Problem 1 - Biochemistry: Concepts and Connections

Question

A random-sequence polyribonucleotide produced by polynucleotide phosphorylase, with CDP and ADP in a \(5: 1\) molar ratio stimulated the incorporation of proline, histidine, threonine, glutamine, asparagine, and lysine in a cell- free translation system in the following proportions: \(100,23.4,20,3.3,3.3\), and \(1.0\), respectively. What does this experiment reveal about the nucleotide composition of coding triplets for these six amino acids?

Step-by-Step Solution

Answer

Proline's codons are rich in cytosine, histidine and threonine also favor cytosine, while glutamine, asparagine, and lysine need more adenosine.

Step 1: Understanding the Experiment

The experiment involves a polyribonucleotide sequence formed from CDP (cytosine diphosphate) and ADP (adenosine diphosphate) in a 5:1 molar ratio, used to understand which codons correspond to certain amino acids. By observing the frequency of amino acid incorporation, we decipher hints about the codons.

Step 2: Analyze Amino Acid Proportions

The amino acids proline, histidine, threonine, glutamine, asparagine, and lysine were incorporated in these respective proportions: 100, 23.4, 20, 3.3, 3.3, and 1.0. High proportion indicates codons rich in the more abundant nucleotide (C), whereas lower proportion ones require more of the lesser abundant nucleotide (A).

Step 3: Deducing Codon Composition for Each Amino Acid

Given the 5:1 ratio favoring cytosine, a high incorporation rate of an amino acid suggests its codon has more C's than A's. Proline, with the highest incorporation, suggests multiple codons are CC-rich. Histidine and threonine likely have codons with more cytosines. Glutamine, asparagine, and lysine, with low incorporation figures, indicate codons requiring more adenosine in their sequences.

Key Concepts

Polyribonucleotide Sequence

Polyribonucleotide sequences are strands of RNA that are composed of various combinations of ribonucleotides. Ribonucleotides are the building blocks of RNA, much like how nucleotides are the building blocks of DNA. Understanding how polyribonucleotide sequences work is crucial because they are used to model mRNA and study protein synthesis.

In this particular experiment, a polyribonucleotide sequence was artificially created using polynucleotide phosphorylase. This enzyme strings together ribonucleotides randomly. The specific sequence was formed from CDP (Cytosine Diphosphate) and ADP (Adenosine Diphosphate) in a 5:1 molar ratio. This controlled creation allows scientists to test how the abundance of certain nucleotides affects protein formation.

This sequence formed has critical implications for understanding how random RNA sequences impact the production of proteins, mimicking natural processes that occur within cells. It allows the tracing of how nucleotides are translated into amino acids, essential for cell functions.

Amino Acid Incorporation

Amino acid incorporation is a fundamental process during protein synthesis where amino acids are added to form a protein. This process relies heavily on the RNA sequence, specifically the codons.

In this study, the polyribonucleotide sequence facilitated the incorporation of several amino acids, namely proline, histidine, threonine, glutamine, asparagine, and lysine. Each of these amino acids was incorporated at different rates, suggesting differences in how their respective codons match the nucleotide composition of the sequence.

Here are the incorporation rates observed:

• Proline: 100
• Histidine: 23.4
• Threonine: 20
• Glutamine: 3.3
• Asparagine: 3.3
• Lysine: 1.0

Amino acids like proline with a higher incorporation rate imply codons that consist mostly of the more abundant nucleotide, cytosine. Lower rates, as seen with lysine, suggest codons requiring more adenosine. This pattern helps scientists deduce which codons are involved in the synthesis of these specific amino acids.

Cytosine and Adenosine Ratio

The ratio of cytosine to adenosine, 5:1 in this sequence, is incredibly significant for deciphering the results of the experiment. This ratio means that there is a predominance of cytosine over adenosine in the polyribonucleotide sequence. This skewed ratio helps predict which amino acids might be synthesized more frequently.

Amino acids whose synthesis relies heavily on cytosine-rich codons will appear in higher frequencies. Proline, for instance, achieved the highest frequency of incorporation, suggesting its codons could be likely composed predominantly or entirely of cytosines.

In contrast, amino acids requiring more adenosine, such as glutamine, asparagine, and lysine, have correspondingly lower incorporation rates. These results suggest their codons have higher adenosine content, which is less frequently encountered in the polyribonucleotide sequence used.

Cell-Free Translation System

A cell-free translation system is an in vitro technique that allows for protein synthesis outside a living cell. It utilizes the cellular components necessary for translation, like ribosomes, tRNA, and amino acids, but is performed without the cell intact.

In this experiment, the cell-free system enables a controlled environment to test how different sequences of RNA can affect protein synthesis. Using such a system bypasses cellular variations that might influence the translation, leading to consistent and repeatable results.

The system functions by translating the input RNA into proteins, allowing scientists to observe how different nucleotide compositions in the RNA dictate which proteins (or amino acids) get synthesized. A significant advantage of this method is the ability to manipulate the input sequence, such as altering nucleotide ratios or specific sequence orders to observe outcomes, essential for understanding basic translation rules and underlying genetic codon assignments.