⚡ Core Concept: The Central Dogma of Molecular Biology
Proposed by Francis Crick in 1958, the Central Dogma describes the directional flow of genetic information: DNA → RNA → Protein. Information encoded in double-stranded genomic DNA is transcribed into messenger RNA (mRNA) and subsequently translated into functional polypeptide chains using the universal genetic code.
1. Three Directional Informational Steps
Figure 1: Central Dogma of Molecular Biology
Core Biological Principle
The canonical Central Dogma pathway: DNA replication and transcription into messenger RNA, translated into functional protein. Dotted arrow indicates reverse transcription (Temin & Baltimore, retroviruses).
2. Cardinal Features of the Universal Genetic Code
| Code Characteristic | Biological Meaning | Molecular Example |
|---|---|---|
| Triplet Nature | 64 codons formed by permutations of 4 bases in groups of 3 (43 = 64). | AUG, UAA, GGG, CCU. |
| Degeneracy / Redundancy | 18 of the 20 amino acids are specified by more than one codon (Wobble hypothesis). | Leucine, Serine, and Arginine are each encoded by 6 distinct codons. |
| Non-Overlapping & Non-Punctuated | Read continuously 3 nucleotides at a time without skips or sharing bases between adjacent triplets. | Single nucleotide deletion causes an entirely altered downstream reading frame. |
| Universal (Near-Universal) | Same codons code for identical amino acids across bacteria, plants, and humans. | Exceptions: Mitochondrial codons (UGA encodes Tryptophan instead of Stop; AUA encodes Met). |
| Start & Stop Signals | One initiator codon; three termination codons signaling release factors. | Start: AUG (Methionine); Stop: UAA (Ochre), UAG (Amber), UGA (Opal). |
🎓 High-Yield Exam Checkpoint
- Crick’s Wobble Hypothesis (1966): The 5′ anticodon base (position 1 of tRNA) pairs flexibly with the 3′ codon base (position 3 of mRNA). Inosine (I) at the wobble position can pair with U, C, or A.
- Shine-Dalgarno vs. Kozak Sequence: In prokaryotes, the 16S rRNA binds the Shine-Dalgarno consensus (5′-AGGAGG-3′) upstream of AUG; in eukaryotes, the 40S ribosomal subunit scans from the 5′ cap to find the Kozak consensus (5′-ACCAUGG-3′).
