Question

Three species of lizard, labeled A, B, and C, are being studied using molecular analysis by two research colleagues in different laboratories. The mitochondrial DNA of lizards A and B are compared and are found to differ by 0.1%. The ribosomal RNA of lizards B and C are found to differ by 0.02%. They compare their findings and conclude that lizard B is more closely related to C than to A. What is the flaw in their reasoning, and how can their analysis be improved?

Answer 1

OK, the discrepancy is that the *mitochondrial DNA* of A and B differ by 0.1% while the *ribosomal RNA* differ by 0.02%. That is, the difference between the ribosomal RNA of B and C is much less than the difference in mitochondrial DNA between A and B.

Mitochondrial DNA is pretty much a left over from the days when mitochondrial precursors were free living bacteria - it encodes a handful of protein products but not many. How essential do you think it is to the cell? How tolerant is the cell of mutations in it?

Ribosomal RNA makes up the RNA subunits of the ribosome. Every organism out there needs ribosomes because every organism out there (except viruses, which are debatably not living organisms) needs to translate mRNA into protein. So it is very essential to the cell? How tolerant do you think the cell is of mutations which affect the sequence and structure of the ribosomal subunits?

Those are very broad hints about the rates of mutation in the two different types of genetic material...