Question

Explain why a \(50 \mathrm{S}\) ribosomal subunit and a \(30 \mathrm{S}\) ribosomal subunit combine to form a \(70 \mathrm{S}\) subunit, instead of an \(80 \mathrm{S}\) subunit.

Short answer

The Svedberg (S) unit is not additive because it depends on the interaction between subunits, altering their density and shape, leading to a 70S ribosomal subunit.

Step-by-step solution

Understand the unit of measurement (Svedberg unit)

The Svedberg unit (S) is a measure of the rate at which particles sediment in a centrifuge. It is not additive because it depends on the size, shape, and density of the particles, as well as the viscosity of the medium.

Combine subunits and their individual properties

When a 50S ribosomal subunit and a 30S ribosomal subunit are combined, the resulting complex is not a simple sum of the two. Instead, the combination of the two subunits results in some changes in their combined shape and density.

Realize why the sedimentation rate differs

The actual sedimentation rate of the combined ribosomal subunit (70S) differs from the arithmetic sum of the individual rates (50S + 30S = 80S) because the interactions between the subunits alter their overall density and shape, which affects their sedimentation velocity.

Conclude the reasoning behind the 70S outcome

The combined 70S ribosomal subunit has a specific shape and density due to the interaction between the 50S and 30S subunits that makes it sediment at the rate measured as 70S, rather than the expected arithmetic sum of 80S.

Key concepts

Svedberg unit

The Svedberg unit (S) measures how fast particles sediment when spun in a centrifuge. It helps scientists understand particle size, shape, and density. The unit is named after Swedish scientist Theodor Svedberg. Unlike other units, Svedberg units are not additive. This means if you combine two particles with sedimentation rates of 50S and 30S, you don't get 80S. The actual rate would be determined by their combined size, shape, and density. This is important for understanding why ribosomal subunits do not simply add up their Svedberg units.

Ribosomal Subunits

Ribosomes, the cell's protein factories, have two main parts called subunits: the large subunit (50S) and the small subunit (30S) in prokaryotes. Each subunit has its own specific proteins and RNA. When these subunits come together, they form a functional ribosome. In prokaryotes, a 50S and 30S subunit combine to make a 70S ribosome. The numbers don't add up to 80S because of how their shapes and densities interact. These interactions change the overall sedimentation rate. This combination is crucial for the ribosome's function in protein synthesis.

Sedimentation Rate

Sedimentation rate is how fast particles settle in a centrifuge. The rate depends on several factors:

• Size of the particle
• Shape of the particle
• Density of the particle
• Viscosity of the medium

The complex interactions between these factors make the Svedberg units non-additive. When two ribosomal subunits combine, changes in their shape and density alter how they sediment. This explains why a 50S and 30S subunit form a 70S complex instead of an 80S one.

Protein Synthesis

Protein synthesis is the cell's process of making proteins. This involves two main steps:

1. **Transcription**: DNA is copied to mRNA.
2. **Translation**: Ribosomes read mRNA to build proteins.

Ribosomes are vital for translation. They link amino acids together to form proteins, based on the sequence of the mRNA. The large ribosomal subunit (50S) and small ribosomal subunit (30S) must combine to form a functional 70S ribosome. This complex makes protein synthesis possible by providing the site where amino acids are assembled into proteins.

Biochemistry

Biochemistry is the study of chemical processes within living organisms. It covers various topics including metabolism, enzyme functions, and cell signaling. One critical aspect is understanding how ribosomes work. Ribosomes, made of RNA and protein, are crucial for translating genetic information into proteins. By studying ribosomal subunits (50S and 30S) and their assembly into a 70S complex, biochemists can learn about the molecular machinery of life. This knowledge helps scientists develop antibiotics and understand genetic diseases.