able to alter their size in accordance with metabolic requirements.

Organelles like chloroplasts and mitochondria in Eukaryotic organisms possess its own ribosomes. Ribosomes inside these organelles much like ribosomes that are found in bacteria, with respect to size. The subunits that make up ribosomes found in mitochondria and chloroplasts have smaller (30S -50S) than the ribosome subunits that are found in the remainder of cells (40S up to 60S).

Characteristics of Ribosome

Bacterial ribosomes

• A little smaller than many of the ribosomes found in the eukaryotes (e.g. animals and plants) with a 20nm the diameter.
• Ribosomes are composed from 65% of the rRNA plus 35 percent Ribosomal proteins
• It is formed in the cell cytoplasm of bacteria.
• The prokaryotic , ribosome (70S) is comprised by 50S (large subunit) and 30S (small subunit)
• In prokaryotes, the 30-S ribosomal subunit is home to 16S rRNA while the 50S subunit of ribosomal ribosomal contains 5S rRNA as well as 23S rRNA.
• The number of nucleotides contained within each rRNA unit are:
  • 16S rRNA is a subunit of 1540 nucleotides. It’s linked with 21 proteins.
  • 5S RNA subunit comprises 120 nucleotides.
  • 23S RNA subunit is comprised of 2900 nucleotides.
  • The 5S and 23S RRNAs are linked to 31 proteins.

Archaeal ribosomes

• The archaeal ribosome’s structure is structurally identical to bacterial Ribosomes.
• They also have 70S and contain two subunits: 50S and 30S similar to the bacteria’s Ribosomes. They are however, like the eukaryotic-derived in ribosomes, when examined at the level of sequence.

Eukaryotic ribosomes

• More massive than prokaryotic ribosomes. approximately 25-30nm in size.
• The process of making ribosomes is a combination of both the cytoplasm as well as the nucleolus.
• The eukaryotic Ribosome (80S) is comprised of 60S (large subunit) and 40S (small subunit).
• In eukaryotic cells the 40S ribosomal subunit is home to the 18S rRNA, whereas the 60S ribosomal subunit has three rRNAs, 5S, 5.8S, and 28S.
• In eukaryotes the semi-autonomous organelles such as chloroplasts and mitochondria also contain 70S ribosomes that are similar to those found in prokaryotes (e.g. bacteria). In this way, it is thought that these organelles of eukaryotic origin have been derived from their ancestors, the bacteria.
• The nucleotides counted within each rRNA unit are:
  • 18S RNA comprises 1900 nucleotides. It’s bound by 33 protein.
  • 5S RNA is composed of 120 nucleotides.
  • 28S RNA comprises 4700 nucleotides.
  • 5.8S The RNA comprises 160 nucleotides.
  • 5S RNA, 28 5S RNA, 28S RNA, as well as 5.8S RNA are linked to 46 proteins.

Structure of Ribosome

Ribosomes are intricate cellular assemblies primarily composed of ribonucleic acid (RNA) and proteins. These components, accounting for 32 to 62% RNA and the remainder as proteins, form the structural and functional core of the ribosome. With a diameter ranging between 150 to 250 Ångströms (A°), ribosomes exhibit a porous and hydrated nature.

Structurally, a ribosome is bifurcated into two distinct subunits:

1. Larger Subunit: This dome-shaped component varies in size based on the type of ribosome. In the 70S ribosome, it is referred to as the 50S subunit, with dimensions spanning 160A° to 180A°. Conversely, in the 80S ribosome, it is termed the 60S subunit.
2. Smaller Subunit: Resembling a cap-like structure, its designation is 30S in the 70S ribosome and 40S in the 80S variant.

The association between these subunits is influenced by the concentration of magnesium ions (Mg++). At higher concentrations (around 0.001M), the subunits adhere, even leading to the formation of dimers. However, a reduction in Mg++ ion concentration results in the detachment of these subunits, rendering them as monosomes. Subsequent aggregation of numerous ribosomes leads to the genesis of polyribosomes or polysomes.

Functionally, ribosomes play a pivotal role in protein synthesis. The two subunits collaboratively engage with messenger RNA (mRNA) to facilitate the translation process. By interpreting the genetic sequence within the mRNA, ribosomes orchestrate the assembly of amino acids into polypeptide chains. Given that the active sites of ribosomes are composed of RNA, they are occasionally termed “ribozymes,” underscoring their enzymatic role in protein synthesis.

