Replication Exhibits Polarity

As has already been noted, DNA molecules are double-stranded and the two strands are antiparallel, ie, running in opposite directions. The replication of DNA in prokaryotes and eukaryotes occurs on both strands simultaneously. However, an enzyme capable of polymerizing DNA in the 3' to 5' direction does not exist in any organism, so that both of the newly replicated DNA strands cannot grow in the same direction simultaneously. Nevertheless, the same enzyme does replicate both strands at the same time. The single enzyme replicates one strand ("leading strand") in a continuous manner in the 5' to 3' direction, with the same overall forward direction. It replicates the other strand ("lagging strand") discontinuously while polymerizing the

Figure 36-15. The RNA-primed synthesis of DNA demonstrating the template function of the complementary strand of parental DNA.

DNA template

RNA primer

Newly synthesized DNA strand

Okazaki fragments

100 bp

10 bp

Figure 36-16. The discontinuous polymerization of deoxyribonucleotides on the lagging strand; formation of Okazaki fragments during lagging strand DNA synthesis is illustrated. Okazaki fragments are 100-250 nt long in eukaryotes, 1000-2000 bp in prokaryotes.

nucleotides in short spurts of 150-250 nucleotides, again in the 5' to 3' direction, but at the same time it faces toward the back end of the preceding RNA primer rather than toward the unreplicated portion. This process of semidiscontinuous DNA synthesis is shown diagrammatically in Figures 36-13 and 36-16.

In the mammalian nuclear genome, most of the RNA primers are eventually removed as part of the replication process, whereas after replication of the mi-tochondrial genome the small piece of RNA remains as an integral part of the closed circular DNA structure.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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