Which is the initiation codon for protein synthesis in higher organisms?

Appeared in: KPSC Staff Nurse - 2015

Explanation

The initiation codon for protein synthesis in both prokaryotic and eukaryotic organisms is AUG.
This codon signals the ribosome to start the process of translation, which is the synthesis of a protein from an mRNA template.
AUG also codes for the amino acid Methionine. Therefore, every protein initially starts with Methionine, although it is often removed later during post-translational modifications.

Option B: UAG is a stop codon, also known as the 'amber' codon. It signals the termination of protein synthesis, not the initiation.
Option C: UGA is a stop codon, also known as the 'opal' codon. It signals the end of the translation process.
Option D: UAA is a stop codon, also known as the 'ochre' codon. Its function is to terminate protein synthesis.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Protein Synthesis Initiation Codon as background academic context rather than a clinical decision trigger.
Understanding the genetic code is fundamental to diagnosing and understanding genetic diseases caused by mutations. A mutation changing an amino acid codon to a stop codon (nonsense mutation) can lead to a truncated, non-functional protein.
Many antibiotics, such as tetracyclines and aminoglycosides, work by selectively inhibiting protein synthesis in bacteria by targeting their ribosomes, while leaving human ribosomes unaffected. This principle relies on the differences between prokaryotic and eukaryotic translation machinery.
What if? - If a mutation occurred that changed the AUG start codon to another sequence, protein synthesis for that specific gene would likely fail to initiate, resulting in a complete absence of the protein product. This is often more severe than a mutation that simply changes one amino acid for another.

How to Approach the Question

First, identify the core concept of the question, which is the 'initiation codon' for 'protein synthesis'.
Recall the central dogma of molecular biology: DNA is transcribed into RNA, and RNA is translated into protein.
Remember that translation has specific start and stop signals encoded in the mRNA sequence.
Differentiate between the single start codon and the multiple stop codons.
The most common start codon is AUG, which codes for Methionine.
The three stop codons are UAA, UAG, and UGA. A mnemonic to remember them is 'U Are Away', 'U Are Gone', 'U Go Away'.

Concept Tested & Keywords

QVySaiKmA7_nohW5pSUiS4

E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 2 — Subpart B (pp 240-463 of 478) pp. 14-16, 15-17

E6 Medicine Davidson Principles Practice 24e p. 60-62

Practise the full KPSC Staff Nurse - 2015

Attempt every question from this paper in a timed mock, then review the full solution for each one.