In essence, ribosomes, with their dual subunit configuration and RNA-protein composition, serve as the cellular machinery responsible for translating genetic information into functional proteins.

Bacterial ribosomes

Bacterial ribosomes are essential cellular structures in prokaryotes responsible for protein synthesis. These ribosomes exhibit distinct characteristics that differentiate them from their eukaryotic counterparts:

1. Size: Bacterial ribosomes are relatively smaller than those found in eukaryotic organisms such as plants and animals. They possess a diameter of approximately 20nm, as noted by Kurland CG in 1960.
2. Composition: Constituting the bacterial ribosome are ribosomal RNA (rRNA) and ribosomal proteins. Specifically, they are composed of 65% rRNA and 35% ribosomal proteins.
3. Location: These ribosomes originate and function within the cytoplasm of bacterial cells.
4. Subunit Configuration: The prokaryotic ribosome, designated as 70S, bifurcates into two subunits:
   • 50S (Large Subunit): This subunit encompasses both 5S rRNA and 23S rRNA. The 5S rRNA subunit consists of 120 nucleotides, while the 23S rRNA subunit comprises 2900 nucleotides. Collectively, these rRNAs are associated with 31 specific proteins.
   • 30S (Small Subunit): This subunit contains the 16S rRNA, which is composed of 1540 nucleotides and is intricately bound to 21 distinct proteins.

In summary, bacterial ribosomes are specialized structures within prokaryotic cells that play a pivotal role in translating genetic information into proteins. Their unique size, composition, and subunit configuration set them apart from eukaryotic ribosomes, underscoring the diversity of cellular machinery across different organisms.

Archaeal ribosomes

Archaeal ribosomes, essential components of the protein synthesis machinery in archaea, exhibit a unique blend of characteristics that bridge the gap between bacterial and eukaryotic ribosomes:

1. Structural Resemblance: Archaeal ribosomes bear a structural similarity to bacterial ribosomes. Both types are designated as 70S and are bifurcated into two subunits: the larger 50S and the smaller 30S.
2. Sequence-Level Comparison: Despite their structural alignment with bacterial ribosomes, archaeal ribosomes share a closer resemblance to eukaryotic ribosomes at the sequence level, as highlighted by Cullen, Katherine E. in 2009.

In essence, archaeal ribosomes provide a fascinating confluence of features, merging structural attributes of bacterial ribosomes with sequence-level characteristics of eukaryotic ribosomes, underscoring the evolutionary intricacies of cellular machinery across diverse life forms.

Eukaryotic ribosomes

Eukaryotic ribosomes are intricate cellular assemblies responsible for protein synthesis within eukaryotic cells. These ribosomes exhibit distinct features that set them apart from their prokaryotic counterparts:

1. Size: Eukaryotic ribosomes are notably larger than prokaryotic ribosomes, with a diameter ranging between 25-30nm, as documented by Wilson DN in 2012.
2. Ribosome Production: The genesis of eukaryotic ribosomes is a coordinated process that involves both the cytoplasm and the nucleolus of the cell.
3. Subunit Configuration: The eukaryotic ribosome, designated as 80S, is bifurcated into two subunits:
   • 60S (Large Subunit): This subunit encompasses three distinct rRNAs: 5S, 5.8S, and 28S. The 5S rRNA consists of 120 nucleotides, the 5.8S rRNA has 160 nucleotides, and the 28S rRNA comprises 4700 nucleotides. Collectively, these rRNAs are associated with 46 specific proteins.
   • 40S (Small Subunit): This subunit contains the 18S rRNA, which is composed of 1900 nucleotides and is intricately bound to 33 distinct proteins.
4. Ribosomes in Organelles: Eukaryotic cells house semi-autonomous organelles, namely chloroplasts and mitochondria, which possess 70S ribosomes. Intriguingly, these 70S ribosomes bear a resemblance to those found in prokaryotes, such as bacteria. This similarity has led to the hypothesis that these eukaryotic organelles have evolutionary roots tracing back to ancestral bacteria.

In summary, eukaryotic ribosomes, with their unique size, composition, and subunit configuration, play a pivotal role in the protein synthesis of eukaryotic cells. Their presence in organelles like chloroplasts and mitochondria further underscores the evolutionary intricacies and interconnectedness of life forms.

