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Prokaryotic Ribosomes – Definition, Subunits, Function

Sourav Bio

What is Prokaryotic Ribosomes?

  • Prokaryotic ribosomes represent fundamental cellular machinery crucial for protein synthesis within prokaryotic organisms, such as bacteria and archaea. These ribosomes are intricate molecular complexes composed primarily of ribonucleic acid (RNA) and proteins. In prokaryotic cells, ribosomes are dispersed throughout the cytosol and play a pivotal role in the translation of genetic information into functional proteins. This article elucidates the structural and functional aspects of prokaryotic ribosomes with a focus on their composition and distribution.
  • Prokaryotic ribosomes are composed of two distinct subunits: the large 50-S subunit and the small 30-S subunit. These two subunits combine to form the 70-S ribosome, with the ‘S’ denoting the Svedberg unit—a measure of the sedimentation coefficient obtained through ultracentrifugation. The 70-S ribosome is a hallmark of prokaryotic cells and serves as the molecular platform for protein synthesis.
  • The structural composition of prokaryotic ribosomes is intricate and comprises both ribosomal RNA (rRNA) and proteins. Remarkably, ribosomes in prokaryotes are dense granular structures found within the cytoplasmic matrix. These ribosomal inclusion bodies consist of approximately 35% proteins and 65% ribosomal RNA. This unique composition underscores their indispensable role in cellular processes, as they facilitate the translation of genetic information encoded in messenger RNA (mRNA) into functional polypeptides.
  • It is essential to note that prokaryotic cells differ from eukaryotic cells in terms of organelle organization. Prokaryotes lack specialized membrane-bound organelles, and as such, their ribosomes are not associated with internal membranes like the endoplasmic reticulum (ER) in eukaryotic cells. Instead, prokaryotic ribosomes are closely associated with the cytoplasmic membrane, positioning them optimally for the translation of mRNA and protein synthesis.
  • In conclusion, prokaryotic ribosomes are integral cellular structures found in prokaryotic organisms. Comprising two subunits, the 70-S ribosome is vital for protein synthesis and is distributed throughout the cytoplasmic matrix. The dense, granular nature of prokaryotic ribosomes, primarily composed of ribosomal RNA and proteins, highlights their universal importance in the translation of genetic information into functional proteins, a process essential for the survival and function of all cells.

Definition of Prokaryotic Ribosomes

Prokaryotic ribosomes are molecular complexes consisting of ribonucleic acid (RNA) and proteins that facilitate protein synthesis within prokaryotic cells, such as bacteria and archaea.

Prokaryotic Ribosomes - Definition, Subunits, Function
Prokaryotic Ribosomes

Ribosome Subunits

Ribosomes, intricate molecular complexes responsible for protein synthesis, consist of two essential subunits in prokaryotic cells, collectively referred to as the 70S ribosome. These ribosomal subunits, known as the 30S and 50S subunits, play distinct yet complementary roles in the translation process, involving both ribonucleic acid (RNA) and various proteins. Here, we delve into the structural composition and functional significance of these ribosomal subunits.

The 30S subunit, representing the smaller of the two, comprises 16S ribosomal RNA (rRNA) and 21 associated proteins. In contrast, the larger 50S subunit consists of both 5S and 23S rRNA along with 31 associated proteins. The molecular weight of a ribosome is approximately 2.7×10^6 Daltons, attesting to its substantial molecular complexity.

When protein synthesis is underway, these individual subunits join forces to form a complete 70S ribosome. These ribosomes are approximately 25 nanometers in width and are present in substantial numbers, with an estimated 15,000 ribosomes per bacterial cell.

Crucially, both the 30S and 50S subunits harbor three distinct sites for the association of transfer RNA (tRNA) molecules:

  1. A site (A): The aminoacyl site accepts incoming aminoacylated tRNA molecules, bearing amino acids ready for incorporation into the growing polypeptide chain.
  2. P site (P): The peptidyl site accommodates tRNA molecules carrying the nascent peptide chain.
  3. E site (E): The exit site holds deacylated tRNA molecules as they exit the ribosome after donating their amino acids.

Ribosomes are the central hubs for protein synthesis, orchestrating the conversion of genetic information encoded in messenger RNA (mRNA) into functional proteins. Both the 30S and 50S subunits contribute significantly to this intricate process. The 30S subunit, for instance, plays a pivotal role in the accurate association of anticodon sites on adapter tRNA molecules with the mRNA strand transcribed from DNA. It ensures precise base pairing between mRNA codons and tRNA anticodons during the decoding process, thereby ensuring fidelity in the translation process.

