AP Biology : Understanding DNA Replication

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #21 : Dna, Rna, And Proteins

Which enzyme creates complementary strands of DNA during replication?

Possible Answers:

DNA polymerase I

Helicase

Primase

DNA polymerase III

Ligase

Correct answer:

DNA polymerase III

Explanation:

DNA polymerase III is the enzyme that attaches to the RNA primer and adds DNA nucleotides complementary to the template strand in order to create the new, growing strand.

Primase is responsible for synthesizing the RNA primer. Helicase unwinds the DNA and creates the replication fork for other enzymes to bind. Ligase repairs breaks in the sugar-phosphate backbone and binds Okazaki fragments together. DNA polymerase I has a number of functions, including replacing the RNA primer with DNA nucleotides.

Example Question #21 : Understanding Dna Replication

Which one of the following proteins is found in the nucleus of eukaryotic cells?

Possible Answers:

Gyrase

All choices are found in the nucleus

DNA polymerase

Helicase

Correct answer:

All choices are found in the nucleus

Explanation:

Helicase, gyrase, and DNA polymerase are all used in the process of DNA replication, which takes place in the nucleus. Helicase is responsible for "unzipping" DNA, separating its two strands and unwinding the double-helix. Gyrase is responsible for relaxing the DNA strands and relieving tensions during unwinding. DNA polymerase synthesizes the the new DNA strands by recruiting nitrogenous bases.

Example Question #22 : Dna, Rna, And Proteins

DNA polymerase can only add nucleotides to which end of the DNA strand?

Possible Answers:

Correct answer:

Explanation:

The polarity of nucleic acids, and therefore DNA molecules, describes the chemical orientation of the macromolecule. Each nucleotide has a  end, which refers to the fifth carbon of the deoxyribose sugar ring that has a phosphate group attached to it, and a  end, which is the third carbon in the sugar ring that has a hydroxyl group attached to it. DNA polymerase can only add nucleotides to the  hydroxyl group of a nucleotide. This results in a new DNA strand elongating in the  to  direction.

Example Question #871 : Ap Biology

Which enzyme is important in the initiation stage of DNA replication?

Possible Answers:

DNA ligase

DNA primase

Telomerase

Topoisomerase

Correct answer:

Topoisomerase

Explanation:

In the initiation stage of DNA replication, a number of enzymes are involved. These include the initiator proteins in the pre-replication complex, DNA helicase, single stranded binding proteins, and topoisomerase. Topoisomerase is an enzyme that helps relieve winding and unwinding tension in DNA that arise from the helical structure of the DNA molecule. The DNA ahead of the replication fork often becomes tangled and/or supercoiled. Topoisomerase cuts the DNA to relieve the stress and allow the DNA to relax by unwinding a few times.Later during the replication process, the DNA rewinds and these breaks are resealed.

Example Question #31 : Dna, Rna, And Proteins

Which of the following are true regarding primers used in DNA replication?

Possible Answers:

The primers are DNA primers 

RNA polymerase adds RNA primers to the new strands

Each newly synthesized strand receives several primers

RNA primers allow DNA polymerase to add nucleotides to the  hydroxyl end 

Correct answer:

RNA primers allow DNA polymerase to add nucleotides to the  hydroxyl end 

Explanation:

In the process of DNA replication, DNA polymerase can only add nucleotides to an already existing DNA strand. To overcome this difficulty in synthesizing new DNA strands, the cell utilizes RNA primers. RNA primers are short strands of RNA synthesized by the enzyme primase. Primase is a type of RNA polymerase that does not need a free  hydroxyl as a substrate, like DNA polymerase. Thus, it can lay down RNA nucleotides using the DNA parent strands as templates to provide DNA polymerase with the substrate it needs to begin elongation. RNA primers are also used in lagging strand synthesis, since DNA polymerase can only add nucleotides to the  end of a nucleotide. Thus all nucleic acid polymerization occurs in the  direction. DNA polymerase I removes the RNA primers, replacing them with DNA, then DNA ligase seals the gaps between DNA on the lagging strand.

