In this chapter, we focused on the translation of mRNA into proteins as well as on protein structure and function. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions: (a) What experimentally derived information led to Holley's proposal of the two-dimensional cloverleaf model of tRNA? (b) What experimental information verifies that certain codons in mRNA specify chain termination during translation? (c) How do we know, based on studies of Neurospora nutritional mutations, that one gene specifies one enzyme? (d) On what basis have we concluded that proteins are the end products of genetic expression? (e) What experimental information directly confirms that the genetic code, as shown in Figure \(13-7,\) is correct? (f) How do we know that the structure of a protein is intimately related to the function of that protein?

Step by step solution

01

(a) Holley's proposal of the two-dimensional cloverleaf model of tRNA

Holley's team isolated and sequenced tRNA molecules. Through their findings, they observed that tRNA had a consistent length of approximately 80 nucleotides and had several base-pairings within the molecule. Combining this with X-ray crystallography data, which showed a cloverleaf pattern of bonded regions, the researchers proposed the two-dimensional cloverleaf model of tRNA. This model helped explain the structural properties of tRNA that allowed it to function in protein synthesis.

02

(b) Experimental information verifying codon specificity in chain termination

Studies involving in vitro translation systems were carried out to verify the role of specific codons in mRNA chain termination. By introducing chemically synthesized mRNA sequences with known codons into the in vitro translation system, researchers observed that particular stop codons like UAA, UAG, and UGA led to the termination of translation, resulting in the release of the newly synthesized polypeptide chain. This demonstrated the specific role of these codons in chain termination during the translation process.

03

(c) One gene-one enzyme hypothesis from Neurospora studies

The one gene-one enzyme hypothesis was proposed based on studies of nutritional mutations in the organism Neurospora. Researchers Beadle and Tatum observed that certain strains of Neurospora could not synthesize specific nutrients due to mutations in their genes. This led to the idea that each gene is responsible for encoding a single enzyme, which in turn is responsible for catalyzing a specific biochemical reaction in the organism. This hypothesis was later modified to the one gene-one polypeptide concept, as more evidence was discovered revealing that some enzymes are composed of multiple polypeptide chains, each encoded by a separate gene.

04

(d) Proteins as the end products of genetic expression

The central dogma of molecular biology, originally proposed by Francis Crick, suggests that genetic information flows from DNA to RNA to proteins. This concept is supported by experimental observations showing that DNA serves as a template for transcription into mRNA, which is then translated by ribosomes into proteins. Since proteins are responsible for many cellular functions, they can be considered as the end products of genetic expression.

05

(e) Confirming the genetic code

To confirm the genetic code as identified in Figure 13-7, several experiments were conducted involving in vitro translation systems and synthesizing tRNA molecules with specific anticodons. These experiments demonstrated that introducing a chemically synthesized mRNA with a known sequence into the translation system resulted in the production of a specific amino acid sequence in the newly synthesized polypeptide chain. The correlation between the mRNA codons, tRNA anticodons, and the corresponding amino acids confirms the accuracy of the genetic code as depicted in the figure.

06

(f) Relationship between protein structure and function

Experimental studies involving denaturing and renaturing of proteins have demonstrated the importance of protein structure in determining their function. When a protein is denatured, it loses its specific structure and subsequently its function. However, in some cases, proteins can regain their structure and function upon renaturing. Additionally, studies on genetic mutations have shown that changes in the amino acid sequence can lead to alterations in protein structure and function. Furthermore, the relationship between protein structure and function is also evident in the active sites of enzymes, which are highly specific in structure and complementary to the substrate molecules they act upon. Overall, these observations point to the intimate relationship between protein structure and its function in determining cellular processes.

Unlock Step-by-Step Solutions & Ace Your Exams!

• Full Textbook Solutions

  Get detailed explanations and key concepts

• Unlimited Al creation

  Al flashcards, explanations, exams and more...

• Ads-free access

  To over 500 millions flashcards

• Money-back guarantee

  We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!