Which of the following is not true of a codon?
a. it may code for the same amino acid as another codon.
b. it never codes for more than one amino acid.
c. it extends from one end of a trna molecule.
d. it is the basic unit of the genetic code.

It extends from one end of a tRNA molecule. (Ans. C)

Explanation:

Codon or genetic code, the sequence of nucleotides in DNA and RNA which determines the amino acid sequence of the proteins. The linear sequence of nucleotides present in DNA has the information for protein sequence, proteins are not directly synthesized from DNA, an mRNA molecule is synthesized from DNA and complete the formation of proteins.

RNA is composed of 4 nucleotides adenine, cytosine, uracil and guanine. Three nucleotides constitute a unite which is called Codon, which helping to codes the amino acids. Example AUG is a codon that specifies the amino acid methionine. It is found at the starting of every mRNA and indicates the start of a protein.

There are 64 codons, 3 of them are not coding for an amino acid but indicate the end of the protein, the remaining 61 codons specify the 20 amino acids which help in makeup the proteins.  

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b. the carbon cycle ensures living things have a source of hydrogen