Svedberg Unit

The Svedberg unit, symbolized as “S,” is a non-additive unit that represents the sedimentation rate of a particle in a centrifuge. This sedimentation rate is primarily influenced by the size and shape of the particle. The Svedberg unit is not a direct measure of mass; rather, it quantifies how fast a particle settles when subjected to centrifugal force.

In the context of ribosomes, the Svedberg unit is employed to differentiate between the subunits of ribosomes based on their sedimentation rates. For instance:

Comparison of Prokaryotic and Eukaryotic Ribosomal Subunits:

• Prokaryotic Ribosomes (70S):
  • Large Subunit (50S): Comprises 5S rRNA and 23S rRNA.
  • Small Subunit (30S): Contains 16S rRNA.
• Eukaryotic Ribosomes (80S):
  • Large Subunit (60S): Encompasses 5S, 5.8S, and 28S rRNAs.
  • Small Subunit (40S): Incorporates 18S rRNA.

It’s crucial to understand that the sum of the Svedberg units for the individual subunits does not equate to the total for the whole ribosome. This is because the Svedberg unit is a measure of sedimentation rate, which is influenced by both size and shape, rather than a direct measure of mass.

What is Plastoribosomes and mitoribosomes?

Plastoribosomes and mitoribosomes are specialized ribosomes found within eukaryotic cells, specifically in plastids and mitochondria, respectively.

Plastoribosomes:

• Location: These are situated within plastids, predominantly in chloroplasts of plant cells.
• Type: They are of the 70S type, akin to prokaryotic ribosomes.
• Function: Plastoribosomes play a pivotal role in protein synthesis within plastids.
• Similarity: Plastoribosomes exhibit a closer resemblance to prokaryotic ribosomes, underscoring the evolutionary lineage of chloroplasts from ancestral cyanobacteria.

Mitoribosomes:

• Location: Mitoribosomes are localized within the mitochondrial matrix.
• Type: They also belong to the 70S category.
• Function: Mitoribosomes are instrumental in synthesizing proteins that are essential for mitochondrial functions, utilizing precursors provided by the mitochondria.
• Similarity: While mitoribosomes share certain characteristics with prokaryotic ribosomes, they are not as closely related as plastoribosomes. This reflects the evolutionary history of mitochondria, which are believed to have originated from an ancestral prokaryote.

In summary, both plastoribosomes and mitoribosomes are eukaryotic cell-specific ribosomes that exhibit similarities with prokaryotic ribosomes, highlighting the endosymbiotic origins of plastids and mitochondria.

High-resolution structure of Ribosomes

The intricate architecture of ribosomes has been elucidated with remarkable precision through high-resolution structural studies. These studies have been instrumental in mapping the atomic resolutions of ribosomes from various organisms, providing invaluable insights into their functional and structural nuances.

High-Resolution Structures of Ribosomes from Various Organisms:

1. Prokaryotic Ribosomes:
   • 50S Subunit: This is the large subunit of prokaryotic ribosomes. High-resolution structures have been determined for this subunit from two organisms:
     • Archaeon Haloarcula marismortui: An extremophilic archaeon known for its resilience in high-saline environments.
     • Bacterium Deinococcus radiodurans: A bacterium renowned for its extraordinary resistance to ionizing radiation.
   • 30S Subunit: Representing the small subunit of prokaryotic ribosomes, its high-resolution structure has been delineated from:
     • Bacterium Thermus thermophilus: A thermophilic bacterium that thrives at elevated temperatures.
2. Eukaryotic Ribosomes:
   • 60S Subunit: This is the large subunit of eukaryotic ribosomes. Its high-resolution structure has been mapped from:
     • Tetrahymena thermophila: A model ciliated protozoan used extensively in molecular and cellular biology research.
   • 40S Subunit: The small subunit of eukaryotic ribosomes has its high-resolution structure determined from:
     • Tetrahymena thermophila.

In essence, the high-resolution structures of ribosomes, derived from diverse organisms, have paved the way for a deeper understanding of ribosomal functions, interactions, and evolutionary relationships. These structures, determined with atomic precision, stand as a testament to the advancements in structural biology and the collaborative efforts of the global scientific community.

Ribosome biogenesis 

Ribosome biogenesis is a complex and coordinated process that encompasses the synthesis and assembly of ribosomal components. In eukaryotic cells, this intricate process occurs both within the nucleus, specifically the nucleolus, and the cytoplasm.

