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Chapter 13: Problem 4

The primer used for DNA replication a. is a short strand of RNA added to the 3 ' end. b. is needed only once on a leading strand. c. remains on the DNA after replication. d. ensures that there will be a free \(5^{\prime}\) end to which nucleotides can be added. e. is added to only one of the two template strands.

Short Answer

Expert verified
The primer is needed only once on a leading strand (option b).

Step by step solution

01

Understand the Role of a Primer

A primer in DNA replication is a short segment of nucleic acid, usually RNA, that provides a starting point for DNA synthesis. It is necessary because DNA polymerases cannot initiate synthesis of a new strand; they can only add nucleotides to an existing strand. Specifically, the primer provides a free 3' hydroxyl group for the elongation of the new DNA strand.
02

Eliminate Incorrect Options

Evaluate the provided options to determine which are incorrect based on typical DNA replication processes: - Option a states that the primer is added to the 3' end, which is correct in that it provides a 3' hydroxyl, but the primer itself is typically added to the 5' end of the new strand to facilitate this. - Option b suggests the primer is needed only once on the leading strand, which is true since continuous synthesis occurs there. - Option c implies the primer remains after replication, which is false because RNA primers are eventually replaced by DNA. - Option d incorrectly states that a primer ensures a free 5' end, but primers provide a free 3' end for nucleotide addition. - Option e suggests the primer is added to only one strand, which is incorrect as both leading and lagging strands require primers.
03

Identify the Correct Option

Considering DNA replication process on leading vs. lagging strands: - On the leading strand, DNA synthesis is continuous, requiring only a single primer initially. - On the lagging strand, primers are needed repeatedly to initiate synthesis of Okazaki fragments. Thus, the correct answer is that a primer is needed only once on the leading strand. This corresponds to option b.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

RNA primer
During DNA replication, an RNA primer plays a crucial role. It is a short strand of RNA that is synthesized by an enzyme called primase. Its purpose is to provide a starting point for DNA synthesis. DNA polymerase, the enzyme responsible for DNA elongation, cannot start a new strand on its own. It needs a starting segment to which it can add nucleotides. This is where the RNA primer comes in.
  • The primer is typically around 10-12 nucleotides long.
  • It provides a 3' hydroxyl group, crucial for DNA polymerase to add nucleotides.
  • Unlike the final DNA product, the primer is made of ribonucleotides, not deoxyribonucleotides.
Without the RNA primer, the replication process would stall since DNA polymerase relies on it to begin laying down new DNA strands.
DNA polymerase
DNA polymerase is a vital enzyme in the process of DNA replication. It adds nucleotides to the new DNA strand being formed, ensuring that each strand forms an accurate copy of the original DNA. However, it requires an existing 3'-hydroxyl group to start this process; hence, the necessity for an RNA primer.
  • DNA polymerase synthesizes new DNA by extending from a 3'-OH end of the RNA primer.
  • Different types of DNA polymerases exist, each performing specific roles in replication and repair.
  • It also exhibits proofreading ability, identifying and correcting errors during DNA synthesis.
The function of DNA polymerase is to ensure that errors are minimized and that the DNA is replicated accurately, maintaining genetic fidelity.
leading strand
The leading strand plays a significant role in DNA replication. It is synthesized continuously in the direction that the replication fork is opening. This happens because DNA polymerase can only add nucleotides in the 5' to 3' direction, and the leading strand aligns perfectly with this.
  • Only a single RNA primer is required on the leading strand to initiate synthesis.
  • DNA polymerase adds nucleotides continuously without interruption as the replication fork progresses.
  • The leading strand is replicated more efficiently than the lagging strand.
By being synthesized in a straightforward linear fashion, the leading strand provides a smoothly replicated section of the DNA, leaving less room for error.
lagging strand
The lagging strand is somewhat more complex than the leading strand due to the direction of replication. Since DNA polymerase synthesizes in the 5' to 3' direction, and the lagging strand is oriented in the opposite direction, replication is discontinuous and occurs in short segments known as Okazaki fragments.
  • Multiple RNA primers are necessary for the lagging strand.
  • Each fragment requires a primer to begin synthesis.
  • Once a fragment is synthesized, another primer is laid down further along the template strand.
  • Eventually, these fragments are joined together by an enzyme called DNA ligase.
The discontinuous nature of the lagging strand makes its replication more intricate, but enzymes ensure the process is seamless and the eventual joining of fragments results in a continuous DNA strand.

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Most popular questions from this chapter

Suppose that Meselson and Stahl had continued their experiment on DNA replication for another ten bacterial generations. Would there still have been any \(^{14} \mathrm{N}-^{15} \mathrm{N}\) hybrid DNA present? Would it still have appeared in the centrifuge tube? Explain.

In semiconservative replication of DNA, a. the original double helix remains intact and a new double helix forms. b. the strands of the double helix separate and act as templates for new strands. c. polymerization is catalyzed by RNA polymerase. d. polymerization is catalyzed by a double-helical enzyme. e. DNA is synthesized from amino acids.

Outline a series of experiments using radioactive isotopes (such as \(^{32} \mathrm{P}\) and \(^{35} \mathrm{S}\) ) to show that during bacterial conjugation it is DNA and not protein that moves from the donor cell to the recipient cell and is responsible for bacterial transformation.

One strand of DNA has the sequence \(5^{\prime}-\) ATTCCG-3 \(^{\prime}\) The complementary strand for this is a. \(5^{\prime}\) -TAAGGC-3' b. \(5^{\prime}-\) ATTCCG-3 \(^{\prime}\) c. \(5^{\prime}-\) ACCTTA-3 \(^{\prime}\) d. \(5^{\prime}-\) CGGAAT-3 \(^{\prime}\) e. \(5^{\prime}-\) GCCTTA-3 \(^{\prime}\)

Which statement about complementary base pairing is not true? a. Complementary base pairing plays a role in DNA replication. b. In DNA, T pairs with A. c. Purines pair with purines, and pyrimidines pair with pyrimidines. d. In DNA, C pairs with G. e. The base pairs are of equal length.
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