Lecture 6 - 4.2 DNA Structure and Function U s i n g m o d e l s(F i g u r e 4 8 W a t s o n a n d C r ic k c o n c lu d e d th a t tw o a n tip a

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4.2 DNA Structure and Function • Using models ( Figure 4.8 ), Watson and Crick concluded that two antiparallel DNA strands form a double helix with the hydrophilic sugar-phosphate backbone facing the exterior and purine-pyrimidine pairs of nitrogenous bases on the interior, packed closely together. • The two DNA strands form complementary base pairs A-T and G-C through hydrogen bonding ( Figure 4.9 ). The G-C pair has three hydrogen bonds, whereas the weaker A-T pair has only two. • The double-helix structure explains Franklin and Wilkins’ molecular measurements ( Figure 4.11 ) and Chargaff’s rules that the number of purines equals the number of pyrimidines and that T=A and G=C.
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DNA Is an Information- Containing Molecule The nitrogenous base sequence of DNA stores information required for all living cells, and complementary base pairing provides a simple mechanism for DNA replication, since each strand can serve as a template for formation of a new complementary strand ( Figure 4.12 ). Is DNA a Catalytic Molecule? • DNA’s stability makes it a reliable store for genetic information; it is less reactive than RNA but more resistant to degradation. Stable molecules such as DNA make poor catalysts. • Because DNA does not appear to be able to catalyze any chemical reaction, biologists think that the first life-form was made of RNA, not DNA. 4.3 RNA Structure and Function • The primary structure of RNA is similar to that of DNA, with two differences: (1) RNA contains uracil instead of thymine, and (2) RNA contains ribose instead of deoxyribose. The presence of the –OH group on ribose makes RNA much more reactive and less stable than DNA. • RNA’s secondary structure, like DNA’s, results from complementary A-U and G-C base pairing. • The bases of RNA typically form hydrogen bonds with complementary bases on the same strand. The RNA strand folds over, forming a hairpin structure where the bases on one side of the fold align with an antiparallel RNA segment on the other side of the fold ( Figure 4.13 ).
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• Sometimes hairpins and other RNA secondary structures have additional folds or are attached to other RNA strands, giving some RNA molecules
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This note was uploaded on 12/06/2009 for the course BIO 225 taught by Professor Pavgi during the Spring '08 term at University of Michigan.

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Lecture 6 - 4.2 DNA Structure and Function U s i n g m o d e l s(F i g u r e 4 8 W a t s o n a n d C r ic k c o n c lu d e d th a t tw o a n tip a

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