Leading and lagging

Equipment operating schedules Confirm that the timing of occupied and unoccupied cycles is compatible with actual occupied periods, and that the building is flushed by the ventilation system before occupants arrive. ASHRAE 62-1989 provides guidance on lead and lag times for HVAC equipment. In hot, humid climates, ventilation may be needed during long unoccupied periods to prevent mold grow th. 

Online boilers may be idle or may run on low- or high-fire and may operate under a variety of conditions, such as steady-state, peak-load, or lead and lag arrangements. 

Lack of BD control High treatment/small boiler Poor design, lead and lag boiler operation scale and ferrous metasilicate FeSi03 Water losses, low inhibitor levels Surging/carryover from high TDS. Excessive TDS in lead boiler. Lack of treatment in lag boiler. 

We have to wait until the controller chapters to see that this function is the basis of a derivative controller and not till the frequency response chapter to appreciate the terms lead and lag. For now, we take a quick look at its time response. 

Figure 11-2. The prokaryotic DNA replication fork. A schematic representation of semi-conservative replication of DNA by different mechanisms on the leading and lagging strands by DNA polymerase III (DNA pol III) is shown. Other enzymes and accessory proteins that participate in initiation, elongation, and ligation phases of the process are indicated, with DNA pol I depicted as having just dissociated from a completed Okasaki fragment. SSBs, single-stranded DNA binding proteins.

SSB. From this point, synthesis of leading and lagging strands is sharply different. 

The replisome promotes rapid DNA synthesis, adding -1,000 nucleotides/s to each strand (leading and lagging). Once an Okazaki fragment has been completed, its RNA primer is removed and replaced with DNA by DNA polymerase I, and the remaining nick is sealed by DNA ligase (Fig. 25-15). 

Leading and Lagging Strands Prepare a table that lists the names and compares the functions of the precursors, enzymes, and other proteins needed to make the leading versus lagging strands during DNA replication in E. coli. 

Figure 27-20 (A) Hypothetical replisome for concurrent replication of leading and lagging strands by a dimeric polymerase associated with helicase dnaB and a primosome. Open arrows indicate directions of movement of DNA, which is forming a loop as the polymerase fills a gap to complete an Okazaki fragment. The primase will then form a new primer and a new loop. From Komberg and Baker.265 (B) Electron micrograph of the primosome bound to covalently closed ( )X174 duplex replicative form. These enzymatically synthesized duplexes invariably contain a single primosome with one or two associated small DNA loops. From A. Komberg in Hubscher and Spadari,266 pp. 9,10.

Compare systhesis of the leading and lagging strands in the elongation phase of DNA replication. Explain why DNA polymerases may have difficulty in replicating the 3 -end of the lagging strand of linear DNA. How has this problem been solved in many bacterial and viral systems In eukaryotic cells ... 

FIGURE 6.12 Static VAR compensator draws optimum amount of leading and lagging currents to maintain required voltage and power factor levels. 

Figure 1.31 A schematic representation of the replication of DNA. The arrows show the directions of the growth of the leading and lagging strands. Hydrogen bonding can only occur between either A and T or C and G. This means that the daughter strand is, in theory, an exact replica of the parent strand. Reproduced by permission of Prentice Hall from Chemistry for Pharmacists and the Life Sciences by G Thomas...

DNA replication requires an RNA primer that is synthesized by an RNA polymerase called primase. This is extended by DNA polymerase III, which makes the DNA for both the leading and lagging strands. DNA polymerase degrades the primer and replaces it with DNA. DNA ligase then joins DNA ends. 

A multienzyme complex simultaneously carries out both leading and lagging strand replication. You can see the best model of the process in the next figure the lagging strand may curl around so it presents the correct face to the enzyme. The two replication forks proceed around the chromosome, until they meet at the terminus. Termination is poorly defined biochemically, but it is known to require some form of DNA gyrase activity. See Figure 8-12. 

Know the roles of DNA polymerase I, II, III, and eukaryotic DNA polymerases, the roles of primers, helicases, single-stranded binding proteins (SSB), topoisomerases (gyrase), ligases, primase, and RNA polymerases, and the differences between the leading and lagging strands of DNA. 

Figure 11.5 Simultaneous synthesis of leading and lagging templates. P = RNA primer SSB = single-stranded binding protein.

High pressure drop causes disruptions to the system hydraulics. Because of the high pressure drop, the lead membranes tend to operate at very high fluxes while the lag membranes operate at low flux. This increases the rate of membrane fouling for both the lead and lag membranes. Lead membranes foul faster because more water is forced to the membrane module faster and the rate of contaminant accumulation in the boundary layer on the membrane surface increases. The lag membranes, on the other hand, experience low flows since most of the water is removed through... 

The structure of replication forks in eukaryotes is essentially the same as in bacteria. Chain growth is continuous on the leading strand and discontinuous on the lagging strand. There are equivalents of the polymerases, helicase, primase, SSB, etc., but there are clearly some differences. For example, two different polymerases, DNA polymerase 8 and DNA polymerase a, function on the leading and lagging strand, respectively. Also, the mitochondrion has its own DNA polymerase. 

What is meant by the terms leading and lagging strands in DNA replication ... 

---