Question

All organisms contain DNA, and every organism's DNA is made of the same four nucleotides. The differences between organisms is simply based on the order of these nucleotides. Since all organisms have the same basic, universal structure for DNA, which of these must also be universal?
A)All organisms have the same proteins.
B)All living things must have the same amount of DNA.
C)All organisms must be genetically identical to each other.
D)All codons in all organisms' DNA code for the same amino acids.

Answer 1

A) All organisms have the same proteins. 

Proteins are composed of amino acids.
The four bases in DNA are adenine, cytosine, guanine, and thymine.

These four are divided into two groups. The pyrimidines, the single-ring nitrogenous bases, is where thymine and cytosine is included. The purines, the double-ring nitrogenous bases, includes adenine and guanine. 

Cytosine will only bond with guanine; they form three hydrogen bonds. Adenine will only bond with thymine forming two hydrogen bonds.

Answer 2

Personally I think it is D) because every organisms follow the so called "recipe book" of making protein, where specific triplet of bases codes for specific amino acid. For example, Codon of GGG codes for proline(pro). This applies to all organism be it human, dog, blue whale and etc.