Problem 2 - Biochemistry

Question

Ribosomes were isolated from bacteria grown in a "heavy" medium \(\left(^{13} \mathrm{C} \text { and }^{15} \mathrm{N}\right)\) and from bacteria grown in a "light" medium \(\left(^{12} \mathrm{C} \text { and }^{14} \mathrm{N}\right)\). These \(60 \mathrm{S}\) ribosomes were added to an in vitro system engaged in protein synthesis. An aliquot removed several hours later was analyzed by density-gradient centrifugation. How many bands of \(70 \mathrm{S}\) ribosomes would you expect to see in the density gradient?

Step-by-Step Solution

Answer

Three bands: all-heavy, all-light, and hybrid ribosomes.

Step 1: Understand Ribosome Composition

Ribosomes are composed of two subunits: the small subunit and the large subunit. In this problem, we are dealing with 60S ribosomes, which will likely become 70S once engaged in protein synthesis.

Step 2: Differences in Isotopic Composition

The ribosomes are distinguished by their isotopes: 'heavy' ribosomes have isotopes of \(^{13}C\) and \(^{15}N\), while 'light' ribosomes have \(^{12}C\) and \(^{14}N\). Density-gradient centrifugation separates molecules based on their density differences due to isotope variation.

Step 3: Mix Ribosomes in an In Vitro System

When mixed in an in vitro system, the heavy and light ribosomes will exist together. Given they are used for protein synthesis, any newly formed ribosome will create a hybrid density, utilizing components from both light and heavy sources.

Step 4: Analyze Density Gradient for Bands

Due to the differences in the isotopic composition and their presence in the system, three distinct bands would be expected: one for all-heavy ribosomes, one for all-light ribosomes, and one for hybrid ribosomes containing subunits from both isotopic compositions.

Key Concepts

Isotopic Labeling

Isotopic labeling is a technique used to track the incorporation of specific atoms into molecules. By replacing atoms in a compound with their isotopic forms, they become "heavy" or "light" based on the added isotopes. For instance, carbon atoms can be found in forms like the stable isotopes, \(^{12}C\) and \(^{13}C\), while nitrogen can be \(^{14}N\) or \(^{15}N\). These isotopes do not alter the chemical behavior but increase the atomic mass, allowing researchers to distinguish between molecules.

• Purpose: Isotopic labeling helps in tracing the pathways and interactions of biological molecules.
• Application in Study: In this exercise, ribosomes from bacteria grown in separate isotopic media carry different carbon and nitrogen isotopes.

This labeling facilitates the understanding of the mixture's density, especially during analytical techniques like density-gradient centrifugation.

Density-Gradient Centrifugation

Density-gradient centrifugation is a powerful laboratory technique used to separate particles based on their density. During the process, particles are spun at high speeds within a centrifuge, causing them to move until they reach a point where their density matches that of the surrounding gradient.

• How It Works: The solution is layered with different densities, creating a gradient (usually made with sucrose or cesium chloride), which establishes a medium to differentiate between molecules.
• Application in the Exercise: By employing this method on the ribosome samples, the differing densities from isotopic labeling allow for clear separation. Each band in the gradient represents distinct populations of ribosomes based on their isotopic compositions – light, heavy, and hybrid.

The outcome is the appearance of different bands, like genetic markers, indicating the isotope mix of the ribosomes.

Protein Synthesis

Protein synthesis is the biological process of building proteins from amino acids, guided by the genetic instructions. Ribosomes play a crucial role in this process, functioning as the site where this synthesis occurs.

• Role of Ribosomes: Ribosomes are molecular machines that read mRNA and translate its sequence into a chain of amino acids to form proteins.
• Impact of Isotopic Labeling: In this context, isotopically labeled ribosomes allow simultaneous visualization of protein synthesis from different ribosomal compositions.
• Study Application: When the ribosomes from different isotopic origins are mixed in an in vitro system, their involvement in protein synthesis creates hybrids. These hybrids are detectable as a unique band in the centrifugation process.

Understanding this intricate dance highlights how ribosomes integrate various components to fulfill the essential task of assembling proteins and how technological advancements elucidate these molecular processes.