Key Aspects of Ribosome Biogenesis:

1. Ribosomal Subunits: Eukaryotic ribosomes are denoted as 80S, comprising two distinct subunits: the larger 60S and the smaller 40S. These subunits are differentiated by their ribosomal RNA (rRNA) and protein constituents.
2. Ribosomal Protein Synthesis: Ribosomal proteins undergo synthesis through a two-step process. Initially, their encoding genes are transcribed within the nucleus. The resultant transcripts are then exported to the cytoplasm, where they undergo translation, yielding ribosomal proteins. Upon maturation, these proteins are transported back to the nucleus, specifically targeting the nucleolus.
3. rRNA Production: The rRNAs, namely 18S, 28S, and 5.8S, are transcribed as a unified precursor, termed 45S pre-rRNA, within the nucleolar organizer region. This transcriptional activity is facilitated by RNA polymerase I. Post-transcriptional modifications lead to the separation of these rRNAs. Conversely, the 5S rRNA is transcribed as pre-5S rRNA by RNA polymerase III in the nucleoplasm, external to the nucleolus. Subsequent processing guides it to the nucleolus.
4. Subunit Assembly: Within the nucleolus, the 5S rRNA associates with the 28S and 5.8S rRNAs, orchestrating the formation of the 60S large subunit. The 18S rRNA, on the other hand, collaborates with ribosomal proteins to constitute the 40S small subunit. These assembled subunits are then channeled from the nucleolus to the cytoplasm.
5. Final Assembly: In the cytoplasm, the 60S and 40S subunits converge, culminating in the formation of the functional 80S ribosome, primed for its role in protein synthesis.

In summation, ribosome biogenesis is a multifaceted process that necessitates the synchronized interplay of transcriptional, translational, and assembly mechanisms. The nucleus, particularly the nucleolus, and the cytoplasm function in tandem to ensure the efficient and accurate formation of ribosomes, underscoring their pivotal role in cellular protein synthesis.

Ribosomes and Protein Assembly

Protein synthesis is triggered by process of transcription and translation. When transcription occurs the genetic code in DNA is translated into an RNA version the code, which is known as messenger RNA (mRNA). The mRNA transcript is transferred from the nucleus into the cell wall where it undergoes translation. As it is translated, a growing amino acid chain sometimes known as a polypeptide chain is created. Ribosomes aid in translating mRNA through binding to the protein molecule and linking amino acids to create the polypeptide chain. The polypeptide chain ultimately becomes fully functional protein. Proteins are extremely important biochemical polymers within our cells, as they play a role in nearly every cell function.

There are some differences in the synthesis of proteins in eukaryotes as well as prokaryotes. Because eukaryotic ribosomes are bigger than those found in prokaryotes and require greater proteins. The other differences are the different amino acid sequences that initiate the process for triggering protein synthesis, and also various elongation and termination mechanisms.

Differences Between eukaryotic ribosome and prokaryotic ribosomes

Ribosomes can be found in organisms like bacteria, parasites and various other animals such as microscopic and lower-level organisms are those that are referred to as prokaryotic ribosomes. The ones that reside inside humans and other animals like higher-level animals are the ones that we refer to as the eukaryotic ribosome.

1. Prokaryotes possess 70S ribosomes consisting of a 30S and 50S subunit. In contrast, eukaryotes possess 80S ribosomes comprised of a 40S as well as a 60S subunit.
2. 70S Ribosomes tend to be smaller than 80S ribosomes, whereas the 80S Ribosomes are significantly larger than 70S ribosomes.
3. Prokaryotes possess a 30S subunit that is an RNA 16S subunit. These comprise 1540 nucleotides attached by 21 proteins. The 50S subunit is made by a subunit of 5S RNA which contains 120 nucleotides. the 23S RNA subunit includes 2900 nucleotides, and 31 proteins.
4. Eukaryotes are characterized by 40S subunits with 18S RNA, along with 300 proteins and 1900 nucleotides. The main subunit has 5S RNA, 120 nucleotides, 4700 nucleotides as well as also 28S RNA. 5.8S RNA and 160 nucleotides and 46 proteins.
5. Eukaryotic cells contain chloroplasts and mitochondria as organelles. Those organelles also have ribosomes 70S. Therefore, eukaryotic cells possess various types of the ribosomes (70S as well as 80S) and prokaryotic cells only contain 70S ribosomes.

