Ribosomes

What are Ribosomes?

Ribosomes are complex molecular machines found inside living cells that synthesize proteins from amino acids, a process known as protein synthesis or translation. They are located within all living cells and play a major role in the production of proteins.

Ribosomes are made of large and small subunits and are made of ribosomal RNA and proteins. They can be free-floating in the cytoplasm or attached to the endoplasmic reticulum to form the rough endoplasmic reticulum.

Discovery of Ribosomes (George Emil Palade)

George Emil Palade, a cell biologist of the 1950s, was the pioneer in presenting a detailed description of ribosomes, for which he was awarded a Nobel Prize in 1974.

Structure of Ribosomes

Ribosomes are made up of rRNA and proteins, which are again represented in the form of two subunits: the large subunit and the small subunit.

Subunits – Large and Small

• A small subunit decodes the sequence within mRNA whereas a large subunit assembles amino acids into a polypeptide chain.

• Ribosomes of prokaryotes (70S) are smaller, comprising two subunits: 50S large subunit and 30S small subunit. On the other hand, eukaryotes have a larger ribosome (80S), consisting of a 60S large subunit and a 40S small subunit.

• These differences represent the complexity and various size differences between these two cell types.

rRNA and Protein Composition

• Ribosomes can be classified as supra molecular structures made up of RNA (40%) and proteins (60%).

• The rRNA molecules are involved in the actual translation of the mRNA message and protein synthesis at the site of the ribosome, whereas proteins serve to stabilize the whole structure of the ribosome.

• Each ribosome has its own specific sequence to properly decode mRNA for translation into a polypeptide. In prokaryotes and eukaryotes, the ribosomal subunits dissociate after protein synthesis to cycle back through the subunit recycling.

Types of Ribosomes

Based on their location and functions, ribosomes are of two types: free ribosomes and bound ribosomes.

Free ribosomes

• Free ribosomes are found throughout the cytosol and most free ribosomes synthesize proteins that function within the cytosol and other fluid parts of the cell.

• Such proteins include those that are involved in the metabolic pathways of respiration, for example, as well as protein enzymes active in these pathways and structural proteins, necessary to maintain the cell's flexibility and tensile strength, for example.

• Free ribosomes account for the majority of proteins synthesized within a cell and are key in the daily synthesis and turnover of a cell's components.

Bound ribosomes

• Bound ribosomes occur together with a portion of the endoplasmic reticulum (ER) called the rough ER.

• These ribosomes make proteins that are destined for outside the cell, for movement across the membrane of the cell, or for importation into the lysosome.

• These bound ribosomes enable the resultant polypeptides to rapidly transfer across the membranes of the ribosomes, entering ER lumens in which they can fold into secondary, tertiary, and quaternary structures.

• This localization to the ER membrane is important for sorting and localization to their final sites.

Ribosome Subunits in Prokaryotes vs Eukaryotes

Prokaryotic and eukaryotic ribosomes differ in complexity and function.

70S Ribosomes (50S + 30S)

• 70S prokaryotic ribosomes are then made up of a 50S large subunit and a 30S small subunit.

• These are small and relatively simple compared to eukaryotic ribosomes.

80S Ribosomes (60S + 40S)

• Eukaryotic ribosomes are generally larger and are designated by the nomenclature 80S.

• The eukaryotic ribosome can be broken down into a 60S large subunit and a 40S small subunit.

• Compared to prokaryotic ribosomes, they are more complex and contain further rRNA and protein molecules.

  Prokaryotic Ribosomes	Eukaryotic Ribosomes
  Occur free in the cytoplasm	Most are attached to the surface of nucleus and ER
  Sedimentation coefficient is 70S	Sedimentation coefficient is 80S
  Made up of 50S and 30S	Made up of 60S and 40S
  Made up of 60% rRNA and 40% ribosomal proteins	Made up of 40% rRNA and 60% ribosomal proteins

Functions of Ribosomes

The functions of ribosomes is described below-

Protein Synthesis and Translation

• Ribosomes are the main place in protein synthesis where translation takes place.

• A ribosome works by placing itself on the sequence of mRNA to decode the codons and put the corresponding amino acids in a polypeptide chain.

• The ribosomes are a reoccurring site for three types of key RNA interactions: mRNA, rRNA, and tRNA. mRNA is the information template's carrying vehicle for identifying the codon sequence.

Role of mRNA, tRNA, and rRNA

• At the initiation phase of translation, the ribosome engages the target mRNA. mRNA is the information template's carrying vehicle for identifying the codon sequence.

• tRNA translocates the correct amino acids. rRNA catalyses peptidyl transferase formation.

Polysomes (Polyribosomes)

• Polysomes, or polyribosomes, are described as groups of ribosomes translating a single mRNA molecule simultaneously.

• Polysomes are very significant as they increase the rate of protein synthesis when required.

• The mechanism of polysomes is the inclusion of several ribosomes spaced along an mRNA strand, with each at a different stage of translating the mRNA.

• It is an effective assembly line, and in certain cells, such as dividing cells or certain somatic cells with high protein demands, the assembly line needs to work smoothly.

Ribosomes Biogenesis

The ribosome biogenesis is explained below-

Role of Nucleolus

Ribosome assembly is a complicated process. It starts further with the nucleolus, a subnuclear highly specialized structure. A nucleolus actively coordinates the synthesis and assembly of ribosomal RNA and ribosomal proteins.

