AP Biology : Understanding DNA Replication

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understanding Dna Replication

Which of the following represents a step necessary to create the lagging strand, but not the leading strand, during DNA replication?

Possible Answers:

Helicase unwinds the parent strand of DNA

DNA polymerase synthesizes template DNA from the parent strand

DNA ligase joins Okazaki fragments together

DNA is synthesized in the 5’ to 3’ direction

Correct answer:

DNA ligase joins Okazaki fragments together

Explanation:

Okazaki fragments are only produced, and subsequently joined together, in the lagging strand to allow for replication in the opposite direction as replication fork movement. The leading strand, however, allows for continual replication.

All other choices reflect aspects of DNA replication for both the leading and lagging strands.

Example Question #1 : Dna Repair And Replication

Point mutations __________.

Possible Answers:

include missense mutations, which only affect the codon in which the mutation occurs

include nonsense mutations, which change an amino acid to a stop codon, affecting the entire polypeptide structure

include silent mutations, which have no effect on the organism's phenotype

All of the answer choices are correct

Correct answer:

All of the answer choices are correct

Explanation:

Point mutations replace a single nucleotide for a different one. This can change a certain codon to code for a different amino acid (missense), the same amino acid (silent), or lead to a stop codon (nonsense). Nonsense mutations are the most severe type of point mutation, as they will cause early termination of the protein.

Example Question #3 : Understanding Dna Replication

How many copies of DNA would you have after ten replication cycles if you start with four copies?

Possible Answers:

Correct answer:

Explanation:

This is really just a math equation. We need to double the amount of DNA each time it goes through a replication cycle.

Begin: 4

Cycle 1: 8

Cycle 2: 16

Cycle 3: 32

Cycle 4: 64

Cycle 5: 128

Cycle 6: 256

Cycle 7: 512

Cycle 8: 1024

Cycle 9: 2048

Cycle 10: 4096

After ten cycles, we would have 4096 copies from our original 4.

A shortcut calculation would be .

This is why PCR amplification is so effective.

Example Question #11 : Dna, Rna, And Proteins

Select the complementary strand of DNA for the following DNA segment.

5'-ACTTGACT-3'

Possible Answers:

5'-AGTCAAGT-3'

5'-TCAGTTCA-3'

5'-AGUCAAGU-3'

5'-TGAACTGA-3'

Correct answer:

5'-AGTCAAGT-3'

Explanation:

The complementary strand will be going in the opposite direction (3'-5'). As a result, you will need to flip the direction in order for it to be complementary to the original strand. When pairing bases, remember that guanine (G) and cytosine (C) are paired with one another, and adenine (A) and thymine (T) are paired.

5'-ACTTGACT-3' Switch the direction.

3'-TCAGTTCA-5' Find the complement pairs.

5'-AGTCAAGT-3'

Example Question #4 : Understanding Dna Replication

What is the function of the single-strand binding protein during DNA replication?

Possible Answers:

Ensure both strands are replicated at similar rates

Prevent the separated strands from reannealing 

Recruit DNA polymerase

Synthesizing RNA primers

Correct answer:

Prevent the separated strands from reannealing 

Explanation:

Single-strand binding protein (SSB) binds the newly separated DNA strands to ensure that it does not reanneal during replication. This keeps the strands separate so that replication can occur.

All of the other answers describe the functions of other proteins. Primase synthesizes the RNA primers, which helps to recruit DNA polymerase. The structural basis for the replication of the leading and lagging strands ensures that replication follows the same rate on both strands.

Example Question #5 : Understanding Dna Replication

Which enzymes are necessary to produce recombinant DNA?

Possible Answers:

Restriction enzymes and DNA polymerase

DNA polymerase and topoisomerase

DNA polymerase and ligase

Restriction enzymes and topoisomerase

Restriction enzymes and ligase

Correct answer:

Restriction enzymes and ligase

Explanation:

Recombinant DNA technology involves combining genes from two sources, such as different species, into a single molecule.

Applying restriction enzymes to DNA will cleave the DNA into fragments, which can be isolated for specific genes. Ligase can then be used to fuse the fragments together into a full recombinant gene.

Topoisomerase is responsible for relieving tension in the winding of the DNA helix. DNA polymerase synthesizes new DNA from individual nucleotides, but would not be useful in fusing two types of DNA together.

Example Question #1 : Understanding Dna Replication

What is the function of a helicase enzyme during DNA replication?

Possible Answers:

Unwind the double-stranded DNA

Fix breaks in the sugar-phosphate backbone

Scan for mismatches

Add primers to the lagging strand

Correct answer:

Unwind the double-stranded DNA

Explanation:

Helicases are enzymes that separate annealed strands of nucleic acids. This function provides the single-stranded template used in replication.

Primase is responsible for adding DNA primers, DNA polymerase I scans for mismatched nucleotides and mutations, and ligase repairs breaks in the DNA backbone.

Example Question #2 : Understanding Dna Replication

What is the purpose of polymerase chain reaction (PCR)?

Possible Answers:

Increase the efficiency of translation

Amplify a template DNA

Isolate tagged proteins

Amplify a template RNA

Correct answer:

Amplify a template DNA

Explanation:

Polymerase chain reaction, or PCR, is commonly used in laboratories to increase the amount of a small biological sample. Given a small sample of DNA, the process replicates the sample to make numerous identical copies. These copies can then be studied directly, used to make protein products, or incorporated into genetic modification.

Other laboratory techniques can be used to achieve the results given by the other answer options.

Example Question #3 : Understanding Dna Replication

What is the purpose topoisomerase during DNA replication?

Possible Answers:

Identify unwanted isomers of nucleotides 

Bring new nucleotides to the replication fork

Hold the DNA polymerases in place during replication

Cut the phosphate backbone ahead of the replication fork to prevent the DNA from getting overwound

Correct answer:

Cut the phosphate backbone ahead of the replication fork to prevent the DNA from getting overwound

Explanation:

DNA topoisomerases are the cell's solution to the "winding" problem. The double helical nature of DNA results in tension during the replication process that would interfere with the process. DNA topoisomerases cut the phosphate backbone to relieve this tension, and allow DNA to replicate properly.

Example Question #9 : Understanding Dna Replication

What would be a direct result of a mutated, nonfunctional gene for primase in a cell?

Possible Answers:

Inability to maintain genetic fidelity

Inability to transcribe DNA

Inability to replicate DNA

Inability to denature double-stranded DNA

Correct answer:

Inability to replicate DNA

Explanation:

Primase is an enzyme that is essential for the process of DNA replication. It synthesizes RNA primers so that DNA polymerase may begin replicating DNA. Mutation to the gene that codes for primase would damage the protein. Without primase, a cell would not be able to go through the process of replication because DNA polymerase would not properly bind the DNA.

RNA polymerase is responsible for transcribing DNA and helicase is responsible for unwinding the DNA double stranded helix.

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