Codons and genetic codes

A codon is a triplet sequence of bases. The tRNA molecule contains an anticodon.

The Shine-Dalgarno sequence is found in the 23S prokaryotic ribosomal RNA. A codon is on the same strand of DNA or mRNA.

Three of the 64 possible codons do not code for specific amino acids; these have been termed nonsense codons. 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 the 20 naturally occurring amino acids. Thus, there is “degeneracy” in the genetic code—that is, multiple codons decode the same amino acid.

B i.e. Hydroxyproline

Terminator Codons
There are three codons that do not code for any particular amino acids. They are “nonsense codons”, more correctly termed as punctuation codons or terminator codons. They put “full stop” to the protein synthesis. These three codons are UAA, UAG, and UGA. UGA is a stop codon; but in special circumstances, it stands for seleno-cysteine

C i.e. 256

A i.e. UAG

C i.e. 3 consecutive nucleotide units

D i.e. Termination of protein synthesis

A i.e. Degeneracy

C i.e. t – RNA

 A i.e. t – RNA

B i.e Stop Codon

C i.e. Missense mutation

Ans. is ‘b’ i.e., Unambiguous

Characteristics of genetic codes

Genetic codes have following characteristics ‑

1) Universal :- Each codon specifically codes for same amino acid in all species, e.g. UCA codes for serine and CCA codes for proline in all organisms. That means specificity of codon has been conserved from very early stages of evolution. Exception to the universality of genetic coder are found in human mitochondria, where the code :-

 – α UGA codes for tryptophan instead of serving as a stop codon.

 – AUA codes for methionine instead of isoleucine.

 – CUA codes for threonine instead of leucine.

 – LI AGA and AGG serve as stop codon instead of coding for arginine.

2) Unambiguous/Specific :- A particular codon always codes for the same amino acid. For example CCU always codes for proline and UGG always codes for tryptophan.

3)  Degeneracy/Redundancy :- A given amino acid may have more than one codon. For example, CCU, CCC, CCA and CCG all four codons code for proline. Therefore, there are 61 codons for 20 amino acids.

4) Stop or termination or nonsense codons:- Three of the 64 possible nucleotide triplets UAA (amber), UAG (Ochre) and UGA (opal) do not code for any amino acid. They are called nonsense codons that normally signal termination of polypeptide chains. Thus, though there are 64 possible triplet codons, only 61 codes for 20 amino acids (as remaining three are non-sense codons).

5) Non overlapping and nonpuntate (Comma less) :- During translation, the code is read sequentially, without spacer bases, from a fixed starting point, as a continuous sequence of bases, taken 3 at a time, e.g. AUGCUA GACUUU is read as AUG/CUA/GAC/UUU without “ponctation” (coma) between codons.

Ans. is ‘c’ i.e., m RNA

The m RNA carries genetic information in the form of codons.

• Codons are a group of three adjacent nucleotides that code for the amino acids of protein.
• Each mRNA molecule is a transcript of antisense or template strand of a particular gene.
• Its nucleotide sequence is complementary to that of antisense or template strand of the gene, i.e. adenine for thyamine, guanine for cytosine, uracil for adenine (as RNA does not contain thymine) and cytosine for guanine.
• For example, if antisense strand of DNA has a gene with sequence 5′-TTACGTAC-3′, its complementary RNA transcript will be 5 ‘-GUACGUAA-3’.

Ans. is ‘d’ i.e., 64 

• The information needed to direct the synthesis of protein is contained in the mRNA in the form of a genetic code, which inturn is transcribed from template strand of DNA and is therefore complementary to it.
• The genetic code is the system of nucleotide sequences of mRNA that determines the sequence of amino acids in protein.
• Codon is a sequence of three adjacent bases that corresponds to one amino acid.
• There are 64 possible codom sequences.
• Because four nucleotide bases A,G, C and U are used to produce the three base codons, there are therefore 64(43) possible codon sequences.

Ans. is ‘c’ i.e., UGA 

Stop codons or nonsense codons or termination codons

• Three of the 64 possible nucleotide triplets UAA (amber), UAG (Ochre) and UGA (opal) do not code for any amino acid.
• They are called nonsense codons that normally signal termination of polypeptide chains.
• Thus, though there are 64 possible triplet codons, only 61 codes for 20 amino acids (as remaining three are non­sense codons).