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Kristin Fracchia

ACT Challenge Question 10: DNA Replication Models – Explanation

Magoosh ACT Challenge Question

Hey Science Superstars! Were you able to solve our ACT Science Challenge Question this week? Check out the question again below (or review the passage here), then scroll below for the answer and explanation.

ACT Challenge Question #10

Consider the following diagrams showing possible mechanisms of DNA replication. Which of the following best describes how the diagrams relate to the hypotheses of Scientists A and B?

Model 1

Model 2

Model 3

  1. Scientist A = Model 1, Scientist B = Model 2
  2. Scientist A = Model 2, Scientist B = Model 1
  3. Scientist A = Model 3, Scientist B = Model 2
  4. Scientist A = Model 2, Scientist B = Model 3



Deoxyribonucleic acid, or DNA, is the molecule within living organisms that encodes the genetic code that informs an organism’s development and functioning. This molecule consists of two long strands, each of which is made up of subunits called nucleotides. These two strands wind around one another, forming the characteristic double helix shape of DNA.

There are four nucleotides used in the production of DNA–guanine, adenine, cytosine, and thymine–that are often referred to as G, A, C, and T, respectively.

Nucleotides pair up in a predictable way as illustrated in the figure: for every “G” on the first strand, there is a “C” on the opposite strand, and for every “A” on the first strand, there is a “T” on the opposite strand. A set of two paired nucleotides is typically referred to as a base pair, and the nucleotides within each pair are held together with hydrogen bonds. Because of the consistency of these pairings, the two individual paired strands can be said to be complementary: the sequence of nucleotides on one strand can be entirely determined if the sequence of bases on the opposite strand is known.

During the process of DNA replication, two identical DNA molecules are produced from a single original piece of DNA through the action of an enzyme known as DNA polymerase. Shortly after DNA’s structure was determined by James Watson and Francis Crick, there was substantial debate in the scientific community regarding how this process occurs. Two of the prevailing viewpoints in this discussion are given below.

DNA Replication

Scientist A

When DNA is not in the process of being replicated, it is found tightly coiled and bound to proteins. Prior to replication, the coiling is loosened and some of the proteins are removed, allowing the two strands of the DNA molecule to be separated. DNA polymerase then acts on each of these individual strands separately, adding new nucleotides by matching them to complementary nucleotides on the original strand. Proteins bind to both new DNA molecules following the completion of replication, facilitating the re-coiling of the DNA.

Scientist B

It is known that intact DNA is typically found bound to proteins known as histones. During DNA replication, the histones remain attached to the DNA molecule, but serve to slightly alter the shape of the double helix, allowing DNA polymerase to access and “read” the base pairs while leaving the overall structure intact. DNA polymerase slides along the length of the DNA-histone molecular complex, producing an entirely new double-stranded DNA molecule bound to new histones as it is synthesized.

Answer and Explanation

ANSWER: A, Scientist A = Model 1, Scientist B = Model 2

Scientist A describes the two original strands as splitting apart, with each one being incorporated into a new DNA double helix, which fits the diagram in Model 1. Scientist B describes the original DNA molecule staying intact, and an entirely new one being formed, which fits the diagram in Model 2. Model 3 is a different model entirely from the ones described in the passage: in this third model, the original double helix is broken into segments that act as templates for the creation of new DNA molecules.

About Kristin Fracchia

Kristin makes sure Magoosh's blogs are chock-full of awesome, free resources for students preparing for standardized tests. With a PhD from UC Irvine and degrees in Education and English, she’s been working in education since 2004 and has helped students prepare for standardized tests, as well as college and graduate school admissions, since 2007. She enjoys the agonizing bliss of marathon running, backpacking, hot yoga, and esoteric knowledge.

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