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Chapter 11: Problem 18

What is an Okazaki fragment? In which strand of replicating DNA are Okazaki fragments found? Based on the properties of DNA polymerase, why is it necessary to make these fragments?

Short Answer

Expert verified
Okazaki fragments are short DNA sequences formed on the lagging strand during DNA replication. They are necessary because DNA polymerase, the enzyme that catalyzes DNA synthesis, can only add nucleotides in the 5' to 3' direction, and thus, while the leading strand can be synthesized continuously, the lagging strand has to be synthesized discontinuously in the form of Okazaki fragments.

Step by step solution

01

Defining Okazaki Fragments

Okazaki fragments are short sequences of DNA nucleotides (approximately 150 to 200 base pairs long in eukaryotes), which are synthesized discontinuously and later linked together by the enzyme DNA ligase to create the lagging strand during DNA replication.
02

Locating Okazaki Fragments in DNA strand

Okazaki fragments are found in the lagging strand of the replicating DNA. So during DNA replication, one strand (leading strand) is replicated continuously in the 3' to 5' direction, while the other strand, known as the lagging strand, is replicated discontinuously in fragments known as Okazaki fragments, in the 5' to 3' direction.
03

Linking Okazaki fragments to the Properties of DNA Polymerase

The reason why DNA polymerase makes these fragments lies in its properties. DNA polymerase can only add nucleotides in the 5' to 3' direction. On the leading strand, replication can proceed continuously, but on the lagging strand, replication is discontinuous, with DNA synthesized in short stretches (Okazaki fragments) moving away from the replication fork. Thus, these fragments are necessary due to the unidirectional nature of DNA polymerase, making it unable to synthesize both strands continuously.

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Most popular questions from this chapter

Explain the proofreading function of DNA polymerase.

Obtain two strings of different colors (e.g., black and white) that are the same length. A length of 20 inches is sufficient. Tie a knot at one end of the black string and another knot at one end of the white string. Each knot designates the \(5^{\prime}\) end of a string. Make a double helix with your two strings. Now tape one end of the double helix to a table so that the tape is covering the knot on the black string. A. Pretend your hand is DNA helicase and use your hand to unravel the double helix, beginning at the end that is not taped to the table. Should your hand be sliding along the white string or the black string? B. As shown in Figure 11.12, imagine that your two hands together form a dimeric DNA polymerase. Unravel your two strings halfway to create a replication fork. Grasp the black string with your left hand and the white string with your right hand. Your thumbs should point toward the \(5^{\prime}\) end of each string. You need to loop one of the strings so that one of the DNA polymerases can synthesize the lagging strand. With such a loop, dimeric DNA polymerase can move toward the replication fork and synthesize both DNA strands in the \(5^{\prime}\) to \(3^{\prime}\) direction. In other words, with such a loop, your two hands can touch each other with both of your thumbs pointing toward the fork. Should the black string be looped, or should the white string be looped?

Single-strand binding proteins keep the two parental strands of DNA separated from each other until DNA polymerase has an opportunity to replicate the strands. Suggest how single-strand binding proteins keep the strands separated and yet do not impede the ability of DNA polymerase to replicate the strands.

Which of the following statements is not true? Explain why. A. A DNA strand can serve as a template strand on many occasions. B. Following semiconservative DNA replication, one strand is a newly made daughter strand and the other strand is a parental strand. C. A DNA double helix may contain two strands of DNA that were made at the same time. D. A DNA double helix obeys the AT/GC rule. E. A DNA double helix could contain one strand that is 10 generations older than its complementary strand.

Sometimes DNA polymerase makes a mistake, and the wrong nucleotide is added to the growing DNA strand. With regard to pyrimidines and purines, two general types of mistakes are possible. The addition of an incorrect pyrimidine instead of the correct pyrimidine (e.g., adding cytosine where thymine should be added) is called a transition. If a pyrimidine is incorrectly added to the growing strand instead of purine (e.g., adding cytosine where an adenine should be added), this type of mistake is called a transversion. If a transition or transversion is not detected by DNA polymerase, a mutation is created that permanently changes the DNA sequence. Though both types of mutations are rare, transition mutations are more frequent than transversion mutations. Based on your understanding of DNA replication and DNA polymerase, offer three explanations why transition mutations are more common.
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