Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 17.1 Genes Specify Proteins via Transcription and Translation
Genes dictate traits by encoding proteins, which act as the link between genotype and phenotype. This process involves two main stages—transcription and translation—and was uncovered through classic experiments in metabolic genetics.
Genes, Proteins, and Traits
- Genes are sequences of DNA that encode instructions for building proteins
- Proteins perform functions that determine traits (e.g., pigment production)
- Gene expression involves:
- Transcription: DNA → RNA
- Translation: RNA → Protein
- Example: A faulty enzyme-coding gene results in albinism due to lack of melanin production
From One Gene–One Enzyme to One Gene–One Polypeptide
- Archibald Garrod first proposed genes control enzyme production, based on inherited diseases like alkaptonuria
- Beadle and Tatum's experiments with Neurospora crassa (bread mold) supported this idea:
- X-ray-induced mutants required extra nutrients to grow
- Each mutant lacked a different enzyme in a biosynthetic pathway
- Led to the one gene–one enzyme hypothesis
- Later modified to one gene–one polypeptide, since:
- Some proteins are made of multiple polypeptides (e.g., hemoglobin)
- Some genes code for functional RNAs, not proteins
- Eukaryotic genes can produce multiple proteins via alternative splicing
Beadle and Tatum’s Experiment
- Mutated Neurospora spores with X-rays
- Identified “nutritional mutants” that could grow only with added nutrients
- Srb and Horowitz further classified mutants using the arginine biosynthesis pathway
- Mutants were grouped by which compound (ornithine, citrulline, arginine) rescued growth
- Conclusion: each gene codes for a specific enzyme in a metabolic pathway
The Central Dogma of Molecular Biology
- Genetic information flows in one direction: DNA → RNA → Protein
- This was coined as the central dogma by Francis Crick
- Transcription creates mRNA, which carries instructions from DNA to the ribosome
- Translation uses mRNA to direct amino acid sequence assembly into proteins
- RNA differs from DNA:
- Single-stranded
- Contains ribose instead of deoxyribose
- Uses uracil (U) instead of thymine (T)
Transcription vs. Translation in Prokaryotes and Eukaryotes
- Prokaryotes:
- Transcription and translation occur simultaneously in the cytoplasm
- No nuclear envelope separates the processes
- Eukaryotes:
- Transcription occurs in the nucleus
- mRNA is processed before exiting to the cytoplasm for translation
- The initial transcript is called pre-mRNA, later modified into mRNA
Codons and the Genetic Code
- Each set of three mRNA nucleotides = a codon
- Each codon specifies one amino acid
- Total of 64 codons: 61 for amino acids, 3 are stop codons
- Start codon = AUG (codes for methionine)
- The genetic code is redundant (multiple codons for same amino acid) but not ambiguous (each codon has one meaning)
- Codons must be read in the correct reading frame
- The near-universality of the genetic code across life forms shows a shared evolutionary origin
In a Nutshell
Genes express their instructions by first being transcribed into RNA and then translated into proteins. This flow of genetic information—DNA → RNA → Protein—is the central dogma of molecular biology. Experiments with fungi and metabolic pathways led to the “one gene–one polypeptide” concept, and decoding the genetic code revealed how nucleotide triplets specify amino acids.