Question

What do the codons AUG, UAG, UAA, and UGA code for?

Short answer

The codon AUG codes for the amino acid Methionine (Met) and acts as the start codon during translation. UAG, UAA, and UGA are stop codons, signaling the end of protein synthesis and do not code for any amino acids.

Step-by-step solution

Understand the genetic code and codons

The genetic code is a set of rules specifying how sequences of nucleotides in DNA and RNA molecules are translated into protein sequences. In the genetic code, a sequence of three nucleotides on an mRNA molecule, called a codon, codes for a specific amino acid during translation. Each codon corresponds to a specific amino acid, and there are 64 different codons in total.

Identify the role of AUG

The codon AUG acts as the start codon during translation, where protein synthesis begins. Specifically, AUG codes for the amino acid Methionine (Met), and the ribosome recognizes it as the starting point for translation. Thus, all proteins synthesized start with Methionine.

Identify the roles of UAG, UAA, and UGA

UAG, UAA, and UGA are known as stop codons in the genetic code. These codons indicate the end of a protein-coding sequence. When a ribosome encounters one of these codons, no amino acid is added; instead, the protein synthesis process terminates. These stop codons are also called nonsense codons since they do not code for any amino acids. In summary: - AUG: Start codon, codes for Methionine (Met) - UAG, UAA, UGA: Stop codons, signal the end of protein synthesis

Key concepts

codons

In the world of genetics, a codon is a sequence of three nucleotides found in messenger RNA (mRNA). These sequences are the "words" of the genetic language, dictating how proteins are constructed. Each triplet, known as a codon, corresponds to a specific amino acid or a signaling function during protein synthesis. There are 64 possible combinations of codons, each coding for one of the 20 amino acids or signaling a start or stop in the process.

Think of codons as instructions read by the ribosome during protein production. As the ribosome "reads" each codon, it assembles amino acids in the order dictated by the sequence, eventually constructing a full protein.

protein synthesis

Protein synthesis is the vital process in which cells build proteins, essential for life and function. This process occurs in two main stages: transcription and translation.

During transcription, the information encoded in a gene within DNA is transcribed to produce a messenger RNA (mRNA) molecule. This mRNA serves as a template for translation, the next step where the ribosome reads the sequence of mRNA codons to synthesize a specific protein.

• Transcription: DNA → mRNA
• Translation: mRNA → Protein

The ribosome plays a key role in translation, moving along the mRNA strand and utilizing transfer RNA (tRNA) to add the corresponding amino acid for each codon, forming a chain that eventually folds into a functional protein.

start codon

The start codon is a crucial part of protein synthesis, acting as the signal to begin translation. The codon AUG is designated as the universal start codon in virtually all organisms, initiating the synthesis process.

AUG not only serves as a signal to start but also codes for the amino acid Methionine. This means every newly synthesized protein initially contains Methionine at its N-terminus, although it may be modified later. The presence of the start codon ensures that the ribosome begins assembling the protein sequence at the right spot, setting the stage for accurate protein construction.

stop codon

Stop codons are integral to the termination of protein synthesis. In the genetic code, the codons UAG, UAA, and UGA act as stop signals that instruct the ribosome to cease protein synthesis. When encountered, these codons do not code for an amino acid but rather signal the ribosome that the newly formed protein is complete.

These are also known as nonsense codons because they do not correspond to any amino acids. The appearance of a stop codon ensures that protein synthesis is both precise and regulated, preventing the addition of unnecessary amino acids and facilitating the release of a fully functional protein.