| Concept | Description |
|---|---|
| Genetic Code | The set of rules by which information encoded in genetic material is translated into proteins. |
| Replication and Transcription | Processes that involve copying a nucleic acid to form another nucleic acid based on complementarity. |
| Translation | Process requiring transfer of genetic information from nucleotides to synthesize a polymer of amino acids. |
| Complementarity | Exists in replication and transcription but not in translation. |
| Evidence of Genetic Code | Changes in nucleic acids lead to changes in amino acids in proteins, suggesting a genetic code. |
| Proposition of Triplet Code | George Gamow proposed that a combination of three nucleotides (triplet) could code for 20 amino acids. |
| Codon | A sequence of three nucleotides that together form a unit of genetic code. |
| Number of Codons | 64 codons (4^3 combinations), with 61 coding for amino acids and 3 as stop codons. |
| Proof of Triplet Codon | Developed by Har Gobind Khorana and Marshall Nirenberg through chemical methods and cell-free systems. |
| Polynucleotide Phosphorylase | An enzyme used by Severo Ochoa to polymerize RNA sequences without a template. |
| Checker-Board for Genetic Code | A table showing the codons for various amino acids. |
| Codon is Triplet | 61 codons code for amino acids and 3 are stop codons. |
| Code is Degenerate | Some amino acids are coded by more than one codon. |
| Codon is Read Contiguously | Codons are read in mRNA without punctuations. |
| Nearly Universal Code | Codons generally code for the same amino acids across different organisms. Exceptions in mitochondria/protozoans. |
| AUG Codon | Codes for Methionine and acts as an initiator codon. |
| Stop Codons | UAA, UAG, and UGA do not code for any amino acid and signal termination of protein synthesis. |
| Wobble Hypothesis | The third nucleotide of a codon is less specific, allowing some tRNA to pair with multiple codons. |
| Redundancy | Multiple codons can code for the same amino acid, reducing the impact of mutations. |
| No Overlapping | Codons are read one after another without overlapping in the genetic sequence. |
| Non-ambiguity | Each codon specifies only one amino acid or a stop signal, ensuring precise protein synthesis. |
| Start Codon | The codon AUG not only codes for Methionine but also indicates the start of translation. |
| Stop Codons Role | UAA, UAG, and UGA signal the end of translation, ensuring the protein chain is correctly terminated. |
| Reading Frame | The way nucleotides are grouped into codons, starting from the start codon. Shifting the frame alters the protein. |
| Codon Usage Bias | Different organisms prefer certain codons over others, influencing gene expression efficiency. |
| Mitochondrial Genetic Code | Mitochondria have a slightly different genetic code, reflecting their evolutionary origin. |
| George Gamow (1954) |
Contribution: Triplet Hypothesis: Physicist George Gamow proposed that the genetic code must be composed of “triplets” of nucleotides (three nucleotides per amino acid). Significance: Basis for Genetic Coding: Gamow’s hypothesis suggested that 20 amino acids could be encoded by 64 possible triplets, laying the groundwork for future experiments to decode the genetic code. |
| Severo Ochoa (1955) |
Contribution: Polynucleotide Phosphorylase: Severo Ochoa discovered an enzyme called polynucleotide phosphorylase. Experiment: RNA Synthesis: This enzyme could synthesize RNA from nucleotides in vitro, facilitating the creation of synthetic RNA sequences for decoding experiments. Significance: Tool for Code Cracking: Ochoa’s enzyme was a vital tool that allowed researchers like Nirenberg and Khorana to produce RNA sequences needed for their experiments to decipher the genetic code. |
| Marshall Nirenberg and Heinrich Matthaei (1961) |
Contribution: First Deciphering Experiment: Nirenberg and Matthaei conducted the first successful experiment to crack the genetic code. Experiment: Poly-U RNA: They synthesized RNA composed solely of uracil (poly-U) and added it to a cell-free system containing ribosomes and other components needed for protein synthesis. Phenylalanine Discovery: This led to the production of a protein composed entirely of phenylalanine, demonstrating that the codon “UUU” coded for phenylalanine. Significance: First Codon-Amino Acid Match: This was the first direct evidence linking a specific nucleotide sequence (UUU) to a specific amino acid (phenylalanine). |
| Har Gobind Khorana (1960s) |
Contribution: Synthetic RNA Sequences: Khorana developed methods to synthesize RNA molecules with defined sequences. Methods: Homopolymers: Khorana synthesized RNAs composed of repeating single nucleotides. For example, poly-C RNA (CCCC…) codes for the amino acid proline. Copolymers: He also created RNAs with alternating sequences. For instance, using a copolymer of UC (UCUCUC…), he discovered that UCU codes for serine and CUC codes for leucine. Experiment: Systematic Deciphering: Using these synthetic RNAs, Khorana was able to systematically determine the sequences of triplets (codons) that corresponded to each of the 20 amino acids. Significance: Complete Codon Table: Khorana’s work was crucial in completing the genetic code table by identifying all the codon assignments for the amino acids. |
| Robert W. Holley (1965) |
Contribution: tRNA Structure: Holley determined the structure of transfer RNA (tRNA), the molecule that carries amino acids to the ribosome for protein synthesis. Significance: Understanding Translation: Knowing the structure of tRNA was essential for understanding how the genetic code is translated into proteins, as tRNAs are the adaptors that read the mRNA codons and bring the corresponding amino acids. |
