Question

The codons UGA, UAA, and UAG do not code for amino acids. What is their role as codons in mRNA?

Short answer

UGA, UAA, and UAG are stop codons that signal the termination of protein synthesis in mRNA.

Step-by-step solution

- Understanding Codons

Codons are sequences of three nucleotides in mRNA that correspond to specific amino acids or signal the termination of protein synthesis.

- Identify Special Codons

UGA, UAA, and UAG are unique because they do not correspond to any amino acids. Instead, they play a special role in the process of translation.

- Role of Stop Codons

The codons UGA, UAA, and UAG are 'stop codons.' They signal the termination of protein synthesis, indicating to the ribosome that the protein chain is complete and should be released.

- Biological Significance

The presence of stop codons ensures that proteins are synthesized correctly, preventing further elongation of the polypeptide chain which could lead to non-functional or harmful proteins.

Key concepts

Codon

A codon is a sequence of three nucleotides in messenger RNA (mRNA). These sequences are the building blocks of genetic code. Each codon corresponds to a specific amino acid or a signal.

Here’s how it works:

• During protein synthesis, the ribosome reads the mRNA sequence codon by codon.
• Each codon matches with a transfer RNA (tRNA) that carries a specific amino acid.
• This process translates the genetic code in mRNA into a chain of amino acids, forming a protein.

It’s like a biological dictionary, where each three-letter word (codon) translates to an amino acid or a functional instruction.

Protein Synthesis

Protein synthesis is the process by which cells make proteins. It involves two main stages: transcription and translation.

Transcription:

• Occurs in the nucleus.
• DNA is copied into mRNA by the enzyme RNA polymerase.
• mRNA then leaves the nucleus and moves to the cytoplasm.

Translation:

• Occurs in the cytoplasm.
• Ribosomes read the mRNA sequence and assemble amino acids in the correct order to produce a protein.

Thus, through transcription and translation, the genetic information in DNA leads to the creation of proteins, which perform numerous crucial functions in the body.

Translation Termination

Translation termination is the final step in protein synthesis. This is an essential stage to ensure that the protein is correctly formed.

Here’s how it happens:

• The ribosome continues to read mRNA codons until it encounters a stop codon (UGA, UAA, or UAG).
• Stop codons do not code for any amino acids.
• When the ribosome reaches a stop codon, it recognizes that the protein chain is complete.
• The newly synthesized protein is then released from the ribosome.

This step is crucial because continuing beyond this point could lead to the addition of incorrect amino acids, resulting in dysfunctional proteins.

Ribosome

Ribosomes are complex molecular machines found within all living cells. They have a critical role in protein synthesis.

Functions of Ribosomes:

• Reading mRNA sequences and translating them into proteins.
• Ensuring the correct assembly of amino acids to form a specific protein.

Ribosomes have two main parts:

• The small subunit, which binds to mRNA.
• The large subunit, which joins amino acids to form a polypeptide chain.

These parts work together to ensure that proteins are built accurately and efficiently, following the instructions encoded in mRNA.