Question

Which of the following describes the experimental strategy that was used in deciphering the genetic code? a. comparing the amino acid sequences of proteins with the base sequence of their genes b. analyzing the sequence of RNAs produced from known DNA sequences c. analyzing mutants that changed the code d. examining the polypeptides produced when RNAs of known sequence were translated

Short answer

The correct answer is option d: examining the polypeptides produced when RNAs of known sequence were translated. This experimental strategy, known as the Nirenberg and Matthaei experiment, played a crucial role in deciphering the genetic code.

Step-by-step solution

Understand the genetic code

The genetic code is the set of rules by which information encoded in DNA sequences is translated into proteins by living cells. It consists of 64 codons (combinations of three nucleotides) that code for 20 amino acids or signalling sequences.

Evaluate each choice

a. Comparing the amino acid sequences of proteins with the base sequence of their genes: This option is related to the efforts to understand the genetic code, but it does not directly describe the experimental strategy used in deciphering the code. b. Analyzing the sequence of RNAs produced from known DNA sequences: This approach played an important role in the process of discovering the genetic code, but it's not the main experimental strategy used. c. Analyzing mutants that changed the code: This method has been valuable in studying variations of the genetic code, but it was not the main strategy used in deciphering the universal genetic code. d. Examining the polypeptides produced when RNAs of known sequence were translated: This was the primary experimental strategy, known as the Nirenberg and Matthaei experiment. By synthesizing artificial mRNA molecules of specific sequences, they could determine which amino acids would be incorporated into the resulting polypeptide, allowing them to decipher the genetic code.

Choose the correct answer

Based on the evaluation of the choices, the experimental strategy that was used in deciphering the genetic code is option d: examining the polypeptides produced when RNAs of known sequence were translated.

Key concepts

Nirenberg and Matthaei experiment

Back in 1961, Marshall Nirenberg and Heinrich Matthaei conducted a groundbreaking experiment at the National Institutes of Health. This experiment was pivotal in cracking the genetic code. They aimed to understand how the sequences of nucleotides in DNA and RNA translate into proteins—the building blocks of life.

They synthesized RNA sequences artificially, using a type of RNA known as poly-U, which consists entirely of the nucleotide uracil. By placing this poly-U RNA in a cell-free system, they observed that it produced a polypeptide composed entirely of the amino acid phenylalanine. This was the first time the correlation between a specific RNA sequence (UUU) and an amino acid was observed.

This experiment was the first concrete evidence that specific sequences of nucleotides (later known as codons) in RNA could dictate the sequence of amino acids in proteins. It set the stage for more discoveries that eventually led to the full understanding of the genetic code.

Polypeptide synthesis

Polypeptide synthesis is a critical process that occurs within every cell, resulting in the formation of proteins. The sequence of amino acids in a polypeptide chain determines its specific function within the cell. This synthesis occurs in two main stages: transcription and translation.

During transcription, the DNA sequence of a gene is copied into messenger RNA (mRNA).

• This mRNA transports the genetic information from the DNA in the nucleus to the ribosome, the cell's protein-making factory.
• At the ribosome, translation occurs, a process where the mRNA's nucleotide sequence is used to arrange amino acids in the correct order to form a polypeptide.

Ribosomes read the sequence of the mRNA three nucleotides at a time, a unit known as a codon. Each codon corresponds to a specific amino acid or a stop signal, dictating where protein synthesis should terminate. This precise operation ensures that the correct proteins are produced, enabling cells to function properly.

Codon-amino acid relationship

The genetic code is essentially a set of instructions contained within your cells, which guides the synthesis of proteins. This code consists of a series of three-nucleotide sequences known as codons in mRNA. Each codon is like a word in a language, where every word represents a different amino acid.

There are 64 possible codons (three nucleotide sequences) due to the combination of four different nucleotides (adenine, uracil, guanine, and cytosine).

• Since there are only 20 amino acids, this means there is some redundancy in the code—several codons can correspond to a single amino acid.
• For example, both UUU and UUC code for the amino acid phenylalanine.
• In contrast, there are unique codons that signal the termination of protein synthesis, known as stop codons.

This fascinating relationship between codons and amino acids is fundamental to understanding how genetic information is translated into functional proteins, enabling life to exist and operate as it does.