Problem 11 - AP Psychology Premium Prep 2021

Question

A biologist systematically removes each of the proteins involved in DNA replication to determine the effect each has on the process. In one experiment, after separating the strands of DNA, she sees many short DNA/RNA fragments as well as some long DNA pieces. Which of the following is most likely missing? (A) Helicase (B) DNA polymerase (C) DNA ligase (D) RNA primase

Step-by-Step Solution

Answer

The correct answer is: (C) DNA ligase. This protein is responsible for joining the short fragments of newly synthesized DNA (called Okazaki fragments) on the lagging strand to create a continuous DNA strand. In its absence, the observation of short DNA/RNA fragments (unjoined Okazaki fragments) and some long DNA pieces (synthesized on the leading strand) would be expected.

Step 1: Understand the roles of the proteins listed in the options

Before selecting the correct answer, it is important to understand what each of the proteins does during DNA replication. (A) Helicase: This enzyme is responsible for separating the two strands of DNA, allowing replication to occur. (B) DNA polymerase: This enzyme synthesizes new DNA strands by adding nucleotides to the growing strand, based on the template strand. (C) DNA ligase: This enzyme joins the short fragments of newly synthesized DNA (called Okazaki fragments) on the lagging strand to create a continuous DNA strand. (D) RNA primase: This enzyme creates a short RNA primer for DNA polymerase to start the synthesis of new DNA strands.

Step 2: Analyze the observed effects

The observation made by the biologist is that there are many short DNA/RNA fragments and some long DNA pieces after removing a protein involved in DNA replication. Based on this information, the missing protein must be the one whose absence would result in the formation of short fragments and long DNA pieces.

Step 3: Choose the correct protein

Now, based on the understanding of each protein's role and the observed effects, we are to eliminate the less likely options: (A) Helicase: Since the strands of DNA are separated in the observation, it means helicase is not the missing protein. (B) DNA polymerase: If DNA polymerase was missing, new DNA strands wouldn't be synthesized, so there would be no short or long DNA pieces observed. (D) RNA primase: If RNA primase was missing, there would be no RNA primers, hindering DNA polymerase from synthesizing new DNA strands. This would also not result in the observed short fragments and long DNA pieces. This leaves us with option (C) DNA ligase as the most likely missing protein. In the absence of DNA ligase, the short fragments (Okazaki fragments) on the lagging strand would not be joined together. Therefore, what would be observed are short DNA/RNA fragments (unjoined Okazaki fragments) and some long DNA pieces (synthesized on the leading strand). The correct answer is: (C) DNA ligase.