SN6
SHORT NOTE
5M
Describe the genetic code with its properties. Add a note on the wobble hypothesis.
Definition
Genetic code = the set of rules by which the sequence of bases in mRNA is translated into the sequence of amino acids in a protein.
Properties
| Property | Meaning | Consequence |
|---|---|---|
| Triplet | 3 bases = 1 codon | 43 = 64 codons for 20 amino acids |
| Degenerate | Several codons per amino acid | A silent mutation may have no effect |
| Unambiguous | One codon → one amino acid only | No confusion in reading |
| Non-overlapping | Each base used once | A single deletion shifts the whole frame |
| Comma-less | Read continuously | The reading frame is critical |
| Universal | Same in nearly all species | Human genes can be expressed in bacteria — the basis of recombinant insulin |
| Has punctuation | Start and stop codons | Defines the boundaries of the message |
Special Codons
| Codon | Meaning |
|---|---|
| AUG | Start and methionine (formyl-methionine in bacteria) |
| UAA, UAG, UGA | Stop — ochre, amber and opal |
| UGG | Tryptophan — only one codon |
| AUG | Methionine — only one codon |
| Leucine, serine, arginine | Six codons each — the most |
The Wobble Hypothesis
- Proposed by Crick
- The third base of the codon pairs loosely with the first base of the anticodon
- Therefore one tRNA can read more than one codon
- Explains why only about 30–40 tRNAs are needed rather than 61
- Inosine in the anticodon can pair with U, C or A — the widest wobble
- Explains why the third base is the most tolerant of mutation
Exceptions to Universality
- Mitochondria use a slightly different code — UGA codes tryptophan instead of stop, and AUA codes methionine
- Mitochondria also need fewer tRNAs (22) because of extended wobble
- Some organisms use selenocysteine (UGA in context), called the 21st amino acid
Applied Aspects
- Degeneracy is protective — many random base changes produce no change in protein
- Frameshift mutations are the most damaging precisely because the code is comma-less and non-overlapping
- Universality made genetic engineering possible — a human gene placed in E. Coli is read correctly, which is how recombinant insulin is made
- Nonsense mutations may be overcome by read-through drugs (ataluren) or suppressor tRNAs — an active area of therapy in cystic fibrosis and muscular dystrophy
All questions in Nucleotide Metabolism & Molecular Biology
- Q1Describe the de novo synthesis of purine nucleotides with its regulation. Explain the salvage pathway and the degradation of purines. ★
- Q2Describe the synthesis and degradation of pyrimidine nucleotides. Compare them with purines and add a note on orotic aciduria. ★
- Q3Describe the process of DNA replication with the enzymes involved. Add a note on telomeres and the inhibitors of replication. ★
- Q4Describe transcription and the post-transcriptional processing of mRNA. Compare it with replication. ★
- Q5Describe the process of translation. Explain the genetic code and the inhibitors of protein synthesis. ★
- SN1Define gout. Describe its biochemical basis, causes and treatment. ★
- SN2Describe the salvage pathway of purines. Add a note on Lesch–Nyhan syndrome. ★
- SN3Describe the principle, steps and applications of the polymerase chain reaction. ★
- SN4Classify mutations with examples. Add a note on DNA repair. ★
- SN5Describe the tools of recombinant DNA technology and its medical applications. ★
- SN6Describe the genetic code with its properties. Add a note on the wobble hypothesis.
- SN7Describe the structure of DNA. Add a note on its denaturation and melting temperature. ★