[Biology Class Notes] on Genetic Code Codons Amino Acids Pdf for EXAM

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.

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)