Saturday , 19 April 2014

Rajasthan University 2014 LDC Vacancy Notification Date

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A single-stranded RNA molecule may fold back on itself to
form a complex structure. (A) The nucleotide sequence showing Watson-Crick base pairs and other nonstandard base
pairings in stem-loop structures. (B) The three-dimensional structure and one important long-range interaction between
three bases. Hydrogen bonds within the Watson-Crick base pair are shown as dashed black lines; additional hydrogen
bonds are shown as dashed green lines
I. The Molecular Design of Life 5. DNA, RNA, and the Flow of Genetic Information
5.3. DNA Is Replicated by Polymerases that Take Instructions from Templates
We now turn to the molecular mechanism of DNA replication. The full replication machinery in cells comprises more
than 20 proteins engaged in intricate and coordinated interplay. In 1958, Arthur Kornberg and his colleagues isolated the
first known of the enzymes, called DNA polymerases, that promote the formation of the bonds joining units of the DNA
backbone.
5.3.1. DNA Polymerase Catalyzes Phosphodiester-Bond Formation
DNA polymerases catalyze the step-by-step addition of deoxyribonucleotide units to a DNA chain (Figure 5.21).
Importantly, the new DNA chain is assembled directly on a preexisting DNA template. The reaction catalyzed, in its
simplest form, is:

Rajasthan University 2014 LDC Vacancy Notification Date

where dNTP stands for any deoxyribonucleotide and PPi is a pyrophosphate molecule. The template can be a single
strand of DNA or a double strand with one of the chains broken at one or more sites. If single stranded, the template
DNA must be bound to a primer strand having a free 3 -hydroxyl group. The reaction also requires all four activated
precursors that is, the deoxynucleoside 5 -triphosphates dATP, dGTP, dTTP, and dCTP as well as Mg2+ ion.
The chain-elongation reaction catalyzed by DNA polymerases is a nucleophilic attack by the 3 -hydroxyl group of the
primer on the innermost phosphorus atom of the deoxynucleoside triphosphate (Figure 5.22). A phosphodiester bridge
forms with the concomitant release of pyrophosphate. The subsequent hydrolysis of pyrophosphate by pyrophosphatase,
a ubiquitous enzyme, helps drive the polymerization forward. Elongation of the DNA chain proceeds in the 5 -to-3
direction.
DNA polymerases catalyze the formation of a phosphodiester bond efficiently only if the base on the incoming


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