DNA replication helicases

Heat of combustion, 113 Heat of hydrogenation, 186 table of, 187 Heat of reaction, 154 Helicase, DNA replication and, 1106 Hell-Volhard-Zelinskii reaction, 849 amino acid synthesis and. 1025 mechanism of, 849 Heme, biosynthesis of, 966 structure of, 946 Hemiacetal, 717 Hemiketal, 717 Hemithioacetal, 1148 Henderson-Hasselbalch equation,... 

Primosome The mobile complex of helicase and primase that is involved in DNA replication. 

Figure 20.14 Preparation for DNA replication. At Least three proteins are involved DNA helicases disrupt hydrogen bonds between bases to allow the two strands to unwind single-strand DNA-binding proteins stabilise the unbound strands in preparation for base pairings DNA topoisomerase relaxes strain in the strands to facilitate polymerisation.

The process of eukaryotic DNA replication closely follows that of prokaryotic DNA synthesis. Some differences, such as the multiple origins of replication in eukaryotic cells versus single origins of replication in prokaryotes, have already been discussed. Eukaryotic single-stranded DNA-binding proteins and ATP-dependent DNA helicases have been identified, whose functions are analogous to those of the prokaryotic enzymes previously discussed. In contrast, RNA primers are removed by RNase H. 

Correct answer = C. Fluoroquinolones, such as ciprofloxacin, inhibit bacterial DNA gyrase—a type II DNA topoisomerase. This enzyme catalyzes the transient breaking and rejoining of the phosphodiester bonds of the DNA backbone, to allow the removal of positive supercoils during DNA replication. The other enzyme activities mentioned are not affected. Primase synthesizes RNA primers, helicase breaks hydrogen bonds in front of the replication fork, DNA polymerase I removes RNA primers, and DNA igase joins Okazaki fragments. 

Cells of patients with Bloom syndrome (BS) have many chromosome breaks and a high frequency of sister chromatid exchanges, perhaps in an effort to correct these breaks. The body is small but well-proportioned.kk A somewhat similar disease, the Werner syndrome (WS), is associated with premature aging.11 The Bloom s protein BLM and the WS gene product WRN are both helicases related to E.coli RecQ. Protein BLM colocalizes with replication protein A as discrete foci in the meiotic synaptonemal complex.1 3 Protein WRN also seems to be associated with DNA replication. Defects... 

The DNA replication process begins at origin points in the parent DNA strand which are targeted by DNA binding proteins resulting in the separation of the strands and the initiation of the replication fork complex via the helicase enzyme. The origin points tend to be A-T rich regions because of... 

DNA helicases are a class of enzymes necessary for fundamental DNA transactions such as DNA replication, transcription, repair, and recombination. Moreover, among the components of the DNA replication, repair, recombination or transcription apparatus, the first that may encounter a site of DNA damage are the DNA helicases. Thus, a complete understanding of the effect of cisplatin lesions on DNA metabolism requires a biochemical analysis of their interaction with this class of proteins. At least three reports have investigated the effects of cisplatin intrastrand lesions on the activity of DNA helicases implicated either in repair or in recombination. 

In addition, three enzymes involved in DNA replication, including DNA primases, prokaryotic DNA topoisomerase I and some hexameric DNA helicases, are also classic zinc-ribbon proteins. In bacteriophage DNA primases, mutations of the zinc-binding residues abrogate the synthesis of RNA primers for lagging strand DNA synthesis. Strikingly, each subunit of the mini-chromosomal maintenance (MCM) protein, a heterohexameric helicase that initiates DNA replication in S. cerevisiae, contains an independently folded zinc-ribbon domain that appears to stabilize the dodecameric structure (a dimer of hexamers) of this replication complex. ... 

Many DNA polymerases proofread the nascent product their 3 —> 5 exonuclease activity potentially edits the outcome of each polymerization step. A mispaired nucleotide is excised before the next step proceeds. In E. coli, DNA polymerase I repairs DNA and participates in replication. Fidelity is further enhanced by an induced fit that results in a catalytically active conformation only when the complex of enzyme, DNA, and correct dNTP is formed. Helicases prepare the way for DNA replication by using ATP hydrolysis to separate the strands of the double helix. 

DNA replication in E. coli starts at a unique origin (oriC) and proceeds sequentially in opposite directions. More than 20 proteins are required for replication. An ATP-driven helicase unwinds the oriC region to create a replication fork. At this fork, both strands of parental DNA serve as templates for the synthesis of new DNA. A short stretch of RNA formed by primase, an RNA polymerase, primes DNA synthesis. One strand of DNA (the leading strand) is synthesized continuously, whereas the other strand (the lagging strand) is synthesized discontinuously, in the form of 1-kb fragments (Okazaki fragments). Both new strands are formed simultaneously by the concerted actions of the highly processive... 

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