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Kristin Fracchia

ACT Challenge Question 9: DNA and Nucleotides

Magoosh ACT Challenge Question

This week and next week, we are turning to an ACT Science challenge! Both questions will be based on the same ACT Science Conflicting Viewpoints Passage, which you will see below the question. Let’s see if you can deserve your O.W.L in Potions: post your answer and how you got it in the comments below, and check back for the answer on Friday!

Based on the description of DNA given in the passage, which of the following could be the nucleotide composition of a DNA double helix?

    A) 20% adenine, 20% guanine, 30% thymine, 30% cytosine
    B) 60% adenine, 20% thymine, 10% guanine, 10% cytosine
    C) 20% adenine, 20% thymine, 30% guanine, 30% cytosine
    D) 60% adenine, 20% guanine, 10% thymine, 10% cytosine



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.

DNA Replication

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.

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.

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About Kristin Fracchia

Kristin makes sure Magoosh's sites are full of awesome, free resources that can be found by students prepping 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 agony and bliss of train running, backpacking, hot yoga, and esoteric knowledge.

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