The Central Dogma and Nucleic Acid Function

The Central Dogma: $\text{DNA} \xrightarrow{\text{Replication}} \text{DNA}$ $\text{DNA} \xrightarrow{\text{Transcription}} \text{RNA} \xrightarrow{\text{Translation}} \text{Protein}$

Exceptions: Reverse transcriptase (retroviruses): RNA → DNA; Prions: protein can template protein misfolding (no nucleic acid).

Transcription (DNA → mRNA):

• Template: antisense (template) strand of DNA, read 3'→5'
• Product: mRNA, synthesised 5'→3' by RNA polymerase
• In eukaryotes: pre-mRNA processed → 5' cap + poly-A tail + splicing → mature mRNA

Translation (mRNA → Protein):

• Ribosome reads mRNA codons (5'→3')
• tRNA anticodon pairs with mRNA codon (complementary + antiparallel)
• Amino acid attached to 3'-CCA of tRNA transferred to growing peptide chain
• Start codon: AUG (methionine)
• Stop codons: UAA, UAG, UGA (no amino acid; release factor binds)

Genetic code properties:

• Triplet: 3 nucleotides = 1 codon
• Degenerate: multiple codons for same amino acid (61 sense codons for 20 amino acids)
• Universal: same code in nearly all organisms
• Non-overlapping: each nucleotide belongs to only one codon
• Unambiguous: each codon specifies only one amino acid

tRNA structure (NEET level):

• Cloverleaf (2D secondary structure): anticodon loop + D-loop + TψC loop + variable loop
• L-shaped (3D tertiary structure)
• 3'-CCA end: amino acid attachment site (aminoacyl-tRNA synthetase charges tRNA)
• Anticodon: read mRNA 3'→5' (antiparallel to mRNA 5'→3' direction)