Central Dogma of Molecular Biology

Flow of Genetic Information

• DNA → RNA → Proteins
• Synthesis of proteins involves two main processes:
  • Transcription: Copying genetic information from DNA to RNA
  • Translation: Converting RNA into a sequence of amino acids (proteins)

Transcription

• DNA and RNA use the same language (nucleotides)
• RNA is a copy of the genetic information from DNA

Translation

• RNA and proteins use different languages
  • RNA: Nucleotides
  • Proteins: Amino acids
• Ribosomes translate nucleotide sequences (RNA) into amino acid sequences (proteins)
• Genetic code: System used by cells to translate RNA sequences into proteins

The Genetic Code

• Utilized to translate nucleotide sequences into amino acids
• Consists of codons (3-nucleotide sequences)

Four Key Facts about the Genetic Code

1. Three-Nucleotide Codons: Each amino acid is encoded by a triplet of nucleotides
2. Non-Overlapping Code: Ribosomes read each codon without overlap
3. Continuous Reading: The code is read sequentially without pauses or skipping
4. Degenerate Code: Multiple codons can encode the same amino acid

Fact 1: Three-Nucleotide Codons

• Single nucleotide cannot encode an amino acid (only 4 nucleotides, 20 amino acids)
• Pairs of nucleotides also insufficient (4^2 = 16 < 20)
• Triplets necessary (4^3 = 64 > 20)
  • Example: UUU (Phenylalanine), UUG (Leucine)

Fact 2: Non-Overlapping Code

• Codons are read one after the other without overlapping
  • Correct: GCC - CCU - CUU
  • Incorrect: GCC, then overlaps CCU

Fact 3: Continuous Reading

• Codons are read sequentially without interruptions

Fact 4: Degenerate Code

• There are 64 possible codons but only 20 amino acids
• Multiple codons can code for the same amino acid (synonyms)
  • Example: Leucine (e.g., UUA, CUU, CUC, etc.)
• Minimizes mutation impact
  • Mutation may still encode the same amino acid
  • Example: UUA to UUG still encodes Leucine

Importance of Degenerate Code

• Reduces the effect of mutations
• Table shows codons for different amino acids and stop signals
  • Example: UAA, UAG, UGA are stop codons
• Stop codons signal the end of protein synthesis