Main Difference – Codon vs Anticodon
Codon and anticodon are nucleotide triplets which specify a particular amino acid in a polypeptide. A specific rule set exists for the storage of genetic information as a nucleotide sequence either on DNA or mRNA molecules in order to synthesise proteins. That specific rule set is referred to as the genetic code. Codon is a group of three nucleotides, especially on the mRNA. Anticodon is present on tRNA molecules. The main difference between codon and anticodon is that codon is the language which represents an amino acid on mRNA molecules whereas anticodon is the complement nucleotide sequence of the codon on tRNA molecules.
This article examines,
1. What is Codon
– Definition, Features
2. What is Anticodon
– Definition, Features
3. What is the difference between Codon and Anticodon
What is a Codon
A codon is a sequence of three nucleotides which specifies one amino acid in the polypeptide chain. Every gene that encodes a specific protein consists of a sequence of nucleotides, which represent the amino acid sequence of that particular protein. Genes utilize a universal language, the genetic code, in order to store the amino acid sequences of proteins. Genetic code consists of nucleotide triplets which are called codons. For example, the codon TCT represents the amino acid serine. Sixty one codons can be identified in order to specify the twenty essential amino acids required by the translation.
Reading Frame
A particular nucleotide sequence in a single-stranded DNA molecule consists of three reading frames in 5′ to 3′ direction of the strand. Considering the nucleotide sequence in figure 1, first reading frame starts from the first nucleotide, A. The first reading frame is shown in blue colour. It contains the codons, AGG TGA CAC CGC AAG CCT TAT ATT AGC. The second reading frame starts from the second nucleotide, G which is shown in red colour. It contains the codons GGT GAC ACC GCA AGC CTT ATA TTA. The third reading frame starts from the third nucleotide, G which is shown in green colour. It contains the codons GTG ACA CCG CAA GCC TTA TAT TAG.
Figure 1: Reading Frames
Since DNA is a double-stranded molecule, six reading frames can be found in the two strands. But, only one reading frame is potential to be translated. That reading frame is referred to as the open reading frame. A codon can only be identified with an open reading frame.
Start/Stop Codon
The open reading frame is defined basically by the presence of a start codon encoded by the mRNA. The universal start codon is AUG which codes for the amino acid, methionine in eukaryotes. In prokaryotes, AUG encodes for formylmethionine. Eukaryotic open reading frames are interrupted by the presence of introns in the middle of the frame. Translation stops at the stop codon in the open reading frame. Three universal stop codons are found on the mRNA: UAG, UGA and UAA. A codon series on an mRNA piece is shown in figure 2.
Figure 2: Codon series on mRNA
Effect of Mutations
Errors occur in the replication process which introduces changes into the nucleotide chain. These changes are called mutations. Mutations may change the amino acid sequence of the polypeptide chain. Two types of point mutations are missense mutations and nonsense mutations. Missense mutations alter the properties of the polypeptide chain by changing the amino acid residue and they may cause diseases like sickle-cell anaemia. Nonsense mutations change the nucleotide sequence of the stop codon and may cause thalassemia.
Degeneracy
The redundancy that occurs in the genetic code is referred to as the degeneracy. For example, the codons, UUU and UUC both specify the amino acid phenylalanine. The RNA codon table is shown in figure 3.
Figure 3: RNA codon tabl
Codon Usage Bias
The frequency which a particular codon occurs in a genome is referred to as the codon usage bias. For example, the frequency of the occurrence of the codon, UUU is 17.6% in the human genome.
Variations
Some variations can be found with the standard genetic code when considering the human mitochondrial genome. Some Mycolasma species also specify the codon UGA as tryptophan rather than the stop codon. Some Candida species specify the codon, UCG as serine.
What is Anticodon
The three nucleotide sequence on the tRNA, which is complementary to the codon sequence on the mRNA is referred to as the anticodon. During translation, anticodon is complementary base paired with the codon via hydrogen bonding. Therefore, each codon contains a matching anticodon on distinct tRNA molecules. The complementary base pairing of anticodon with its codon is shown in figure 4.
Figure 4: Complementary Base pairings
Wobble Base Pairing
The ability of a single anticodon to base pair with more than one codon on the mRNA is referred to as wobble base pairing. The wobble base pairing occurs due to the loss of the first nucleotide on the tRNA molecule. Inosine is present in the first nucleotide position on the tRNA anticodon. Inosine can form hydrogen bonds with different nucleotides. Due to the presence of wobble base pairing, an amino acid is specified by the third position of the codon. For example, glycine is specified by GGU, GGC, GGA and GGG.
Transfer of RNA
Sixty one distinct types of tRNA can be found in order to specify the twenty essential amino acids. Due to wobble base pairing, the number of distinct tRNA is reduced in many cells. The minimum number of distinct tRNAs required by the translation is thirty one. The structure of a tRNA molecule is shown in figure 5. The anticodon is shown in grey colour. The acceptor stem, which is shown in yellow colour contains a CCA tail at the 3′ end of the molecule. The specified amino acid is covalently bound to the CCA tails’ 3′ hydroxyl group. The amino acid-bound tRNA is called aminoacyl-tRNA.
Figure 5: Transfer of RNA
Difference Between Codon and Anticodon
Location
Codon: Codon is located on the mRNA molecule.
Anticodon: Anticodon is located in the tRNA molecule.
Complementary Nature
Codon: Codon is complementary to the nucleotide triplet in the DNA.
Anticodon: Anticodon is complementary to the codon.
Continuity
Codon: Codon is sequentially present on the mRNA.
Anticodon: Anticodon is individually present on tRNAs.
Function
Codon: Codon determines the position of the amino acid.
Anticodon: Anticodon brings the specified amino acid by the codon.
Conclusion
Codon and anticodon are both involved in the positioning of amino acids in the correct order in order to synthesise a functional protein during translation. Both of them are nucleotide triplets. Sixty one distinct codons can be found specifying the twenty essential amino acids required for the synthesis of a polypeptide chain. Thus, sixty one distinct tRNAs are required in order to complementary base pair with the sixty one codons. But, due to the presence of wobble base pairing, the number of tRNAs required is reduced to thirty one. The anticodon complementary base pairs with the codon are considered as a universal feature. Therefore, the key difference between codon and anticodon is their complementary nature.
Reference:
“Genetic code”. Wikipedia, the free encyclopedia, 2017. Accessed 03 March 2017
“Transfer RNA”. Wikipedia, the free encyclopedia, 2017. Accessed 03 March 2017
Image Courtesy:
“Reading Frame” By Hornung Ákos – Own work (CC BY-SA 3.0) via Commons Wikimedia
“RNA-codon” By The original uploader was Sverdrup at English Wikipedia – Transferred from en.wikipedia to Commons., Public Domain) via Commons Wikimedia
“06 chart pu” By NIH – (Public Domain) via Commons Wikimedia
“Ribosome” By pluma – Own work (CC BY-SA 3.0) via Commons Wikimedia
“TRNA-Phe yeast 1ehz” By Yikrazuul – Own work (CC BY-SA 3.0) via Commons Wikimedia
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