📌Definition Table

Term	Definition
Genetic Code	The set of rules by which nucleotide sequences are translated into amino acid sequences in proteins.
Codon	A triplet of nucleotides in mRNA that codes for a specific amino acid or a stop signal.
Start Codon	AUG; signals the start of translation and codes for methionine.
Stop Codon	UAA, UAG, UGA; signals the end of translation.
Degeneracy	Multiple codons can code for the same amino acid.
Reading Frame	The way nucleotides are grouped into codons during translation.

📌Introduction

The genetic code is a universal molecular language linking DNA sequences to protein structures. It ensures accurate transfer of genetic instructions from DNA to functional proteins through transcription and translation.

📌 Properties of the Genetic Code

• Triplet Nature: Each amino acid is specified by a three-nucleotide codon.
• Degenerate: Most amino acids have more than one codon, reducing mutation effects.
• Unambiguous: Each codon codes for only one amino acid.
• Non-Overlapping: Codons are read sequentially without sharing bases.
• Universal: Shared by almost all organisms (with minor exceptions in mitochondria).
• Start/Stop Signals: AUG starts translation; UAA, UAG, UGA stop it.

🧠 Examiner Tip: Always specify the start codon (AUG) and stop codons in answers about translation.

📌 Role in Transcription

• DNA sequence is copied into mRNA by RNA polymerase.
• Uses complementary base pairing (A–U, G–C).
• Only one strand (template strand) is transcribed.
• Promoter regions control where transcription begins.
• mRNA produced is complementary to the DNA template strand.
• Errors in transcription can lead to incorrect proteins.

🧬 IA Tips & Guidance: Measure transcription rates under different temperature or pH conditions using in vitro transcription systems.

📌 Role in Translation

• Ribosomes read mRNA codons in the 5′ → 3′ direction.
• tRNA molecules carry specific amino acids to the ribosome.
• Anticodon in tRNA pairs with codon in mRNA.
• Peptide bonds form between amino acids.
• Process continues until a stop codon is reached.
• Produces a specific polypeptide chain.

🌐 EE Focus: Investigate how codon bias affects translation efficiency in different organisms.

📌 Start and Stop Codons

• Start: AUG — codes for methionine; establishes reading frame.
• Stops: UAA, UAG, UGA — do not code for amino acids.
• Misreading start/stop sites can produce non-functional proteins.
• Mutations in these sites can disrupt protein synthesis.
• Alternative start codons exist in rare cases.

❤️ CAS Link: Create a classroom activity where students decode amino acid sequences from mRNA codons.

📌 Degeneracy and Mutation Protection

• Reduces harmful effects of point mutations.
• Silent mutations often occur at the third codon position.
• Codon families group codons with the same amino acid outcome.
• Evolutionary adaptation to minimize protein errors.
• Some codons are more common (codon bias).

📌 Universality and Evolution

• Shared by most life forms — evidence for common ancestry.
• Minor variations exist in mitochondrial and some microbial codes.
• Supports evolutionary conservation of molecular machinery.
• Changes in code are rare due to complexity of translation system.

🔍 TOK Perspective: Universality of the genetic code raises questions about whether life shares a single origin or if convergent evolution could produce the same code.

📝 Paper 2: Data Response Tips: Expect codon tables, mRNA sequences to translate, or mutation impact questions.