..il T-Mobile 1:32 PM Back 1. The origin of E. coli, oriC, is 245 base pairs and
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..il T-Mobile 1:32 PM Back 1. The origin of E. coli, oriC, is 245 base pairs and contains two elements that play a significant role in the initiation of replication a) methylated GATC/CTAG repeats and b) 9 and 13 mer base repeats. Select one of these elements and explain their role/significance in the initiation of replication. (4pts) 2. Since SSB does not have an enzymatic function, why is it essential for replication? (3 pts) 3. How does repair DNA synthesis differ from replication DNA synthesis? (3 pts) 4. Why would a DNA polymerase enzyme need to have nuclease activity? (3 pts) Figure 13.14 Refer to figure 13.14 when answering questions 5 and 6 5. The figure shows two core enzymes attached by two tau subunits. Why are two core enzymes required? Why can't replication take place with just one core enzyme unit? (4 pts)Explanation / Answer
9 and 13mer base repeats:
In E.coli, single origin of the replication presen in the chromosome is referred to as oriC. It contains two short repeat motifs, one of 9 nucleotides and the other of 13 nucleotides. The nine-nucleotide repeat, five copies of which are dispersed throughtout oriC, is the biniding site for a protein called DnaA. The result of DnaA binding is that the double helix opens up (melts) within the tandem array of three AT-rich, 13-nucleotide repeats located at one end of the oriC sequence. The 13mer region is then denatured, and this open complex serves as a replication start site.
The single-stranded DNA-binding protein (SSB) of Escherichia coli lack enzymatic activity and contribute to the ‘three Rs’ of DNA metabolism (Replication, Recombination and Repair) mainly through their shared ability to bind with high affinity to single-stranded DNA (ssDNA) and with low affinity to double-stranded DNA (dsDNA). During replication, the separated strands of DNA are inhibited from subsequently reannealing by the SSB proteins, which binds to both separated strands.
DNA repair synthesis aims to maintain the integrity of the genomes of the cells. The repair system recognizes mismatched bases that are incorporated during DNA replication and removes them and replaces them with the proper bases. This repair synthesis is carried out by DNA polymerases that synthesize new bases in the place of the removed mismatched bases. Whereas in DNA replication synthesis, a whole new strand of DNA is synthesized by the replication mechanism.
The nuclease activity of the enzyme DNA polymerase helps in the proof reading mechanism. The function of DNA polymerase is not quite perfect, with the enzyme making about one mistake for every billion base pairs copied. During the proof reading process, the nuclease activity allows the incorrect base pair to be excised and then the correct base pair can be re-inserted and the replication proceedes.
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