AP Biology : Understanding DNA Replication

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #11 : Dna Repair And Replication

Why must there be a lagging strand during DNA synthesis?

Possible Answers:

The lagging strand ensures replication of both strands occurs at a similar rate

DNA is antiparallel 

The lagging strand helps conserve energy

DNA is a polyanion

Correct answer:

DNA is antiparallel 

Explanation:

The lagging strand exists because DNA is antiparallel and replication always occurs in the 5' to 3' direction. One strand of DNA will be replicated in the 5' to 3' direction toward the replication fork, following in the same direction as the DNA is "unzipped." This is the leading strand, which can be replicated fluidly. The lagging strand is oriented in the 3' to 5' direction, and must be read backward (away from the replication fork).

Having a lagging strand does not help the cell conserve energy. DNA is a polyanion, but this is due to the phosphate groups in the backbone. If anything, having a lagging strand actually makes it more difficult to maintain a similar rate of replication between strands since they cannot be replicated in the same direction.

Example Question #12 : Dna Repair And Replication

What are Okazaki fragments?

Possible Answers:

Short fragments of DNA synthesized on the lagging strand

Small fragments of RNA used to silence genes

DNA that has been cleaved by nucleases

Small proteins used to prevent DNA fragments from reannealing

Correct answer:

Short fragments of DNA synthesized on the lagging strand

Explanation:

Okazaki fragments are the cell’s solution to replicating DNA in the opposite direction of the replication fork. They are small fragments of DNA synthesized on the lagging strand. While the leading strand can be continuously synthesized toward the replication fork, the lagging strand must be made in small pieces opposite from the replication fork.

Using small fragments of RNA to silence genes is a process known as RNA interference. DNA that has been cleaved by nucleases is not related to Okazaki fragments. Single-strand binding proteins are small proteins used to prevent DNA from reannealing during replication.

Example Question #13 : Dna Repair And Replication

What is the function of DNA ligase?

Possible Answers:

Transcribe DNA into RNA

Synthesize RNA primers

Separate double-stranded nucleic acids

Create phosphodiester bonds 

Correct answer:

Create phosphodiester bonds 

Explanation:

DNA ligase is an enzyme responsible for repairing nicks in the sugar-phosphate backbone of DNA and for fusing Okazaki fragments during DNA replication. It accomplishes this task by resynthesizing the phosphodiester bonds that hold the backbone together

The other answers describe the functions of other proteins involved in DNA replication or DNA transcription. Helicase is responsible for unwinding double-stranded nucleic acids and is essential for producing the replication fork during DNA synthesis. Primase synthesizes RNA primers as attachment points for DNA polymerase during replication. RNA polymerase is responsible for transcribing a DNA template into RNA products.

Example Question #21 : Dna Repair And Replication

Which term best describes DNA replication?

Possible Answers:

Semiconservative

Random

Dispersive

Conservative

Correct answer:

Semiconservative

Explanation:

DNA is replicated in a semiconservative manner. This implies that each parental strand serves as the template for a newly replicated strand. Each daughter DNA helix is thus composed of one complete parental strand and one complete new strand.

Parent: (PP)

Replication: (PD)(DP)

The other answer choices refer to other theories of DNA replication, which have since been proven incorrect.

Conservative replication results in a newly synthesized molecule of DNA that does not contain either parental strand. Each daughter helix would be composed of only parental strands or only new strands.

Parent: (PP)

Replication: (PP)(DD)

Random and dispersive actually refer to the same process, and imply that DNA is replicated such that the resulting strands are made up of bits and pieces of both newly replicated DNA and the parental DNA. Neither strand is fully conserved in this theory of replication, and instead two hybrid strands are produced.

Example Question #21 : Dna, Rna, And Proteins

Which base pair sequences would you expect to find near the replication origin? 

Possible Answers:

Cytosine and adenine

Adenine and thymine

Guanine and thymine

Adenine and guanine

Guanine and cytosine

Correct answer:

Adenine and thymine

Explanation:

An adenine-thymine sequence would be more likely to be found near the replication origin. Adenine and thymine pair with two hydrogen bonds, while cytosine and guanine pair with three hydrogen bonds. This makes adenine and thymine regions easier to break apart. Since helicase must break the hydrogen bonds in order to create the replication fork at the replication origin, it makes sense that this event would occur in a region where there were weaker forces between the two DNA strands.

