Genetic Code: Meaning, Codons and Characteristics

The below mentioned article provides a short note on the Genetic Code:- 1. Meaning of Genetic Code 2. Codons –Genetic Words 3. Characteristics  4. Wobble.

Meaning of Genetic Code:

The genetic code is composed of words formed by a sequence of nucleotide bases and a sequence of amino acids. Each word in the code is composed of three nucleotide bases. The genetic words are called codons.

Codons –Genetic Words:

i. The codons are usually presented in the messenger RNA.

ii. The four nucleotide bases [Adenine (A), Guanine (G), Cytosine (C), and Uracil (U)] are used to form the three-base codons. Therefore, 64 different combination of bases, taken three at a time (4³), are formed.

iii. The nucleotide sequences are always writ­ten from the 5′-end to the 3′-end.

(a) Translation of codons:

Sixty one of 64 codons code for the 20 common amino acids. For example, the codon 5′-CAU-3′ codes for histidine, whereas 5’AUG-3′ codes for methio­nine.

(b) Termination (stop or nonsense) codons:

(1) Three codons (UAG, UGA, and U AA) do not code for amino acids and hence, termed as termination codons.

(2) If one of these codons appears in an mRNA sequence, it signals the complete synthesis of the peptide chain coded for by that mRNA.

Characteristics of Genetic Code:

i. Degeneracy:

(a) Three codons (UAA, UAG, UGA) do not code for specific amino acids, these have been termed nonsense codons. At least 2 of these nonsense codons are utilized in the cell as termination signals, they specify where the polymerization of amino acids into a protein molecule is to stop. The remaining 61 codons code for 20 amino acids. So there must be degeneracy in the genetic code.

(b) Every amino acid except methionine is represented by several codons.

(c) Codons that represent the same amino acid are called synonyms.

ii. Unambiguity:

For any specific codon, only a single amino acid is indicated. So the genetic code is unambigu­ous. With few exceptions, given a specific codon, only a specific amino acid will be incorporated al­though, given a specific amino acid, more than one codon may call for it.

iii. Non-overlapping:

All codons are independent sets of 3 bases. There is no overlapping.

iv. Universality:

(a) In all the living organisms the genetic code is the same.

(b) The codon AUA is read as Met and UGA codes for Trp in mammalian mitochon­dria, codons AGA and AGG are read as stop or chain terminator codons. So mito­chondria require only 22 tRNA molecules to read their genetic code, whereas cyto­plasmic translation system passes a few complement of 31 tRNA species. These exceptions represent that the genetic code is universal.

v. Commalessness:

The genetic code is read from a fixed starting point as a continuous sequence of bases taken three at a time. If one or two nucleotides are added to or deleted from the interior of a message sequence, a frame-shift mutation occurs and the reading frame is changed.

It is to be noted that three nucleotides, if are added, a new amino acid is added to the pep­tide, or if three nucleotides are deleted, an amino acid is lost. In such cases, the reading frame is not affected.

Wobble:

The wobble hypothesis describes the mechanism by which tRNAs can recognize more than one codon for a specific amino acid. In this hypothesis, the base at the 5′-end of the anticodon is not de­fined as the other two bases. The movement of the first base allows nontraditional base pairing with the 3′-base of the codon. This movement is called wobble.

Nucleoid:

A Prokaryotic organism contains a single, double- stranded, supercoiled, circular chromosome. Each chromosome is associated with his- tone-like proteins and RNA that can condense the DNA to form a nucleoid.

Plasmids:

(i) In addition to nucleoid, most species of bacteria also contain small, circular, extra-chromosomal DNA molecules called plasmids.

(ii) Plasmid DNA carries genetic information and undergoes replication that may or may not be related to chromosomal division.

(iii) Plasmid may carry genes that cause anti­biotic resistance to the host bacterium and may facilitate the transfer of genetic in­formation from one bacterium to another.

(iv) Plasmids are used as vectors in recombinant DNA technology.

Semiconservative Replication:

Each strand of the DNA as a result of the separation of the two strands of the DNA double helix can act as a tem­plate for the replication of a new complementary strand producing two daughter molecules each of which contains two DNA strands with an antiparallel orientation. This process is called semiconservative replication.