Amino Acid Codon Table
Inheritance of traits is possible due to the transfer of genetic information from a parent to the offspring. The process of transfer of genetic information starts with the replication and then transcription and translation of DNA. In the transcription process, the genetic information which is stored in the DNA gets copied into a form of RNA. The process of translation is completed by the ribosome which pairs amino acids as specified by mRNA (messenger RNA), with the help of tRNA (transfer RNA) molecules to carry amino acids and read the mRNA three nucleotides at one given time. The amino acid codon table has 64 entries and is very similar in all organisms; i.e., the genetic code is the same for the tiniest organism to the largest one. The genetic code table is a summarisation of an organism’s genetic code, a set of relationships between codons and amino acids.
Genetic Code
The genetic code is the set of complete information of the protein manufactured from RNA. Therefore, even a minor change in the sequence can lead to the alteration in the formation of amino acids. It was Dr George Gamow who observed that there were 43 therefore 64 probable permutations of the 4 DNA bases, if we consider 3 at a time, then it would be lowered down to 20 distinct combinations provided that the order was irrelevant. He also proposed that these 20 combinations may code for the 20 amino acids which may be the only constituents of every protein.
Genetic Code Example
The genetic code is stored on any one of the two strands of DNA molecule as a straight, well-spread and non-overlapping sequence of the nitrogenous bases namely, Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). These abbreviation letters are used for the code words. The genetic code words consist of a three-lettered word called codons. These letters are written in a sequence with the length of the DNA strand. Each codon is a unique combination of these letters. Once they form a polypeptide chain, they will be interpreted as a single amino acid. In total, they can form up to 64 codons with the unique combinations of four words. Cells read their nucleotides in codons and decode mRNAs. Most codons specify amino acid, three stop codons together signify the end of a protein, one AUG codon which is a start codon signifies the beginning of a protein along with encoding the amino acid methionine. Cells read codons in a messenger RNA (mRNA) during translation starting with a start codon and until reaching the stop codon. The cells read the mRNA from 5’ to 3’. The mRNA specifies the order of the amino acids in a protein from N-terminus to C-terminus. Each codon is known to code only one and specific type of amino acid. Some codons together code one amino acid.
Amino Acid Codon Table
A table of the various combinations of the 3 nucleotides present that come together to form different types of amino acids is given below.
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What are the different Types of Amino Acids?
In the codon table, we see the different types of amino acids produced based on the codons translated by the cell. So the table shows 20 types of amino acids produced overall (see footer of the image)
The abbreviations and names of all 20 amino acids are listed below.
Ala= Alanine Arg= Arginine Asn= Asparagine Asp= Aspartic acid
Cys= Cysteine Glu= Glutamic acid Gln= Glutamine Gly= Glycine
His= Histidine Ile= Isoleucine Leu= Leucine Lys= Lysine
Met= Methionine Phe= Phenylalanine Pro= Proline Ser= Serine
Thr= Threonine Trp= Tryptophan Tyr= Tyrosine Val= Valine
So, the below codons specify the amino acid ARG or Arginine are CGU, CGC, CGA, CGG, AGA, and AGG (see red markings in below figure)
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FAQs on Genetic Code Codons Amino Acids
1. What is the genetic code, and what are codons?
The genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells. This code is read in units of three nucleotide bases, called codons. Each codon corresponds to a specific amino acid or a stop signal, acting as the fundamental language for protein synthesis.
2. How many of the 64 possible codons specify amino acids?
Out of the 64 possible codons in the genetic code, 61 codons specify the 20 standard amino acids. The remaining three codons (UAA, UAG, and UGA) are stop codons, which signal the termination of protein synthesis. The codon AUG serves a dual function: it codes for the amino acid Methionine and also acts as the start codon.
3. How do you read a codon table to find the corresponding amino acid?
To read a standard mRNA codon table, you find the amino acid for a codon (e.g., AUG) by following these steps:
- First Base: Locate the first letter of the codon (A) in the leftmost column of the table.
- Second Base: Find the second letter (U) in the top row of the table. The intersection of this row and column points to a specific box.
- Third Base: Find the third letter (G) in the rightmost column, corresponding to the correct row within that box.
4. What are the key properties of the genetic code as per the CBSE Class 12 syllabus?
The genetic code has several key properties that are fundamental to its function:
- Triplet Code: The code is read in groups of three nucleotides (codons).
- Degenerate: An amino acid can be coded by more than one codon (e.g., Leucine is coded by six different codons).
- Unambiguous: Each specific codon codes for only one specific amino acid.
- Non-overlapping: The code is read sequentially in continuous blocks of three without any bases being shared between consecutive codons.
- Universal: The same genetic code is used by nearly all organisms, from bacteria to humans, with very few exceptions.
- Start and Stop Signals: It has specific codons (AUG for start; UAA, UAG, UGA for stop) to initiate and terminate translation.
5. What is the specific role of the 'start' and 'stop' codons in protein synthesis?
The start codon (AUG) is crucial as it marks the exact point on the mRNA where translation into a protein should begin. It sets the reading frame and codes for the first amino acid, Methionine. The stop codons (UAA, UAG, UGA) do not code for any amino acid. Instead, they signal the ribosome to terminate protein synthesis, causing the newly made polypeptide chain to be released.
6. Why is the genetic code considered both 'degenerate' and 'unambiguous'?
This might seem contradictory, but the terms describe two different aspects. The code is degenerate because multiple codons can specify the same amino acid (e.g., both UCU and UCC code for Serine). This provides a buffer against some mutations. However, the code is also unambiguous because a single codon will only ever code for one specific amino acid; for example, UCU will always code for Serine and nothing else. There is no confusion about which amino acid a particular codon represents.
7. Why are there 64 possible codons but only 20 standard amino acids?
The mismatch between 64 codons and 20 amino acids is explained by the degeneracy of the genetic code. With four nucleotide bases (A, U, G, C) and a triplet codon system, there are 4x4x4 = 64 possible combinations. Rather than having a unique codon for each amino acid and leaving many unused, the system evolved so that most amino acids are specified by more than one codon. This redundancy can help protect against harmful mutations, as a change in the third base of a codon often does not change the resulting amino acid.
8. How does a single DNA base mutation affect the resulting amino acid?
A mutation in a single DNA base can have several outcomes on the amino acid sequence. This is known as a point mutation. If the changed codon still codes for the same amino acid (due to degeneracy), it's a silent mutation with no effect. If it codes for a different amino acid, it's a missense mutation, which might alter the protein's function. If the mutation changes an amino acid codon into a stop codon, it's a nonsense mutation, which prematurely terminates the protein.
