The Ribosomes in human Mitochondria are of:

Question

Accepted Answer

The correct option is 3: 70 S types

The correct answer is 70 S types. Ribosomes are essential cellular machinery responsible for protein synthesis (translation), and their type is classified by their sedimentation coefficient, measured in Svedberg units (S). The presence of 70 S ribosomes in human mitochondria is a crucial piece of evidence for the Endosymbiotic Theory. This theory posits that mitochondria originated from ancient, free-living prokaryotic organisms that were engulfed by a host cell. Because prokaryotes, such as bacteria, have 70 S ribosomes, the organelle that evolved from them—the mitochondrion—retains this characteristic to this day. This organelle-specific protein-synthesizing machinery is responsible for creating the small number of proteins encoded by the mitochondrial DNA, maintaining its semi-autonomous status within the eukaryotic cell. The 70 S ribosome itself is composed of two subunits: a large 50 S subunit and a small 30 S subunit, which associate only when protein synthesis is initiated.

The incorrect option is 2: 80 S types

The 80 S ribosomes are characteristic of the entire *eukaryotic cytoplasm* of human cells. These large ribosomes are found attached to the endoplasmic reticulum or freely floating in the cytosol, and they are the machines responsible for translating the vast majority of the cell's mRNA (the one transcribed from nuclear DNA). The 80 S ribosome is structurally larger and more complex than the 70 S type, being composed of a 60 S large subunit and a 40 S small subunit. Therefore, while human cells possess 80 S ribosomes, they are strictly located outside the confines of the mitochondria and the chloroplasts (in plant cells).

The incorrect option is 1: 60 S types

The 60 S type refers specifically to the large ribosomal subunit of the complete eukaryotic 80 S ribosome. A ribosome is only fully functional when both its large and small subunits are joined together for the process of translation. While this 60 S subunit is a component of the cell’s cytoplasmic machinery, it is not the designation for a complete, functional ribosome in its own right. The 70 S mitochondrial ribosome, by contrast, has a large subunit designated as 50 S. Thus, the 60 S designation refers to the wrong cellular location and the wrong component size.

The incorrect option is 4: 30 S types

The 30 S type refers exclusively to the *small ribosomal subunit* of the complete 70 S ribosome (which is found in mitochondria and prokaryotes). The primary role of the 30 S subunit is to bind the mRNA template and the initiator tRNA, which is the crucial first step in protein synthesis. However, it cannot perform the entire process of translation alone, as the large subunit (50 S, in this case) is required for the peptidyl transferase activity and the elongation of the polypeptide chain. Therefore, 30 S is only a part, or a subcomponent, and not the correct designation for the complete, functional ribosome type.