Question

Certain mutations called amber in bacteria and viruses result in premature termination of polypeptide chains during translation. Many amber mutations have been detected at different points along the gene coding for a head protein in phage \(\mathrm{T} 4\) How might this system be further investigated to demonstrate and support the concept of colinearity?

Short answer

Answer: Amber mutations in the phage T4 system can be used to verify and support the concept of colinearity by comparing the linear sequence of nucleotides in the mutated gene coding for the head protein and the resulting truncated polypeptide chains due to premature terminations caused by amber mutations. After identifying and sequencing the mutated regions, expressing the mutated genes in a bacterial system, and determining the lengths of the polypeptide chains, researchers can compare the observed amino acid sequence and lengths to the corresponding nucleotide sequences in the mutated genes. By suppressing the amber mutations and observing the restoration of the full-length polypeptide, direct evidence of colinearity can be obtained.

Step-by-step solution

Understanding amber mutations

Amber mutations are a type of point mutation, where a single nucleotide is changed in a DNA strand. In these mutations, a codon that codes for an amino acid (for example, UAC for tyrosine) is altered into a stop codon (UAG, which is called an amber stop codon). This results in the premature termination of polypeptide chain synthesis during the translation process.

Relating the gene coding for a head protein in phage T4 to colinearity

Colinearity is the concept that the linear sequence of nucleotides in a gene corresponds to the linear sequence of amino acids in the resulting polypeptide chain. The gene coding for a head protein in phage T4 is an ideal system to investigate colinearity since it contains multiple instances of amber mutations at different points along the gene. These premature termination points in the polypeptide chain can be compared to the sequence of nucleotides in the coding DNA, providing evidence for the correspondence between the gene and the polypeptide sequences.

Proposing an experimental approach to investigate colinearity

One method to demonstrate and support the concept of colinearity using the amber mutations in a phage T4 system would involve the following steps: 1. Identify and sequence the specific regions with amber mutations in the gene coding for the head protein in phage T4, using DNA sequencing techniques. 2. Express the mutated genes in a suitable bacterial system, resulting in different truncated polypeptide chains due to amber mutations' premature termination effects. 3. Determine the length of the polypeptide chains produced in the bacterial system by using techniques such as SDS-PAGE. 4. Compare the observed amino acid sequence and lengths, as well as the corresponding nucleotide sequences in the mutated genes, to confirm that the stop codons and the polypeptide chain terminations coincide. 5. Lastly, verify that the premature stop codons are the direct cause of the truncated polypeptide chains by the suppression of the amber mutations, either through the use of suppressor tRNAs or by reversing the mutation. Observe the restoration of the full-length polypeptide chain synthesis, providing direct evidence supporting the colinearity concept. By following this experimental approach, we can further investigate the concept of colinearity and demonstrate its validity using the amber mutations in the phage T4 system.

Key concepts

Amber Mutations

Amber mutations play a critical role in the study of genetics, particularly when examining the impact of genetic variations on protein synthesis. Simplified, an amber mutation is a specific type of point mutation within a gene that transforms a codon, which originally coded for an amino acid, into a 'stop' codon. The result is a truncated polypeptide chain, as this new 'stop' codon signals the cellular machinery to halt translation prematurely.

This phenomenon has significant implications for our understanding of genetic coding and expression. Specifically, with regards to phage T4, the presence of amber mutations can be seen as a natural experiment illuminating the direct ties between a gene's nucleotide sequence and its respective protein product. For instance, when the tyrosine codon UAC is mutated into UAG (an amber stop codon), the affected genes will produce incomplete proteins. This is observable under laboratory conditions and provides tangible evidence to support theories of genetic structure and function.

Polypeptide Chain Synthesis

Polypeptide chain synthesis is a fundamental process in molecular biology where a sequence of amino acids is assembled into a protein, based on the instructions encoded within a gene. This process, more commonly known as translation, takes place within the ribosomes in cells and follows a template provided by messenger RNA (mRNA). The order of nucleotides in the mRNA dictates which amino acids are added to the growing polypeptide chain through the use of transfer RNAs (tRNAs) that match specific codons to their assigned amino acids.

In the context of phage T4 genetics and amber mutations, the premature termination caused by these mutations starkly demonstrates the tight coordination required for accurate protein synthesis. Each amino acid added is a direct translation of the nucleotide sequence up to the point of mutation, providing a clear map of the gene-to-protein pathway and how disruptions can lead to incomplete and potentially nonfunctional proteins. Understanding polypeptide synthesis is thus essential for grasping the implications of mutations and their effects on organismal biology.

Phage T4 Genetics

Phage T4, a bacteriophage that infects E. coli, is an excellent model organism for genetic research. Its relatively simple genetic structure allows for detailed analysis of fundamental genetic principles such as colinearity. As demonstrated by the occurrence of amber mutations, which induce premature stops in protein synthesis, the genetics of phage T4 provide insights into the precise relationship between DNA sequences and their resultant proteins.

Investigating the genetic makeup of phage T4 involves understanding how its viral proteins are expressed. For example, the head protein gene, which is critical to the assembly of the virus, can be studied to observe the effects of individual genetic changes on phenotype. By strategically using this system to highlight where mutations occur and the impact they have on the final polypeptide product, researchers can strengthen the argument for the colinearity hypothesis, which asserts that there is a direct linear relationship between the nucleotide sequence and the amino acid sequence in proteins.