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Ch. 11 - DNA Replication and Recombination
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of GeneticsISBN: 9780135564776당신이 사용하는 게 아니라요?교과서 변경
모든 교과서Klug 12th EditionCh. 11 - DNA Replication and Recombination문제 29
11장, 문제 29

Reiji and Tuneko Okazaki conducted a now classic experiment in 1968 in which they discovered a population of short fragments synthesized during DNA replication. They introduced a short pulse of ³H-thymidine into a culture of E. coli and extracted DNA from the cells at various intervals. In analyzing the DNA after centrifugation in denaturing gradients, they noticed that as the interval between the time of ³H-thymidine introduction and the time of centrifugation increased, the proportion of short strands decreased and more labeled DNA was found in larger strands. What would account for this observation?

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Understand the context of the experiment: Reiji and Tuneko Okazaki were studying DNA replication in E. coli. They used ³H-thymidine, a radioactive form of thymidine, to label newly synthesized DNA strands. The goal was to observe the dynamics of DNA strand synthesis over time.
Recall the mechanism of DNA replication: DNA replication occurs in a semi-discontinuous manner. On the leading strand, synthesis is continuous, while on the lagging strand, synthesis occurs in short fragments called Okazaki fragments. These fragments are later joined together by DNA ligase.
Interpret the experimental observations: The short fragments observed immediately after the pulse of ³H-thymidine correspond to Okazaki fragments on the lagging strand. Over time, these fragments are joined into longer strands, which explains the decrease in the proportion of short fragments and the increase in labeled larger strands.
Explain the role of enzymes: DNA polymerase synthesizes the Okazaki fragments, and DNA ligase seals the gaps between them to form a continuous strand. The observed shift from short fragments to longer strands reflects the action of DNA ligase over time.
Conclude the significance of the experiment: The findings provided key evidence for the semi-discontinuous nature of DNA replication and the existence of Okazaki fragments, which are essential for lagging strand synthesis.

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주요 개념

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DNA Replication

DNA replication is the biological process by which a cell duplicates its DNA before cell division. It involves the unwinding of the double helix and the synthesis of new strands using existing strands as templates. This process is crucial for genetic continuity and is facilitated by enzymes such as DNA polymerase, which adds nucleotides to form new strands.
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Steps to DNA Replication

Okazaki Fragments

Okazaki fragments are short sequences of DNA synthesized on the lagging strand during DNA replication. They are formed because DNA polymerase can only add nucleotides in a 5' to 3' direction, necessitating the synthesis of these fragments in short bursts as the replication fork opens. These fragments are later joined together by the enzyme DNA ligase to create a continuous strand.
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Steps to DNA Replication

Centrifugation and Denaturing Gradients

Centrifugation is a technique used to separate components of a mixture based on their density by spinning them at high speeds. In the context of DNA analysis, denaturing gradients can be employed to separate DNA fragments of different lengths. As DNA is subjected to varying conditions, shorter fragments migrate differently than longer ones, allowing researchers to analyze the size distribution of DNA synthesized during replication.
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Cell-cell interactions
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교과서 질문

Consider the drawing of a dinucleotide below.

Suppose that the molecule was cleaved with the enzyme spleen phosphodiesterase, which breaks the covalent bond connecting the phosphate to C-5'. After cleavage, to which nucleoside is the phosphate now attached (A or T)?

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교과서 질문

Consider the drawing of a dinucleotide below.

Is the arrow closest to the 5' or the 3' end?

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교과서 질문

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.

Consider the information in your sketch and speculate as to why proofreading would be problematic.

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교과서 질문

Assume that the sequence of bases shown below is present on one nucleotide chain of a DNA duplex and that the chain has opened up at a replication fork. Synthesis of an RNA primer occurs on this template starting at the base that is underlined.

If the RNA primer consists of eight nucleotides, what is its base sequence?

3'.......GGCTACCTGGATTCA....5'

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교과서 질문

Assume that the sequence of bases shown below is present on one nucleotide chain of a DNA duplex and that the chain has opened up at a replication fork. Synthesis of an RNA primer occurs on this template starting at the base that is underlined.

In the intact RNA primer, which nucleotide has a free 3'-OH terminus?

3'.......GGCTACCTGGATTCA....5'

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교과서 질문

Consider the drawing of a dinucleotide below.

Is it DNA or RNA?

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