First, the rRNA genes transcribe into precursor rRNA, which is later processed into the mature rRNA. Afterwards, rRNA combines with ribosomal proteins, which are cytoplasmically imported, to form both the small and the large ribosome subunits. Assembled subunits are then moved out of the nucleolus into the cytoplasm, where they will be paired up into their functional ribosomal form for the synthesis of proteins.

Regulation of Ribosome Production

The production of ribosomes is highly regulated and under strong control by the availability of nutrients, in response to cellular stress and growth signals. Ribosome synthesis and biogenesis are important in cell function and growth to meet the need for these organelles to synthesize proteins required in cellular functions.

Thus, cells must regulate the number of ribosomes in accordance with their metabolic activity without wasting the resources for this process. Disruption of ribosome biogenesis and dysregulation of the ribosomal pathways provide a basis for several human diseases, including cancer and ribosomopathies.

Ribosome Function in Different Cellular Contexts

The various aspects of the function of ribosomes are explained below-

Prokaryotic Ribosomes and Antibiotics

• An important property of prokaryotic ribosomes is their sensitivity to particular antibiotics, including tetracyclines and streptomycin, which can prevent protein biosynthesis by the particular zone in bacterial ribosomes.

• Such selective inhibition is important for the effectiveness of certain antibiotics to treat bacterial infections without harming eukaryotic cells.

Eukaryotic Ribosomes

• Generally, for eukaryotic cells, the ribosomes synthesize proteins that are needed in the cells, like all the various enzymes, structural proteins, and signalling proteins.

• Further, eukaryotic unique inhibitors, for example, cobomycin or cycloheximide, are narrow-spectrum inhibitors that specifically target the eukaryotic protein that synthesizes it, thereby indicating the difference in ribosomal structure and function of prokaryotes and eukaryotes.

Mitochondrial and Chloroplast Ribosomes

• The mitochondria and chloroplasts possess their own ribosomes. Their function goes together with the synthesis of proteins required for mitochondrial and chloroplast functions.

• The similarity of these ribosomes with the prokaryotic ones indicates their common origin in the process of evolution. Prokaryotic antibiotics may lead to potential alterations in these organelles.

What Distinguishes Ribosomes from Other Organelles?

Ribosomes perform protein synthesis and lack a membrane that would differentiate the structures' boundaries.

• In terms of structure, ribosomes are, in fact, complexes of ribosomal RNA and proteins in the way that a particle is composed of smaller subunits.

• Functionally, they serve to translate the genetic code of nucleic acids from the form of "messenger RNA" to the form of the amino acids that make up protein molecules, serving a diverse function from that of the other organelles.

• Endoplasmic reticulum and Golgi are closely interlinked with ribosomes.

• The membrane-bound ribosomes of the rough endoplasmic reticulum are involved in the synthesis of proteins whose destination is either outside the cell or inside the membranes.

• These proteins are transferred to the Golgi apparatus, which further processes, packs, and dispatches these products to their final destinations.

• Such a close collaboration of functions is of critical importance to support cellular function and facilitate the execution of complex biochemical pathways.

Ribosomes NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Structure of Ribosomes

• Difference between Prokaryotic and Eukaryotic Ribosomes

• Ribosomes vs other Organelles

Practice Questions for NEET

Q1. Ribosomes present in prokaryotes have

1. 60S large subunit and 40S small subunit

2. 60S large subunit and 20S small subunit

3. 50S large subunit and 50S small subunit

4. 50S large subunit and 30S small subunit

Correct answer: 4) 50S large subunit and 30S small subunit

Explanation:

Prokaryotes have 70S type of ribosomes which can be further broken into larger and smaller subunits as 50S and 30S respectively. It is composed of 60% RNA and 40% proteins. The ‘S’ denotes the Svedberg unit for sedimentation rate that measures the shape, size, and density of the ribosomes. The main function of the ribosome is to perform the translation of the mRNA to form the polypeptide chain.

Hence, the correct answer is option 4) 50S large subunit and 30S small subunit.

Q2. The function of ribosomes is to

1. Synthesize polypeptide chain

2. Synthesize nucleotide chain

3. Synthesize glycosidic chain

4. Synthesize polysaccharide chain

Correct answer: 1) Synthesize polypeptide chain

Explanation:

Ribosomes are known as the protein factories of the cell and are responsible for synthesizing proteins by translating messenger RNA (mRNA) into amino acid sequences. These organelles can be free-floating in the cytoplasm or attached to the endoplasmic reticulum, forming the rough ER, and playing a central role in gene expression and cellular function.

Hence, the correct answer is option 1) Synthesize polypeptide chain

Q3. 70S ribosome is present in

1. Prokaryotes

2. Mitochondria

3. Plastids

4. All of these

Correct answer: 4) All of these

Explanation:

Prokaryotes, which include bacteria and archaea, have the 70S ribosome, which is essential for protein synthesis. It is also found in eukaryotic cells' mitochondria and chloroplasts, which are thought to have developed through endosymbiosis from prokaryotic predecessors. The 50S big subunit and the 30S small subunit make up the 70S ribosome, which is smaller than the 80S ribosome present in the eukaryotic cytoplasm. Its evolutionary significance in cellular biology is shown by the fact that it is found in both prokaryotes and some eukaryotic organelles.

Hence, the correct answer is option 4) All of these.

Also Read:

• MCQs on Ribosomes

• Difference between Prokaryotic and Eukaryotic Cells

• Cell Wall

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