What is Heterogeneous ribosomes?

Ribosomes have a compositional heterogeneity that is shared by species and even within same cell, as demonstrated by the presence of mitochondrial and cytoplasmic Ribosomes in the same eukaryotic cell. Some researchers have proposed that the diversity in the composition of the ribosomal proteins of mammals is crucial in the regulation of genes, i.e., the special ribosome hypothesis. This hypothesis is controversial and a subject for ongoing study.

Heterogeneity in the composition of ribosomes was first suggested to play a role in the controlling the translation of protein production in the work of Vince Mauro and Gerald Edelman. They suggested the ribosome filter hypothesis in order to explain the functions of regulation the ribosomes. Evidence suggests that specialized ribosomes with a specificity to distinct cell populations could alter the way that gene expression is controlled. Certain ribosomal proteins exchange the complex assembled with the cytosolic copies HTML1suggesting structures of in living the ribosome could be altered without the necessity of creating a new ribosome.

Certain ribosomal protein are essential for the life of a cell, and others aren’t. In the budding yeast, 14/78 of ribosomal proteins are not essential to grow, whereas in humans, this is contingent on the specific cell. Other examples of heterogeneity are post-translational modifications to ribosomal proteins like the acetylation process, methylation as well as phosphorylation. In Arabidopsis and Viral, internal ribosome entrance sites (IRESs) can facilitate translation by ribosomes with distinct compositions. For instance 40S ribosomal units that lack the eS25 in mammalian cells and yeast cells aren’t able to attract CrPV IGR. CrPV IGR.

Heterogeneity in ribosomal DNA modifications is a key factor in maintaining structural integrity or function. Most modifications to mRNA are located in high-conserved areas. The most commonly used changes to rRNA include the pseudouridylation as well as 2′-O of methylation of ribose.

Functions of Ribosome

Ribosomes are intricate molecular machines that play a pivotal role in cellular protein synthesis, a process termed translation. Their primary function is to facilitate the decoding of messenger RNA (mRNA) sequences into polypeptide chains, which will subsequently fold into functional proteins.

Key Functions of Ribosomes:

1. Protein Synthesis: Ribosomes are the primary sites of protein biosynthesis in cells. They read the sequence of the mRNA and, using transfer RNA (tRNA) molecules, assemble amino acids in the precise order dictated by the mRNA, resulting in the formation of a polypeptide chain.
2. Catalytic Activities: Ribosomes exhibit enzymatic properties, particularly during peptidyl transfer and peptidyl hydrolysis, essential steps in the elongation of the polypeptide chain.
3. mRNA Protection: During translation, the mRNA strand is positioned between the large and small ribosomal subunits, safeguarding it from degradation by nucleases.
4. Protection of Nascent Polypeptide: The emerging polypeptide chain is shielded from proteolytic enzymes, ensuring its proper folding and maturation.
5. Coordination of tRNA Binding: Ribosomes possess three distinct tRNA binding sites:
   • A (Aminoacyl) Site: Binds the tRNA carrying the next amino acid to be added to the growing polypeptide chain.
   • P (Peptidyl) Site: Holds the tRNA with the growing polypeptide chain.
   • E (Exit) Site: Where discharged tRNAs exit the ribosome.
6. Translocation: After the addition of an amino acid to the polypeptide chain, the ribosome moves along the mRNA, ensuring the correct positioning of tRNAs and facilitating the continuation of protein synthesis.
7. Termination: Upon encountering a stop codon on the mRNA, release factors such as RF1 and RF2 are recruited, leading to the hydrolysis of the bond between the polypeptide and tRNA. Consequently, the newly synthesized protein is released.
8. Target for Antibacterial Agents: The unique structure and biochemical properties of ribosomes have been exploited in the development of antibiotics that inhibit protein synthesis. Examples include aminoglycosides, macrolides, and tetracyclines, which specifically target either the 50S or 30S ribosomal subunits, disrupting bacterial protein synthesis and thereby inhibiting bacterial growth.

In summary, ribosomes are essential cellular organelles responsible for the synthesis of proteins. They ensure the accurate translation of genetic information from DNA to mRNA and subsequently into proteins, which are vital for the myriad of cellular functions. The understanding of ribosomal structure and function has not only provided insights into cellular biology but has also paved the way for the development of therapeutic agents targeting protein synthesis.

