# DNA, Genes and Protein Synthesis
Genes are sections of DNA that code for polypeptides. The central dogma: DNA → mRNA (transcription) → protein (translation). Gene expression can be regulated at multiple levels.
1. The Genetic Code
- Triplet code: 3 bases (codon) = 1 amino acid
- Degenerate: most amino acids coded by multiple codons
- Non-overlapping: each base read only once
- Universal: same code in nearly all organisms
- Start codon: AUG (methionine); Stop codons: UAA, UAG, UGA
2. Transcription
- RNA polymerase binds to promoter region on DNA
- DNA unwinds; H-bonds between bases break
- RNA polymerase reads template strand (3' → 5')
- Builds mRNA (5' → 3') using complementary base pairing (A-U, T-A, C-G, G-C)
- Pre-mRNA processed: introns spliced out; exons joined; 5' cap and poly-A tail added
- Mature mRNA leaves nucleus through nuclear pore
3. Translation
- mRNA attaches to ribosome
- tRNA with complementary anticodon brings specific amino acid
- Codon-anticodon base pairing at ribosome
- Peptide bond forms between adjacent amino acids
- Ribosome moves along mRNA; tRNA released
- Continues until stop codon reached
- Polypeptide released; folds into functional protein
4. Gene Mutations
| Type | Description | Effect |
|---|---|---|
| Substitution | One base replaced | May change amino acid (missense); may create stop codon (nonsense); may have no effect (silent — degenerate code) |
| Deletion | One base removed | Frameshift — all codons after shift change; usually serious |
| Insertion | One base added | Frameshift |
5. Regulation of Gene Expression
- Transcription factors: proteins that bind to DNA promoter to start/stop transcription
- Epigenetics: heritable changes in gene expression without DNA sequence changes
- DNA methylation: methyl groups on C → gene silenced
- Histone modification: acetylation loosens DNA → gene expressed; deacetylation → gene silenced
- RNA splicing: alternative splicing produces different proteins from same gene
- siRNA: small interfering RNA degrades specific mRNA
6. Practice Questions
- Describe the process of transcription.
- Explain how a deletion mutation differs from a substitution.
- What is alternative splicing and why is it important?
- How does DNA methylation affect gene expression?
- Describe the role of tRNA in translation.
Want to check your answers and get step-by-step solutions?
Summary
- Genetic code: triplet, degenerate, non-overlapping, universal
- Transcription: DNA → mRNA (in nucleus)
- Translation: mRNA → protein (at ribosome)
- Mutations: substitution, deletion, insertion; frameshift most damaging
- Regulation: transcription factors, epigenetics (methylation, histone modification), RNA interference