Molecular Genetics
🔬 Why Scientists Originally Believed Protein Was Genetic Material
Proteins were:
Structurally complex (20 amino acids vs. DNA’s 4 bases)
Known to perform diverse cellular functions
DNA was thought too simple to carry complex genetic information.
🧪 Key Scientists & Contributions
Frederick Griffith (1928) | How bacteria cause pneumonia | Discovered transformation; genetic info could be transferred | Streptococcus pneumoniae, mice |
Oswald Avery (1944) | Identify Griffith’s transforming principle | DNA is the transforming principle | Streptococcus pneumoniae |
Hershey & Chase (1952) | DNA vs. protein as genetic material | DNA enters cells and carries genetic info | Bacteriophage, E. coli |
Archibald Garrod (1902) | Inherited disorders | Genes control enzymes; “inborn errors of metabolism” | Humans |
Beadle & Tatum (1941) | Gene function | One gene → one enzyme (later: polypeptide) | Neurospora crassa (mold) |
Francis Crick (1958) | Genetic information flow | Proposed the Central Dogma: DNA → RNA → Protein | Theoretical (multiple organisms) |
🧬 Key Terms
Transformation: Uptake of external DNA by a cell, changing its traits (Griffith’s experiment).
Transcription: Process where DNA is used to make RNA (mRNA).
Translation: Process where mRNA is used to build a protein (at the ribosome).
đź“ś Central Dogma of Molecular Biology
DNA → RNA → Protein
Genetic information flows in one direction:
DNA is transcribed into RNA
RNA is translated into Protein
🔤 Why Codons Have 3 Nucleotides
A codon = 3 nucleotides = 1 amino acid
4 nucleotides → 4³ = 64 combinations (enough to code 20 amino acids)
2-letter codons (4² = 16) = not enough combinations