Erakina

Palade granules (Ribosomes) – Erakina

Published Date : August 4, 2022

George Palade discovered granular, macromolecular structures in animal cells. These macromolecular machines were later christened ribosomes.

Ribosomes: An overview

Ribosomes are macromolecular structures present within all cells. The main function of ribosomes is the synthesis of proteins within the cell. George Palade first observed ribosomes using an electron microscope and described them as dense, granular entities. Proteins are essential for the functioning of any biological cell. Even ribosomes require proteins for the further synthesis of more proteins. Ribosomes are essentially made up of RNA and proteins that facilitate the joining together of amino acid molecules for the formation of long-chain peptides i.e., proteins. 

Structure 

Ribosomes are divided into two components- a large subunit and a small subunit. The structure of ribosomes can be elucidated as follows-

  • Each ribosome is composed of ribosomal RNA and distinct proteins that vary from organism to organism.
  • The two ribosomal subunits are of different sizes, attributing to the disproportionate length of the ribosomal axis.
  • The (30S) or the small subunit mainly decodes the DNA sequence that is responsible for the encoding of amino acids of a particular protein. It is bound to mRNA.
  • The (50S) or large subunit is responsible for catalysis and is bound to tRNA strands that are aminoacylated.

It is important to note that the ribosomal structure is very similar in bacteria, archaebacteria and eukaryotes. They are organized into distinct tertiary structures or motifs such as pseudoknots in which coaxial stacking is observed. However, they do differ along the following lines-

  • Size
  • Structure 
  • The ratio of protein to RNA

It is this difference in the ribosomal structure among eukaryotes and bacteria that allows antibiotics to take over the ribosomal machinery of specific bacteria without affecting the ribosomes present in human cells.

Yeast ribosome structure
3D molecular structure of a yeast ribosome.

Functioning 

Ribosomes are responsible for the synthesis of proteins within biological cells. The RNA and associated proteins present within a ribosome are employed for the decoding of genetic sequences, and subsequent peptide bond formation. Ribosomes are either found floating freely within the cytoplasm or can be found attached to the Endoplasmic Reticulum (RER). The primary functions of ribosomes are as follows-

  • Decoding of genetic codes into associated amino acid sequences.
  • Conversion of amino acid monomers into protein polymers.
  • Ribosomes act as catalysts in two significant cellular processes- peptidyl transfer and peptidyl hydrolysis.

To summarize, ribosomes traverse each codon provided by mRNA and subsequently pair each codon with its associated amino acid. These amino acids are provided by aminoacyl tRNA. Due to the utilization of conformational changes by ribosomes, tRNA can be recognized efficiently and accurately.

Mechanisms of protein synthesis by ribosomes.
Structural depiction of the ribosomal subunits.

Ribosomal locations

Based on their location within the cell, ribosomes can be categorised as either “free” or “membrane-bound”. Both types of ribosomes are identical in structure and differ only in the context of spatial distribution. Due to the absence of a phospholipid membrane around ribosome granules, they are often referred to as non-membranous organelles.

  • Free ribosomes- these ribosomes are allowed to move freely within the cell cytosol and are barred only from the nucleus and cell organelles. Proteins containing disulfide bonds cannot be synthesized by free ribosomes, due to the reducing nature of the cell cytosol.
  • Membrane-bound ribosomes- are found in association with the endoplasmic reticulum. These ribosomes synthesise proteins that are transported within the cell via the secretory pathway. They usually become membrane-bound during the process of protein synthesis. These proteins are often used within the cell or expelled outside via the process of exocytosis.
Ribosome structure determined by electron microscopy.
Ribosomes found in an E.coli cell.

Biogenesis of ribosomes

The process of ribosome synthesis is different in prokaryotic and eukaryotic cells.

  • Prokaryotes- ribosomes are synthesised in the cytoplasm through the transcription of multiple ribosomal gene operons present within the cell.
  • Eukaryotes- the process of ribosomal cell synthesis takes place within both, the cell cytoplasm and the nucleolus. 

The process of ribosomal synthesis is a result of the coordinated functioning of more than 200 proteins and the processing of the four types of rRNA.

Rough endoplasmic reticulum and ribosomes.
Microscopy of ER with its associated ribosomes.

Origin of ribosomes

Studies suggest that ancient ribosomes existed when the only biological molecules available were RNA. These entities could have been self-replicating in nature and then subsequently developed the ability to synthesise proteins once amino acids started being environmentally synthesised. Studies have hypothesised the existence of cells with self-replicating properties that possessed only RNA. These cells are called ribocytes.

Conclusion 

Ribosomes are granular entities that are present within all biological cells. These ribosomes are responsible for the decoding of DNA sequences that code for amino acids and the synthesis of peptide bonds. In the context of composition, ribosomes are heterogeneous entities. This heterogeneity is observed not only across different cellular species but also within a single cell. This heterogeneity of ribosomes can be exemplified by observing the compositional differences between cytoplasmic ribosomes and mitochondrial ribosomes in a eukaryotic cell.

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