The genetic code is a universal set of rules that translate RNA sequences into proteins. It consists of codons made up of three-nucleotide sequences, which represent specific amino acids for protein synthesis.
Understanding the genetic code is essential for decoding DNA instructions, studying gene-protein interactions, and understanding gene functions in living organisms.
The genetic code is the set of rules that defines how nucleotide triplets, known as codons, correspond to specific amino acids or serve as stop signals in protein synthesis.
The Establishment of The Genetic Code
Here’s a summary of the establishment of the genetic code:
- Discovery of DNA and RNA: Scientists discovered the structure of DNA and RNA in the middle of the 20th century and realized that these molecules carry genetic information.
- Role of Nucleotides: It was discovered that the sequence of nucleotides that make up the DNA or RNA molecule—adenine (A), thymine (T), guanine (G), and cytosine (C)—or, in the case of RNA, uracil (U)—contains the genetic information.
- Codons and Amino Acids: Codons, which are collections of nucleotides, are thought to be responsible for encoding particular amino acids, according to scientific theory. There are 64 potential codons (4 nucleotides raised to the power of three) and 20 amino acids found in proteins. Each codon is made up of three nucleotides.
- Experimental Studies: Biochemical experiments were conducted using various organisms, including bacteria and viruses, to decipher the relationship between codons and amino acids. Scientists performed experiments involving mutation, genetic recombination, and translation to decode the codon-to-amino acid assignments.
- Universal Nature: It was discovered that the genetic code is universal, meaning that the same codons correspond to the same amino acids across different species. This universality provided further evidence for the existence of a standard genetic code.
- Decoding the Code: Through extensive research and experimentation, scientists gradually deciphered the assignments of codons to amino acids. They determined which codons corresponded to each amino acid and which codons served as stop signals to terminate protein synthesis.
- Redundancy and Wobble Hypothesis: It became apparent that the genetic code is degenerate or redundant, meaning that multiple codons can code for the same amino acid. Additionally, the “wobble hypothesis” proposed that the third nucleotide in a codon (the “wobble position”) can sometimes vary, allowing certain tRNA molecules to bind to multiple codons.
- Completion of the Genetic Code: The majority of the genetic code had been cracked by the middle of the 1960s. The final codon assignments were established, and all of the links between codons and amino acids were established.
Contribution of Nirenberg and Har Gobind Khorana in Discovery of Genetic Code
Nirenberg and Har Gobind Khorana were two prominent scientists who made significant contributions to the deciphering of the genetic code.
The majority of the genetic code had been cracked by the middle of the 1960s. The final codon assignments were established, and all of the links between codons and amino acids were established.
Marshall W. Nirenberg
Marshall Nirenberg was an American biochemist who conducted groundbreaking research on the genetic code during the 1960s. His work, alongside Heinrich Matthaei, led to the deciphering of the first codon and the identification of several codons that specify specific amino acids.
Nirenberg’s key contributions include:
- Demonstrating that the sequence of three nucleotides in RNA (known as codons) corresponds to specific amino acids.
- Conducting in vitro translation experiments using synthetic RNA molecules with known sequences to decipher the genetic code.
- Identifying the codons UUU (uracil-uracil-uracil) and UUC (uracil-uracil-cytosine) as encoding the amino acid phenylalanine.
- Collaborating with other scientists to expand the understanding of codon assignments for various amino acids.
Har Gobind Khorana
Har Gobind Khorana was an Indian-American biochemist and Nobel laureate. He made significant contributions to the synthesis of nucleic acids and the deciphering of the genetic code.
Khorana played a crucial role in expanding the understanding of the genetic code by synthesizing RNA molecules with specific sequences and determining their corresponding amino acids.
Khorana’s notable achievements include:
- Synthesizing short RNA molecules with known sequences, such as polyribonucleotides, to study their translation.
- Demonstrating that RNA codons can be chemically synthesized and used to produce specific amino acids during translation.
- Decoding several codons, including those for alanine, lysine, and isoleucine, using synthetic RNA sequences.
- Collaborating with Nirenberg and other scientists to establish a comprehensive understanding of the genetic code.
For his groundbreaking work on the genetic code, Marshall Nirenberg, along with Robert W. Holley and Har Gobind Khorana, was awarded the Nobel Prize in Physiology or Medicine in 1968.
Genetic Code Table
Properties of The Genetic Code
Here are some key features of the genetic code:
Codons are groups of three nucleotides (triplets) found in RNA that make up the genetic code. Each codon either codes to a particular amino acid or acts as a signal for the start or end of translation.
Example: AUG (codes for methionine).
The genetic code is nearly universal, meaning that the same codons code for the same amino acids across different species. This universality is essential for the transfer of genetic information.
Example: The codon GGA codes for glycine in bacteria, plants, and humans.
Each codon has a specific assignment to an amino acid. For example, the codon AUG codes for the amino acid methionine, while the codon GUA codes for valine. This specificity allows for the accurate translation of the genetic code.
Example: The codon CAG codes for glutamine.
The genetic code is degenerate or redundant, meaning that multiple codons can code for the same amino acid.
For instance, the amino acid leucine can be specified by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG. This redundancy provides some flexibility and robustness to the genetic code.
Example: Both GGU and GGC code for glycine.
5. Total Number
The genetic code consists of 64 possible codons. Three of these codons, UAA, UAG, and UGA, serve as stop codons, indicating the termination of protein synthesis. The remaining 61 codons encode the 20 standard amino acids found in proteins.
6. Stop Codon
The stop codons (UAA, UAG, and UGA) signal the end of protein synthesis. When a ribosome encounters a stop codon, it releases the newly synthesized protein and dissociates from the mRNA template.
UAG is also called Ochre, UAA is Amber and UGA is Opal.
7. Start Codon
The start codon initiates protein synthesis. The codon AUG serves as the start codon in most cases and codes for the amino acid methionine. However, in some instances, it can specify formylmethionine or other modified amino acids.
The codons of the genetic code are read in a non-overlapping manner means they do not overlap each other. Each codon is considered individually and does not overlap with the adjacent codons during translation.
Example: AUGGCUAA is read as AUG, GGC, UAA, with each codon representing an individual amino acid or a start/stop signal.
The genetic code is comma-less, meaning that there are no punctuation marks or gaps between codons. The codons are read consecutively in a continuous sequence.
Example: The mRNA sequence AUGGCCUAA is read as AUG, GCC, UAA without any breaks.
Exceptions to the Genetic Code
Codon reassignment or codon redefinition is an exception in the genetic code. This happens when a codon that ordinarily codes for a particular amino acid gets reassigned to encode a different amino acid or acts as a stop codon in particular animals or conditions.. Here are a few examples of codon reassignment:
- Mycoplasma bacteria: Mycoplasma is a genus of bacteria that has a reduced genome and limited genetic material. In some species of Mycoplasma, the codon UGA, which is typically a stop codon in most organisms, is reassigned to encode the amino acid tryptophan.
- Ciliate protozoa: Ciliates are a group of protozoa that exhibit a unique genetic code alteration. In certain ciliate species, the codon UAG, usually a stop codon, is reassigned to encode the amino acid glutamine.
- Human mitochondria: Mitochondria have their own independent genome. The genetic code in human mitochondria differs from the universal genetic code in a few instances. For example, the codon UGA, which serves as a stop codon in the universal genetic code, is reassigned to encode the amino acid tryptophan in human mitochondria.
- Start Codon: Although GUG is intended to code for valine. It may code for methionine as a starting codon
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