MBG PART II: CHAPTER NOTES CHAPTER 10 Synthesis of DNA 1. Chain initiation 2. Chain elongation 3. Chain termination Watson and Crick Proposed that the two complementary strands of the double helix unwind and separate Each strand guides the synthesis of a new complementary strand The sequences of bases in each parental strand is used as a template, and the base pairing restrictions within the double helix dictate the sequence of bases in the newly synthesized strand Adenine in parent strand will serve as a template via its hydrogen-bonding potential for the incorporation of thymine in the nascent complementary strand This is called the semiconservative replication (parental molecule is half conserved) Conservative replication : parental double helix would be conserved, and a new progeny double helix would be synthesized o Would not produce any DNA molecules with hybrid density o After one generation of conservative replication of heavy DNA in light medium, half of the DNA still would be heavy and the other half would be light Dispersive replication: segments of both strands of the parental DNA molecule would be conserved and used as templates for the synthesis of complementary segments that would subsequently be joined to produce progeny DNA strands o Shift of DNA from heavy toward light in each generation Half heavy after one generation, quarter heavy after two generations Origins of Replication (oriC) One unique origin per chromosome This origin controls replication of a unit of DNA called a replicon o Most prokaryotic chromosomes contain a single replicon o Eukaryotic chromosomes contain many replicons One 13-bp sequence is present as three tandem repeats These three repeats are rich in A:T base pairs, facilitating the formation of a localized region of strand separation referred to as the replication bubble A:T base pairs are held together by only two hydrogen bonds as opposed to three in G:C base pairs
Two strands of AT-rich regions of DNA come apart more easily with the input of less energy The formation of a localized zone of denaturation is an essential first step in the replication of all double-stranded DNA Another conserved component of oriC is a 9-bp sequence that is repeated four times and is interspersed with other sequences These four sequences are binding sites for a protein that plays a key role in the formation of the replication bubble The multiple origins of replication in eukaryotic chromosomes also appear to be specific DNA sequences Bidirectional Replication Replication of the E.coli chromosome occurs bidirectionally from the unique origin of replication Each Y-shaped structure is a replication fork, and the two replication forks move in opposite directions sequentially around the circular chromosome The cohesive ends of a lambda chromosome can thus base-pair to form a hydrogen-bonded circular structure After lambda chromosome is injected into a host cell, its converted to a
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- DNA, RNA molecules