Problem 74 - Objective Biology for NEET

Question

In 70 S prokaryotic ribosomes, what are the sub units present in them? (a) \(50 \mathrm{~S}\) and \(20 \mathrm{~S}\) (b) \(50 \mathrm{~S}\) and \(30 \mathrm{~S}\) (c) \(40 \mathrm{~S}\) and \(30 \mathrm{~S}\) (d) \(60 \mathrm{~S}\) and \(30 \mathrm{~S}\)

Step-by-Step Solution

Answer

The correct answer is (b) 50S and 30S. This is the subunit combination that forms the 70S prokaryotic ribosome.

Step 1: Overview of Ribosome subunits

Ribosomes are made up of two subunits. In prokaryotes, these are the 70S ribosomes, named based on their sedimentation coefficients. The 'S' stands for Svedberg units.

Step 2: Identify the subunits of the 70S ribosome

The 70S ribosome is made up of two subunits: the larger 50S subunit and the smaller 30S subunit.

Step 3: Match with the given options

Based on step 2, the 70S prokaryotic ribosome is made up of subunits 50S and 30S. Thus, the right answer is (b) 50S and 30S. The other options do not match the correct combination of ribosome subunits.

Key Concepts

Ribosome Subunits

Ribosomes play a pivotal role in the cellular process of translating mRNA into proteins. They are complex molecular machines found within all living cells and composed of RNA and proteins. Prokaryotic ribosomes, such as those in bacteria, are smaller than their eukaryotic counterparts. They are referred to as 70S ribosomes and are made up of two distinct subunits - a large subunit and a small subunit.

In the specific context of 70S prokaryotic ribosomes, these two subunits are designated as 50S and 30S. The 50S is the larger subunit that facilitates the peptidyl transferase reaction, an essential part of protein synthesis. The 30S subunit, on the other hand, binds mRNA and is responsible for the decoding of the message into a polypeptide chain. It's important to note that the molecular composition of these subunits involves a combination of ribosomal RNA (rRNA) and various ribosomal proteins, which work together to carry out protein synthesis efficiently.

Svedberg Units

Understanding the Svedberg unit is critical for comprehending how scientists measure ribosome subunits. The Svedberg unit, abbreviated as 'S', is not a measure of size or mass but rather of a ribosomal subunit's sedimentation rate during ultracentrifugation. This rate depends on various factors, including size, shape, and density.

The sedimentation coefficient of a particle is a measure of how fast it settles to the bottom of a solution when spun at high speed. Larger and denser particles tend to settle faster, which means they have higher Svedberg values. However, Svedberg units do not add up linearly due to the difference in shape and density between different subunits. For instance, the 70S prokaryotic ribosome is composed of 50S and 30S subunits, but they do not add up to 80S because of their unique structural interactions and the hydration effects during centrifugation.

70S Ribosomes

70S ribosomes are characteristic of prokaryotic cells, such as bacteria and archaea. The '70S' designation reflects their sedimentation rate in Svedberg units, emphasizing that these ribosomes sediment faster than individual subunits due to their complexed state. The 'S' represents their high speed of sedimentation, a reflection of their role in the efficient synthesis of proteins.

The existence of the 70S ribosomes, comprising of 50S and 30S subunits, is a defining feature of the prokaryotic translational machinery. These ribosomes are smaller and slightly simpler than their 80S eukaryotic counterparts, which consist of 60S and 40S subunits. The functional implication of these differences lies in the susceptibility of 70S ribosomes to antibiotics, which can specifically target bacterial protein synthesis without affecting eukaryotic cells. Understanding the structure and function of 70S ribosomes is crucial for the development of antibacterial drugs and studying the evolution of translational mechanisms across different domains of life.