On the other hand, the 50S subunit takes charge of binding the acceptor arm of tRNA molecules. This action initiates the formation of the polypeptide chain in accordance with the instructions encoded in the mRNA. It facilitates the pairing between incoming amino acids carried by the tRNA at the A site and the nascent peptide chain attached to the tRNA at the P site.

In summary, the prokaryotic ribosome is a highly structured and functionally sophisticated molecular machine composed of the 30S and 50S subunits. These subunits, with their precise arrangement of RNA and proteins, play vital roles in the translation of genetic information, ensuring the accurate synthesis of proteins essential for cellular function. The detailed 3D structure of ribosomes also reveals specific binding sites for tRNA, mRNA, and antibiotics targeting ribosomes, shedding light on their remarkable versatility and significance in cellular processes.

Significance of Ribosome

Ribosomes, intricate macromolecular complexes composed of ribonucleic acid (RNA) and proteins, hold profound significance in the realm of cellular and molecular biology. Their multifaceted roles encompass the study of RNA, proteins, and antibiotic interactions, making them invaluable tools for scientific exploration and medical research. This article delves into the profound significance of ribosomes, emphasizing their pivotal functions in various biological processes.

  1. Elucidating RNA Structure and Interactions: Ribosomes offer a unique window into the world of RNA. Their intricate structure reveals the spatial arrangement of ribosomal RNA (rRNA) and its interactions with associated proteins. This structural insight is instrumental in advancing our understanding of RNA folding, stability, and the intricate networks of molecular interactions that underpin cellular processes.
  2. Antibiotic Research and Development: The study of ribosomes is of paramount importance in the field of antibiotic research and development. Many antibiotics exert their antimicrobial effects by targeting ribosomes and disrupting protein synthesis in pathogenic microorganisms. Investigating these interactions provides a foundation for the design and development of antibiotics with enhanced efficacy and reduced resistance.
  3. Antibiotic Mechanism Exploration: Ribosomes serve as a critical platform for deciphering the mechanisms of antibiotics. By studying how antibiotics interact with ribosomal components, researchers gain insights into the precise molecular mechanisms through which these drugs inhibit translation. This knowledge not only aids in the development of new antibiotics but also informs strategies to combat antibiotic resistance.
  4. Fundamental Role in Protein Synthesis: At its core, the ribosome’s primary function is to orchestrate protein synthesis. Proteins are the fundamental building blocks of all living cells, and their synthesis is a fundamental biological process. Ribosomes facilitate the precise pairing of messenger RNA (mRNA) with transfer RNA (tRNA) during translation, ensuring the accurate assembly of amino acids into functional polypeptides. This process is vital for the survival and function of all cells.

In conclusion, ribosomes are not mere cellular structures but indispensable tools for scientific inquiry and medical advancement. Their significance extends from elucidating the intricacies of RNA structure and interactions to informing antibiotic research, development, and mechanism exploration. Furthermore, ribosomes play a foundational role in the synthesis of proteins, the essential molecules that underpin the biochemistry of life. Thus, the study of ribosomes continues to enrich our understanding of the molecular mechanisms that govern the cellular processes that sustain all living organisms.

Function of Ribosome

The ribosome, a paramount cellular organelle, serves as the pivotal site for protein synthesis in both prokaryotic and eukaryotic organisms. Comprising two distinct subunits, the 30S and 50S subunits, the ribosome carries out a multitude of functions in a highly orchestrated manner. Here, we delve into the specific functions of these ribosomal subunits with precision and clarity.

30S Subunit

The 30S subunit plays a critical role in the initiation of protein synthesis. It acts as a binding site for the transcribed messenger RNA (mRNA), facilitating the assembly of the ribosome-mRNA complex. Furthermore, the smaller subunit ensures the fidelity of translation by promoting specific base-pairing between the codons on the mRNA and the anticodons on the transfer RNA (tRNA). This precise codon-anticodon interaction dictates the accurate selection of amino acids for incorporation into the growing polypeptide chain.

50S Subunit

Conversely, the 50S subunit assumes a central role in the elongation phase of protein synthesis. It mediates the formation of peptide bonds during the peptidyl transfer reaction, an indispensable step in polypeptide synthesis. This catalytic activity arises from the presence of the peptidyl transferase center, a specific site within the 50S subunit.

Moreover, the 50S subunit is a key target for various antibiotics, including macrolides, chloramphenicol, clindamycin, and the pleuromutilins. These antibiotics exert their inhibitory effects by binding to the 50S subunit, thereby interfering with its normal functioning. By impeding peptide bond formation or inhibiting the translocation of tRNA molecules, these antibiotics disrupt protein synthesis, making them valuable tools in medicine.

Beyond its catalytic and antibiotic binding functions, the 50S subunit serves as a safeguard against premature polypeptide hydrolysis. It also acts as a binding site for G-protein factors, which regulate essential aspects of translation, including initiation, elongation, and termination. Additionally, the 50S subunit contributes to the intricate process of protein folding that occurs immediately after translation. This folding is crucial for the protein’s functional conformation and biological activity.

