Unformatted text preview: Chapter 6 109 6-10. a. The A—T base pairs have only two hydrogen bonds, so it takes less heat energy to denature
these base pairs. G-C base pairs have three hydrogen bonds holding them together. It thus takes
more energy to break the bonds between Cs and Gs. Remember that the DNA of different species
can vary a great deal in the proportions of A-T base pairs relative to C-G base pairs. Moreover,
this ratio can be very different in different regions of the same chromosome. In most organisms,
the regions between genes have a higher proportion of A-T base pairs (they are "A-T rich") than
the genes themselves. In the early stages of research on genomes, scientists sometimes tried to
locate genes by looking for regions of DNA that were more resistant to heat denaturation, and
thus had a higher G-C content. b. The denatured single-stranded DNA must contain stretches of nucleotides that are
complementary to a nearby sequence but in an inverted orientation. These "stem—and-loop"
structures are regions where the single strand of DNA formed a double-stranded region. The loops and the strings holding the stems together are still single-stranded DNA. 6—11. 5'...CAGAATGGTGCTCTGCTAT...3'. This could also be written backward as
3'. . . ..TATCGTCTCGTGGTAAGAC ..... 5'. It is best to indicate polarity by showing the 5' and 3'
directions on a strand of DNA or RNA. If no polarity is indicated, the 5' direction is at the left end of the sequence. The dots show that this is a short region of a much longer nucleotide chain. Section 6.3—DNA Stores Information
6—12. A note about nomenclature: depending on the context, the letters A, C, G, and T can be used to
represent either the nitrogenous bases or the nucleosides or the nucleotides containing those bases.
Although it is not of great importance for most genetics courses, you might ﬁnd it useful to know that
organic chemists distinguish between bases, nucleosides, and nucleotides with a complicated
nomenclature. The name of the base is altered slightly to indicate a nucleoside (for example adenine
becomes adenosine) and the name "deoxy," or more precisely "2-deoxy," is added to the nucleoside
name if the sugar is deoxyribose. To name a nucleotide, the number of phosphate groups it contains
and the position of their connection to the sugar are also speciﬁed. Thus, 2'-deoxythymidine 5'-
monophosphate signiﬁes a nucleotide containing deoxyribose, the base thymine, and a single
phosphate group connected to the 5'—carb0n atom of deoxyribose. See if you can ﬁnd this nucleotide
in Feature Fig. 6.7.
a. Each of the four building blocks is a nucleotide. Each nucleotide is made of the sugar
deoxyribose and one of the four nitrogenous bases (A, C, G, or T) and one or more of the phosphate groups (see Feature Fig. 6.7). Deoxyribose plus a base makes a nucleoside. ...
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- Spring '08