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Ch. 11 - DNA Replication and Recombination
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of GeneticsISBN: 9780135564776Non è quello che usi tu?Cambia libro di testo
Capitolo 11, Problema 20b

Several temperature-sensitive mutant strains of E. coli display the following characteristics. Predict what enzyme or function is being affected by each mutation.
Okazaki fragments accumulate, and DNA synthesis is never completed.

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Understand the biological context: Okazaki fragments are short DNA sequences synthesized on the lagging strand during DNA replication. Normally, these fragments are joined together to form a continuous strand.
Identify the process involved: The joining of Okazaki fragments requires the removal of RNA primers and the sealing of gaps between fragments. This involves enzymes such as DNA polymerase I and DNA ligase.
Consider the effect of mutation: If Okazaki fragments accumulate and DNA synthesis is never completed, it suggests a failure in either removing RNA primers or sealing the fragments together.
Focus on the key enzymes: DNA polymerase I removes RNA primers and fills in the gaps with DNA, while DNA ligase seals the nicks between Okazaki fragments to create a continuous strand.
Predict the affected enzyme or function: The mutation likely affects either DNA polymerase I's exonuclease activity (removal of RNA primers) or DNA ligase's ability to join fragments, preventing completion of lagging strand synthesis.

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Okazaki Fragments and Lagging Strand Synthesis

Okazaki fragments are short DNA segments synthesized discontinuously on the lagging strand during DNA replication. Their proper joining is essential for continuous DNA synthesis. Accumulation of these fragments indicates a defect in processing or joining steps of lagging strand replication.
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Steps to DNA Replication

Role of DNA Ligase

DNA ligase is the enzyme responsible for sealing nicks between Okazaki fragments by forming phosphodiester bonds. If DNA ligase is defective or inactive, Okazaki fragments accumulate because they cannot be joined into a continuous strand, halting DNA synthesis completion.
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DNA Proofreading

Temperature-Sensitive Mutations

Temperature-sensitive mutations produce proteins that function normally at permissive temperatures but lose activity at restrictive (higher) temperatures. Studying these mutants helps identify essential enzymes by observing which functions fail when the protein is inactivated.
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Mutations and Phenotypes
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Several temperature-sensitive mutant strains of E. coli display the following characteristics. Predict what enzyme or function is being affected by each mutation.

Newly synthesized DNA contains many mismatched base pairs.

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Several temperature-sensitive mutant strains of E. coli display the following characteristics. Predict what enzyme or function is being affected by each mutation.

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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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Suppose that E. coli synthesizes DNA at a rate of 100,000 nucleotides per minute and takes 40 minutes to replicate its chromosome.

(a) How many base pairs are present in the entire E. coli chromosome?

(b) What is the physical length of the chromosome in its helical configuration—that is, what is the circumference of the chromosome if it were opened into a circle?

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Why is DNA synthesis expected to be more complex in eukaryotes than in bacteria? How is DNA synthesis similar in the two types of organisms?

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Several temperature-sensitive mutant strains of E. coli display the following characteristics. Predict what enzyme or function is being affected by each mutation.

No initiation occurs.

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