In summary, the ribosome, with its 30S and 50S subunits, is a multifaceted molecular machine essential for protein synthesis. The 30S subunit facilitates mRNA binding and ensures the accuracy of codon-anticodon interactions, while the 50S subunit catalyzes peptide bond formation, serves as a target for antibiotics, prevents premature polypeptide hydrolysis, and participates in vital regulatory processes. The collaboration of these subunits within the ribosome underscores the precision and complexity of protein synthesis in the cell.


  1. What are the two subunits that make up prokaryotic ribosomes?
    a) 40S and 60S
    b) 30S and 60S
    c) 40S and 50S
    d) 30S and 50S
  1. Which molecule primarily constitutes the smaller subunit of prokaryotic ribosomes?
    a) DNA
    b) RNA
    c) Protein
    d) Lipid
  1. The sedimentation coefficient of the 70S ribosome is measured in which unit?
    a) Dalton
    b) Newton
    c) Svedberg (S)
    d) Hertz
  1. What percentage of ribosomal RNA (rRNA) is typically found in prokaryotic ribosomes?
    a) 10%
    b) 25%
    c) 50%
    d) 65%
  1. How many subunits combine to form a complete 70S ribosome during protein synthesis in prokaryotes?
    a) 1
    b) 2
    c) 3
    d) 4
  1. Which subunit of prokaryotic ribosomes is a target for antibiotics such as chloramphenicol and erythromycin?
    a) 30S
    b) 40S
    c) 50S
    d) 60S
  1. What is the primary role of the 30S subunit during translation in prokaryotic cells?
    a) Formation of peptide bonds
    b) mRNA binding
    c) tRNA binding at the A-site
    d) Protein folding
  1. The 50S subunit possesses which enzymatic activity important for protein synthesis?
    a) DNA replication
    b) DNA repair
    c) Peptidyl transferase
    d) RNA transcription
  1. How many nucleotide pieces of RNA are typically found in the 30S ribosomal subunit of prokaryotic ribosomes?
    a) 100
    b) 500
    c) 1540
    d) 2900
  1. What is the primary function of ribosomes in prokaryotic cells?
    a) DNA replication
    b) ATP synthesis
    c) Protein synthesis
    d) Lipid synthesis


What are prokaryotic ribosomes?

Prokaryotic ribosomes are molecular complexes found in prokaryotic cells, such as bacteria and archaea, responsible for protein synthesis. They consist of two subunits, 30S and 50S, which combine to form the 70S ribosome.

How do prokaryotic ribosomes differ from eukaryotic ribosomes?

Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S). Additionally, prokaryotic ribosomes lack membrane-bound organelles, while eukaryotic ribosomes are associated with the endoplasmic reticulum.

What is the significance of the 70S ribosome in prokaryotes?

The 70S ribosome is essential for protein synthesis in prokaryotic cells. It ensures accurate translation of genetic information from mRNA to proteins, a fundamental process for cell function and survival.

What is the composition of prokaryotic ribosomes?

Prokaryotic ribosomes consist of ribosomal RNA (rRNA) and proteins. The 30S subunit contains 16S rRNA and associated proteins, while the 50S subunit contains 5S and 23S rRNA along with associated proteins.

How do antibiotics target prokaryotic ribosomes?

Many antibiotics, like chloramphenicol and erythromycin, inhibit protein synthesis by binding to specific sites on the 50S subunit of prokaryotic ribosomes. This disrupts the translation process in bacteria.

What is the function of the 30S subunit in prokaryotic ribosomes?

The 30S subunit is responsible for mRNA binding and ensuring accurate base-pairing between mRNA codons and tRNA anticodons during translation, thereby maintaining fidelity in protein synthesis.

What is the role of the 50S subunit in prokaryotic ribosomes?

The 50S subunit facilitates peptide bond formation during protein synthesis. It also serves as a target for antibiotics and plays a role in protein folding after translation.

How many ribosomes are typically found in a bacterial cell?

Bacterial cells can contain thousands of ribosomes. The exact number varies depending on factors like the cell’s metabolic activity and growth rate.

Can prokaryotic ribosomes be a target for drug development?

Yes, prokaryotic ribosomes are a target for developing antibiotics to combat bacterial infections. Understanding their structure and function aids in designing antibiotics with high specificity and effectiveness.

Are prokaryotic ribosomes found only in bacteria?

No, prokaryotic ribosomes are also present in archaea, a group of microorganisms distinct from bacteria. Both archaea and bacteria have prokaryotic ribosomes, although there may be some differences in their ribosomal RNA sequences.

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