Quiz

Which of the following is the primary function of ribosomes?
a) DNA replication
b) Photosynthesis
c) Protein synthesis
d) Lipid metabolism

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In which cellular location would you primarily find ribosomes?
a) Nucleus
b) Mitochondria
c) Cytoplasm
d) Golgi apparatus

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Which type of RNA is primarily associated with ribosomes?
a) mRNA
b) tRNA
c) rRNA
d) snRNA

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The ribosomes found in prokaryotic cells are of which type?
a) 60S
b) 80S
c) 70S
d) 90S

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Which subunit of the ribosome reads the mRNA?
a) Large subunit
b) Small subunit
c) Both subunits
d) Neither subunit

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Which antibiotic specifically targets bacterial ribosomes?
a) Penicillin
b) Tetracycline
c) Aspirin
d) Ibuprofen

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Ribosomes are made up of which two main components?
a) Lipids and proteins
b) Sugars and lipids
c) Proteins and rRNA
d) mRNA and tRNA

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Which organelle is closely associated with ribosomes for protein synthesis and modification?
a) Lysosome
b) Endoplasmic reticulum
c) Peroxisome
d) Vacuole

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Which of the following is NOT a function of ribosomes?
a) Catalyzing peptide bond formation
b) Assisting in DNA replication
c) Reading the genetic code
d) Facilitating the binding of tRNA to mRNA

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In eukaryotic cells, ribosomes can be found attached to which of the following?
a) Mitochondrial membrane
b) Nuclear envelope
c) Rough endoplasmic reticulum
d) Plasma membrane

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FAQ

References

• Nobel Prize Outreach AB. (1974). George E. Palade – Facts. Retrieved from https://www.nobelprize.org/prizes/medicine/1974/palade/facts/
• McQuillen, K., Roberts, R. B., & Britten, R. J. (1959). SYNTHESIS OF NASCENT PROTEIN BY RIBOSOMES IN ESCHERICHIA COLI. Proceedings of the National Academy of Sciences of the United States of America, 45(9), 1437–1447. https://doi.org/10.1073/pnas.45.9.1437
• Noller, H. F., Hoffarth, V., & Zimniak, L. (June 1992). Unusual resistance of peptidyl transferase to protein extraction procedures. Science, 256(5062), 1416–9. https://doi.org/10.1126/science.1604315
• Noller, H. F. (2012). Evolution of protein synthesis from an RNA world. Cold Spring Harbor Perspectives in Biology, 4(4), a003681. https://doi.org/10.1101/cshperspect.a003681
• Burma, D. P. (1992). RNA world and ribosomes. Current Science, 63(9/10), 550–560. Retrieved from http://www.jstor.org/stable/24094570
• Palade, G. E. (January 1955). A small particulate component of the cytoplasm. The Journal of Biophysical and Biochemical Cytology, 1(1), 59–68. https://doi.org/10.1083/jcb.1.1.59
• Kurland, C. G. (1960). Molecular characterization of ribonucleic acid from Escherichia coli ribosomes. Journal of Molecular Biology, 2(2), 83–91. https://doi.org/10.1016/s0022-2836(60)80029-0
• Wilson, D. N., & Doudna Cate, J. H. (May 2012). The structure and function of the eukaryotic ribosome. Cold Spring Harbor Perspectives in Biology, 4(5), a011536. https://doi.org/10.1101/cshperspect.a011536
• Cullen, K. E. (2009). Archaeal Ribosomes. In Encyclopedia of Life Science (pp. 1–5). New York: Facts On File. https://doi.org/10.1002/9780470015902.a0000293.pub3
• Schmitt, E., Coureux, P. D., Kazan, R., Bourgeois, G., Lazennec-Schurdevin, C., & Mechulam, Y. (2020). Recent Advances in Archaeal Translation Initiation. Frontiers in Microbiology, 11, 584152. https://doi.org/10.3389/fmicb.2020.584152
• Alberts, B. et al. (2002). The Molecular Biology of the Cell (4th ed.). Garland Science. ISBN 978-0-8153-3218-3.
• Wilson, D. N. (2013). Ribosome-targeting antibiotics and mechanisms of bacterial resistance. Nature Reviews Microbiology, 12, 35-48.