from which


has a


The melting temperature (7^) of DNA depends on its base sequence. The three hydrogen bonds of a G - C base pair require more energy to break than do the two hydrogen bonds of an A - T pair; so a molecule with a higher percentage of G - C pairs will have a higher Tm. Species I has the highest G - C content of the three species; so it should exhibit the highest 7m.

4. Certain repeated sequences in eukaryotes are flanked by short direct repeats, suggesting that they originated as transposable elements. These same sequences lack introns and possess a string of thymine nucleotides at their 3' ends. Have these elements transposed through DNA or RNA sequences? Explain your reasoning.

The absence of introns and the string of thymine nucleotides (which would be complementary to adenine nucleotides in RNA) at the 3' end are characteristics of processed RNA. These similarities to RNA suggest that the element was originally transcribed into mRNA, processed to remove the introns and to add a poly (A) tail, and then reverse transcribed into a complementary DNA that was inserted into the chromosome.

5. Which of the following pairs of sequences might be found at the ends of an insertion sequence?

The correct answer is d and f. The ends of insertion sequences always have inverted repeats, which are sequences on the same strand that are inverted and complementary. The sequences in part a are direct repeats, which are generated on the outside of an insertion sequence but are not part of the transposable element itself. The sequences in part b are inverted but not complementary. The sequences in part c are complementary but not inverted. The sequences in part d are both inverted and complementary. The sequences in part e are complementary but not inverted. Interestingly, the sequences in part f are both inverted complements and direct repeats.

The New Genetics

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