Example Question #22 : Understanding Dna Replication

Which of the following is not true regarding lagging stand synthesis?

Possible Answers:

It is synthesized continuously 

Due to parent template strand orientation, synthesis runs in the opposite direction that the replication fork opens

It requires many RNA primers

It is synthesized in Okazaki fragments 

Correct answer:

It is synthesized continuously 

Explanation:

The lagging strand is the strand of parent DNA that runs in the opposite direction that the replication fork opens. Because DNA polymerase adds nucleotides in  direction, RNA primers are added along the length of the newly single parent DNA strand to provide a  hydroxyl group onto which DNA polymerase adds nucleotides. DNA polymerase adds nucleotides to the RNA primer until encountering another primer. These segments of newly synthesized DNA are called Okazaki fragments. DNA polymerase I removes RNA primers and replaces them with DNA. Ligase seals the gaps between DNA, forming a continuous strand.

Example Question #31 : Dna, Rna, And Proteins

Without the action of telomerase, what would happen to telomeres after consecutive rounds of DNA replication?

Possible Answers:

They would lengthen

The sequence changes

They would be maintained

They would shorten

Correct answer:

They would shorten

Explanation:

Telomeres are the regions at the ends of chromosomes that contain repetitive sequences of DNA. Telomeres are non-coding, and instead serve to protect the chromosome from deterioration and degradation. In DNA replication, there exists an “end replication problem”, which describes the inability to replicate the entire chromosome. The lagging strand cannot be copied in its entirety due to a lack of strand to attach another primer. Thus, the newly synthesized DNA molecule is shorter than the parent molecule. Because the telomeres are at the ends of chromosomes, telomere sequences are shortened with each consecutive round of DNA replication. However, telomerase solves this "end replication problem." It is a ribonucleoprotein, which means it contains RNA and protein to carry out its enzymatic function. It adds repetitive sequences to the ends of the DNA strand so that the chromosomes do not shorten over time.

Example Question #31 : Dna, Rna, And Proteins

Which of the following describes how telomeres can be linked to cancer?

Possible Answers:

Continued telomere shortening

Increased telomerase activity 

Decreased telomerase activity

Cell senescence

Correct answer:

Increased telomerase activity 

Explanation:

Telomerase catalyzes the lengthening of chromosomes. Without telomerase, chromosomes would shorten with each round of replication, until the chromosome shortens, cutting into an important gene. At this time, the cell would not be able to carry out replication and/or make a gene product essential to its survival. If telomerase is overactive, cells' chromosomes would not naturally shorten over time, and they may continue to lengthen and divide uncontrollably (cancer).

Example Question #31 : Dna, Rna, And Proteins

What is the role of DNA ligase?

Possible Answers:

To join the sugar-phosphate backbone of DNA

Aids DNA polymerase in the addition of nucleotides

Adds primers

Unwinds the double helix 

Correct answer:

To join the sugar-phosphate backbone of DNA

Explanation:

DNA ligase joins the sugar-phosphate backbone of DNA strands through catalyzing the formation of phosphodiester bonds. The nicks in the backbone arise from Okazaki fragments and the action of topoisomerase.

Example Question #21 : Understanding Dna Replication

What model describes DNA replication?

Possible Answers:

Dispersive

Conservative

Semiconservative

Any of these, depending on the cell type

Correct answer:

Semiconservative

Explanation:

There are three models of DNA replication: the dispersive model, the conservative model, and the semiconservative model. The dispersive model postulates that parental DNA cut into segments, each of which acts as a template for newly synthesized fragments. Together, DNA helices reassemble, containing daughter and parental DNA segments mixed together. The conservative model hypothesizes that the parental DNA double helix acts as a template for the daughter DNA molecule. This model results in one parental double helix and one daughter DNA molecule. The model that describes DNA replication is the semiconservative model. In this model, parental DNA strands separate and act as templates for daughter strands, resulting in two DNA molecules each with one parent and one daughter DNA strand. Thus, each newly synthesized DNA molecule has one parent strand bound to one daughter strand.

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