Ribosomes and tRNA

Ribosome is defined as a complex molecular machine found within all living cells that serves as the site of biological protein synthesis (translation). Ribosomes are essential for converting the genetic information encoded in mRNA into a corresponding sequence of amino acids, ultimately leading to the formation of proteins. 

Prokaryotic ribosomes are smaller than eukaryotic ribosomes and have a distinct composition and structure.

1. Subunits and Size

• Overall Size: Prokaryotic ribosomes are referred to as 70S ribosomes based on their sedimentation coefficient in a sucrose gradient. This coefficient indicates the rate at which they sediment during ultracentrifugation, which is influenced by both size and shape.

• Subunits: The 70S ribosome is composed of two subunits:

  • 50S Large Subunit

  • 30S Small Subunit

2. Large Subunit (50S)

• Composition: The 50S subunit is composed of ribosomal RNA (rRNA) and proteins.

  • rRNA Components:

    • 23S rRNA (~2904 nucleotides)

    • 5S rRNA (~120 nucleotides)

  • Proteins: Contains about 31 different ribosomal proteins (L1 to L31).

• Structure: The large subunit has a complex structure that includes the peptidyl transferase center (PTC), which is responsible for forming peptide bonds between amino acids.

3. Small Subunit (30S)

• Composition: The 30S subunit also consists of rRNA and proteins.

  • rRNA Component:

    • 16S rRNA (~1542 nucleotides)

  • Proteins: Contains about 21 different ribosomal proteins (S1 to S21).

• Structure: The small subunit has the decoding center, which interacts with mRNA and ensures proper codon-anticodon pairing during translation.

Chemical Composition

• Ribosomal RNA (rRNA): The bulk of the ribosome's mass is made up of rRNA. These RNA molecules play a critical role in the catalytic activity of the ribosome and in maintaining its structural integrity.

• Ribosomal Proteins: The proteins of the ribosome stabilize the rRNA structure and are involved in various functional interactions necessary for protein synthesis. They are typically highly basic, facilitating binding to the negatively charged rRNA.

Ribosome Assembly and Function

• Assembly: The 30S and 50S subunits are assembled separately in the nucleoid region of prokaryotic cells and only come together during translation.

• Function:

  • The small subunit (30S) binds to the mRNA and ensures correct codon-anticodon interaction.

  • The large subunit (50S) catalyzes peptide bond formation between amino acids.

  • Together, they coordinate the sequential addition of amino acids to a growing polypeptide chain based on the sequence of codons on the mRNA.

Key Points

• Size: 70S ribosomes in prokaryotes, with 50S and 30S subunits.

• rRNA and Proteins: Composed of three rRNA molecules (16S, 23S, 5S) and approximately 52 different proteins.

• Function: Central to the process of translation, converting genetic information into functional proteins.

Eukaryotic ribosomes are distinct from their prokaryotic counterparts in terms of size, structure, and composition.

1. Subunits and Size

• Overall Size: Eukaryotic ribosomes are designated as 80S ribosomes based on their sedimentation coefficient in a sucrose gradient, indicating they sediment faster than the smaller 70S prokaryotic ribosomes.

• Subunits: The 80S ribosome consists of two main subunits:

  • 60S Large Subunit

  • 40S Small Subunit

2. Large Subunit (60S)

• Composition: The 60S subunit is composed of ribosomal RNA (rRNA) and proteins.

  • rRNA Components:

    • 28S rRNA (~4700 nucleotides)

    • 5.8S rRNA (~160 nucleotides)

    • 5S rRNA (~120 nucleotides)

  • Proteins: Contains around 49 different ribosomal proteins, typically named L1 to L49.

• Structure: The large subunit includes the peptidyl transferase center (PTC), which is crucial for catalyzing the formation of peptide bonds during protein synthesis.

3. Small Subunit (40S)

• Composition: The 40S subunit is also made up of rRNA and proteins.

  • rRNA Component:

    • 18S rRNA (~1900 nucleotides)

  • Proteins: Contains about 33 different ribosomal proteins, usually designated S1 to S33.

• Structure: The small subunit contains the decoding center responsible for accurate matching of mRNA codons with tRNA anticodons, ensuring the correct sequence of amino acids in the resulting protein.

Chemical Composition

• Ribosomal RNA (rRNA): The majority of the ribosome's mass is constituted by rRNA. These RNA molecules are not only structural components but also have catalytic roles, particularly in peptide bond formation and the interaction with mRNA and tRNA.

• Ribosomal Proteins: The ribosomal proteins help stabilize the rRNA structure and participate in the various functional processes of the ribosome, including the initiation, elongation, and termination of protein synthesis. They are generally highly basic, which facilitates their interaction with the negatively charged rRNA.

Ribosome Assembly and Function

• Assembly: In eukaryotic cells, ribosome assembly occurs in the nucleolus, a substructure within the nucleus. Pre-ribosomal particles are formed in the nucleolus and then exported to the cytoplasm, where they mature into functional ribosomes.

• Function:

  • The 40S small subunit binds to the mRNA and facilitates the correct pairing of mRNA codons with tRNA anticodons.

  • The 60S large subunit catalyzes the formation of peptide bonds between amino acids, effectively growing the polypeptide chain.

  • Together, they work to translate the genetic information encoded in mRNA into a sequence of amino acids, creating proteins.

Eukaryotic Ribosomes: Additional Details

• Localization: Eukaryotic ribosomes can be found freely in the cytoplasm or bound to the rough endoplasmic reticulum (RER), which plays a role in the synthesis of membrane-bound and secretory proteins.

• Mitochondrial and Chloroplast Ribosomes: Eukaryotic cells also contain 70S ribosomes within mitochondria and chloroplasts, similar to prokaryotic ribosomes, reflecting their evolutionary origins from endosymbiotic bacteria.

Key Points

• Size: Eukaryotic ribosomes are 80S, with 60S and 40S subunits.

• rRNA and Proteins: Consist of four rRNA molecules (18S, 5S, 5.8S, and 28S) and approximately 82 different ribosomal proteins.

• Function: Central to translating genetic information into functional proteins, crucial for cellular function and gene expression.