Remember, guanine always pairs with cytosine and adenine always pairs with thymine.

Example Question #11 : Understanding Dna Replication

A random mutation occurs in the DNA of a cell, altering one of the enzymes necessary for DNA replication and making it nonfunctional. As a result, the DNA remains supercoiled, preventing replication from taking place. Which enzyme has been altered?

Possible Answers:

DNA ligase

DNA helicase

DNA polymerase

Telomerase

Topoisomerase

Correct answer:

Topoisomerase

Explanation:

The role of the topoisomerase enzyme is to unwind the DNA, allowing enzymes such as DNA helicase access to the nucleotide sequence. DNA helicase is responsible for breaking the hydrogen bonds responsible for holding double-stranded DNA together. This creates the replication fork and allows DNA polymerase access to the nitrogenous base sequence. The role of DNA polymerase is to place nucleotides once the DNA has been unwound, synthesizing the daughter DNA strands. DNA ligase binds nucleotide fragments together during synthesis. Telomerase is responsible for lengthening the telomeres at the ends of chromosomes.

Example Question #21 : Dna Repair And Replication

Which of the following proteins is responsible for the fusing of Okazaki fragments?

Possible Answers:

RNA polymerase

DNA polymerase

DNA ligase

Helicase

Primase

Correct answer:

DNA ligase

Explanation:

Okazaki fragments are found on the lagging strand during replication. Because these fragments will not be attached together following strand synthesis, a protein is required to combine the fragments. DNA ligase will follow DNA polymerase on the lagging strand, and combine the fragments in order to create a complete strand.

DNA polymerase is responsible for recruiting and joining nucleotides in the 3'-to-5' direction, but cannot fuse Okazaki fragments on the lagging strand. Primase lays down an RNA primer to recruit DNA polymerase prior to replication. Helicase unwinds the DNA helix in order to expose the template strands. RNA polymerase is involved in transcription, and plays no active role in DNA replication.

Example Question #11 : Understanding Dna Replication

Frameshift mutations _________.

Possible Answers:

affect multiple codons

often extend the length of the resulting polypeptide

only affect the codon in which the mutation occurred

can lead to degeneracy, and have no effect on phenotype

None of the other choices are correct

Correct answer:

affect multiple codons

Explanation:

Frameshift mutations involve the insertion or deletion of a nucleotide in a DNA sequence, changing the reading frame of the entire nucleotide sequence after the mutation. As a result, every subsequent codon is also affected, creating a change in the organism's phenotype.

Oftentimes, this results in a premature stop codon, which causes the protein product to be shorter than an unaffected polypeptide.

Example Question #21 : Dna Repair And Replication

Which of the following statements is true concerning DNA replication?

Possible Answers:

Replication takes place via a conservative mechanism

The strand opened in the 5'-to-3' direction will be replicated more slowly than the 3'-to-5' strand

DNA polymerase reads the template strands in the 5'-to-3' direction

The strand opened in the 3'-to-5' direction will be replicated more slowly than the 5'-to-3' strand

Correct answer:

The strand opened in the 5'-to-3' direction will be replicated more slowly than the 3'-to-5' strand

Explanation:

DNA polymerase always reads DNA strands in the 3'-to-5' direction, creating a complimentary 5'-to-3' strand. As a result, the parent strand oriented in the 3'-to-5' can be replicated seamlessly, but the strand that is unraveled in the 5'-to-3' direction will require multiple attachment points for DNA polymerase so that the whole strand can be replicated in the reverse direction.

These multiple segments of replication are called Okazaki fragments, and can only be found on the lagging strand, which is replicated more slowly.

Example Question #11 : Understanding Dna Replication

Which of the following proteins are not essential to DNA replication?

Possible Answers:

DNA ligase

Helicase

DNA polymerase III

Cohesin

Correct answer:

Cohesin

Explanation:

Cohesin is a protein that is not involved in DNA replication. It has diverse functions, including regulating sister chromatids during mitosis.

During replication, helicase is responsible for unwinding DNA strands to expose the site for replication. DNA polymerase III functions primarily to add new nucleotides, while DNA polymerase II proofreads and corrects errors in replication. DNA ligase is responsible for joining breaks in the DNA backbone.

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