Question

What experimental evidence indicates that genes control the amino acid sequences in proteins?

Short answer

Experimental evidence supporting the idea that genes control amino acid sequences in proteins includes Beadle and Tatum's one gene-one enzyme hypothesis derived from their studies on Neurospora crassa, which demonstrated individual gene mutations affecting enzymatic function. Additionally, advancements in DNA sequencing techniques unveiled the genetic code and confirmed the concept of one gene-one protein, as specific codon sequences in mRNA code for specific amino acids. This evidence, rooted in both early experiments and molecular biology, solidifies the understanding that genes determine the amino acid sequences composing proteins.

Step-by-step solution

Recap of the Central Dogma of Molecular Biology

The Central dogma of Molecular Biology is the process by which the instructions in DNA are converted into a functional product. The two main stages are transcription, where a copy of DNA sequence, called RNA is made, and then translation, in which the RNA sequence are used to create a sequence of amino acids to produce proteins.

Understanding Genes and Protein Synthesis

A gene is a sequence of nucleotides in DNA that codes for a functional product, typically a protein. The order of these nucleotides determines the order of amino acids in a protein sequence in a process known as protein synthesis. During transcription, the gene's DNA sequence is copied to make an RNA molecule. This RNA molecule is then translated into a sequence of amino acids to form a protein. Each group of three nucleotides, known as a codon, encodes for one specific amino acid. So the sequence of nucleotides in the gene dictates the sequence of amino acids in the protein.

Evidence from Beadle and Tatum's Experiments

The first experimental evidence indicating that genes control the amino acid sequences in proteins comes from the work of Beadle and Tatum in 1941. They proposed the one gene-one enzyme hypothesis, which states that each gene is responsible for producing a single enzyme. They demonstrated this through studies on the bread mould Neurospora crassa. By inducing mutations, they could disrupt individual steps in metabolic pathways. Each of these effects could be traced back to mutations in single genes. Hence, they concluded that each gene coded for a distinct enzyme, which later updated to one gene-one protein concept after understanding that not all proteins are enzymes.

Evidence from Molecular Biology

Later advancements in DNA sequencing techniques provided further evidence for genes controlling the amino acid sequences in proteins. Scientists were able to decipher the genetic code and confirm the one gene-one protein theory. They found that specific sequences of three nucleotides (codons) in mRNA code for specific amino acids. Therefore, the sequence of nucleotides in a DNA gene determines the sequence of amino acids in the protein it codes for. In summary, both early experiments and molecular biology techniques show us that the sequence of nucleotides in a gene is transcribed and translated to produce a protein with a specific sequence of amino acids. This confirms that genes control the amino acid sequences in proteins.

Key concepts

Gene Expression

Gene expression is the process by which the information from a gene is used to create a functional product, usually a protein. This journey begins with transcription, where the DNA sequence of a gene is copied into RNA.
This RNA, often messenger RNA (mRNA), then leaves the cell nucleus and goes into the cytoplasm. Here, ribosomes read the mRNA sequence and translate it into a chain of amino acids, forming a protein.
The central dogma of molecular biology outlines this flow of information, from DNA to RNA and then to protein. Each step in this process is crucial for the correct expression of genes, ensuring cells function properly and respond to environmental changes effectively.
Understanding gene expression helps scientists comprehend how genetic information dictates phenotypes and how mutations can lead to genetic disorders.

Protein Synthesis

Protein synthesis is the process of creating proteins from the information encoded in genes. There are two main stages: transcription and translation.

• Transcription: During this phase, an RNA molecule is synthesized from the DNA template of a gene. Specifically, the sequence of nucleotides in DNA is transcribed to make a complementary RNA strand, forming mRNA.
  
• Translation: In this stage, the mRNA is decoded by ribosomes to produce a specific sequence of amino acids. Each set of three nucleotides on the mRNA, known as a codon, corresponds to a specific amino acid or a stop signal during protein synthesis.
  

The sequence in which amino acids are assembled is determined by the nucleotide sequence in the mRNA, originally dictated by the DNA sequence.
Protein synthesis is crucial for maintaining cellular structure and function, as proteins serve both structural and catalytic roles within a cell.

Beadle and Tatum Experiments

The groundbreaking experiments by George Beadle and Edward Tatum in the 1940s demonstrated that genes control specific biochemical reactions, linking genes to proteins directly. They designed experiments using the red bread mold, Neurospora crassa.
By exposing these molds to radiation, they induced mutations and observed that some mutants could not synthesize essential nutrients.
These mutants could grow only when the missing nutrient was added externally, indicating that each mutated gene caused a specific metabolic block.

• Beadle and Tatum's studies supported the idea that each gene specifies the production of a single enzyme, a concept later revised to the "one gene-one protein hypothesis," acknowledging the diverse roles genes play in encoding proteins, not just enzymes.
• Their innovative work laid the ground for proving the genetic basis of metabolic pathways.

One Gene-One Protein Hypothesis

The one gene-one protein hypothesis evolved from Beadle and Tatum's initial "one gene-one enzyme hypothesis." This revised understanding recognizes that not all proteins are enzymes but that every distinct protein corresponds to a single gene.
Initially, Beadle and Tatum demonstrated that specific genes were responsible for individual enzymes in Neurospora. However, as scientists learned more about proteins, they understood that the relationship was not limited to enzymes.
With the discovery of non-enzyme proteins and structural proteins, scientists broadened the hypothesis to "one gene-one polypeptide." This adaptation reflects proteins' diverse functions:

• These insights highlight how specific gene mutations can lead to changes in protein structure and function, causing various genetic disorders.
• This hypothesis remains fundamental in molecular biology, helping researchers identify how genetic information determines an organism's traits and capabilities.