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Ch. 11 - DNA Replication and Recombination
All textbooksKlug 12th EditionCh. 11 - DNA Replication and RecombinationProblem 25
Chapter 11, Problem 25

The genome of D. melanogaster consists of approximately 1.7x10⁸ base pairs. DNA synthesis occurs at a rate of 30 base pairs per second. In the early embryo, the entire genome is replicated in five minutes. How many bidirectional origins of synthesis are required to accomplish this feat?

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1
Calculate the total number of base pairs in the D. melanogaster genome, which is given as 1.7 \times 10^8 base pairs.
Determine the total time available for replication, which is 5 minutes. Convert this time into seconds: 5 \text{ minutes} \times 60 \text{ seconds/minute}.
Calculate the total number of base pairs that can be synthesized by a single origin in the given time. Use the rate of DNA synthesis: 30 \text{ base pairs/second} \times \text{total seconds}.
Since DNA replication is bidirectional, each origin synthesizes DNA in two directions. Therefore, divide the total number of base pairs synthesized by a single origin by 2 to find the effective synthesis per origin.
Divide the total number of base pairs in the genome by the effective synthesis per origin to find the number of bidirectional origins required.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Genome Size

The genome size refers to the total amount of genetic material contained within an organism's DNA, measured in base pairs. In the case of D. melanogaster, the genome consists of approximately 1.7x10⁸ base pairs. Understanding genome size is crucial for calculating the time and resources needed for DNA replication.
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DNA Replication Rate

DNA replication rate is the speed at which DNA polymerases synthesize new DNA strands, typically measured in base pairs per second. For D. melanogaster, this rate is approximately 30 base pairs per second. This concept is essential for determining how long it will take to replicate the entire genome and how many replication origins are needed.
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Bidirectional Origins of Synthesis

Bidirectional origins of synthesis are specific sites on the DNA where replication begins, allowing the process to occur in two directions simultaneously. This mechanism significantly increases the efficiency of DNA replication. To replicate the entire genome within a limited time, calculating the number of these origins is necessary based on the total genome size and replication rate.
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Related Practice
Textbook Question
Several temperature-sensitive mutant strains of E. coli display the following characteristics. Predict what enzyme or function is being affected by each mutation. Supercoiled strands remain after replication, which is never completed.
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While many commonly used antibiotics interfere with protein synthesis or cell wall formation, clorobiocin, one of several antibiotics in the aminocoumarin class, inhibits the activity of bacterial DNA gyrase. Similar drugs have been tested as treatments for human cancer. How might such drugs be effective against bacteria as well as cancer?
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Textbook Question
In 1994, telomerase activity was discovered in human cancer cell lines. Although telomerase is not active in most human adult cells, all cells do contain the genes for telomerase proteins and telomerase RNA. Since inappropriate activation of telomerase may contribute to cancer, why do you think the genes coding for this enzyme have been maintained in the human genome throughout evolution? Are there any types of human body cells where telomerase activation would be advantageous or even necessary? Explain.
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Textbook Question
Assume a hypothetical organism in which DNA replication is conservative. Design an experiment similar to that of Taylor, Woods, and Hughes that will unequivocally establish this fact. Using the format established in Figure 11.5, draw sister chromatids and illustrate the expected results establishing this mode of replication.
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Textbook Question
At the end of the short arm of human chromosome 16 (16p), several genes associated with disease are present, including thalassemia and polycystic kidney disease. When that region of chromosome 16 was sequenced, gene-coding regions were found to be very close to the telomere-associated sequences. Could there be a possible link between the location of these genes and the presence of the telomere-associated sequences? What further information concerning the disease genes would be useful in your analysis?
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Textbook Question
DNA polymerases in all organisms add only 5' nucleotides to the 3' end of a growing DNA strand, never to the 5' end. One possible reason for this is the fact that most DNA polymerases have a proofreading function that would not be energetically possible if DNA synthesis occurred in the 3' to 5' direction. Sketch the reaction that DNA polymerase would have to catalyze if DNA synthesis occurred in the 3' to 5' direction.
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