Genetic code is a set of nucleotides that code for specific amino acids during protein synthesis. These are in the form of triplets that are known as codons.

mRNA is a type of RNA that acts as a messenger of DNA to send information or message for the formation of proteins. mRNA is synthesized from the DNA by a process known as transcription. This takes place in the nucleus. The mRNA then moves to the cytoplasm towards the ribosomes and synthesize proteins by the process known as translation.

The central dogma states that genes in the DNA specify the sequence of mRNA molecules, which in turn specify the sequence of proteins. Because the information stored in DNA is so central to cellular function, the cell keeps the DNA protected and copies it in the form of RNA. This is known as transcription. An enzyme adds one nucleotide to the mRNA strand for every nucleotide it reads in the DNA strand. This is known as translation. The translation of this information to a protein is more complex because three mRNA nucleotides (one codon) correspond to one amino acid in the polypeptide sequence. Each codon instructs the addition of a specific amino acid to a polypeptide chain. A total of 64 codons are present. Three of them are stop codons and don’t encode for any amino acid. Sixty-one of the codons encode twenty different amino acids.

Each codon is formed of three nucleotides. A total of 64 codons are present in our body out of which three codons are known as stop codons as they stop the process of translation. They do not encode for any amino acid. Rest 61 codons encode for 20 amino acids. Out of these 61 codons, one codon acts as an initiation codon as well. This means that some amino acids are formed from multiple codons.  These codons are similar in all organisms and thus are considered as universal. One codon initiates or starts the process of translation i.e. synthesis of proteins (amino acids). This is known as the start codon and is AUG. This codon also codes for an amino acid methionine. Three codons are known as stop codons as they terminate or stop the process of translation. They are UAA, UAG, and UGA. These do not code for any amino acid.

Anticodons are the triplets of nucleotides that are complementary to these codons. The anticodons are present on the tRNA. When tRNA binds to the mRNA, this occurs due to these complementary triplets i.e. due to codon-anticodon relationship. This ensures that correct amino acid is transferred from the tRNA to the mRNA and protein synthesis occurs with the help of desired amino acids.

There are 64 codons that code for 20 amino acids. Some codons code for the same amino acid. This is why these codons are said to degenerate as multiple codons code for a single amino acid. For example, codons UCU, UCC, UCA, and UCG code for a single amino acid serine. Thus, serine is not coded by a single codon. During the pairing of the anticodon of tRNA with the codons of mRNA, the pairing is loose at the third amino acid. This results in the production of the same amino acids from some genetic codons that differ only in the last nucleotide. For example, codons UCU, UCC, UCA, and UCG code for a single amino acid serine. This is known as wobble pairing.

Due to mutation, missense and nonsense codons may be formed. Missense codons are the codons that code for an incorrect amino acid while nonsense codon does not code for any amino acid and act as a stop codons.

 

Practice Questions

Khan Academy

Early experiments on the genetic code

Genetics of Myotonic dystrophy

Molecular genetics and microbiology of Zaire Ebolavirus

 

MCAT Official Prep (AAMC)

Key Points

• DNA encodes RNA and then RNA encodes protein.
• Transcription is the process of converting a specific sequence of DNA into RNA. mRNA is formed from one strand of DNA by transcription.
• The translation is the process where a ribosome decodes mRNA into a protein.
• Out of 64 codons, three are stop codons. Rest 61 codons encode twenty different amino acids.
• The genetic code is universal because it is the same among all organisms.
• Three codons UAA, UAG, and UGA are known as stop codons as they stop the process of amino acid formation (translation).
• AUG is also known as a start codon as it initiates the process of translation. It also encodes for methionine amino acid.
• Codons and anticodons are complementary to each other
• Codons on the mRNA bind to the anticodons on the tRNA during protein synthesis.
• The genetic codes are degenerate because an amino acid is not always coded from a specific codon. Some amino acids can be coded from multiple codons.
• Missense codons code for incorrect amino acids or act as stop codons.
• Nonsense amino acids act as stop codons and terminate the translation process prematurely.
• The wobble base pairing is due to the loose pairing between the third nucleotide of the codon of mRNA and the anticodon of tRNA during translation.
• Wobble base pairing allows coding of the same amino acid from codons that differ only in the third nucleotide.

Key Terms

 

• codon: a sequence of three adjacent nucleotides, which encode for a specific amino acid during protein synthesis or translation
• proteins: Molecules formed by joining of multiple amino acids (polypeptides).
• translation: A process occurring in the ribosome, in which a strand of messenger RNA (mRNA) guides the assembly of a sequence of amino acids to make a protein.
• amino acid: Any organic compound containing both an amino and a carboxylic acid functional group.
• tRNA: Transfer RNA that transfers amino acids to mRNA
• anticodon: The triplet of nucleotides in tRNA that transfer amino acid to mRNA during translation.