Problem 3 - AP Psychology Premium Prep 2021

Question

DNA replication occurs through a complex series of steps involving several enzymes. Which of the following represents the correct order beginning with the earliest activity of enzymes involved in DNA replication? (A) Helicase, ligase, RNA primase, DNA polymerase (B) DNA polymerase, RNA primase, helicase, ligase \(\quad\) (C) RNA primase, DNA polymerase, ligase, helicase \(\quad\) (D) Helicase, RNA primase, DNA polymerase, ligase

Step-by-Step Solution

Answer

The correct order of enzyme activities in DNA replication is (D) Helicase, RNA primase, DNA polymerase, Ligase.

Step 1: Understanding the role of different enzymes in DNA replication

In DNA replication, various enzymes work together to ensure proper and accurate replication of DNA molecules. Here, we'll briefly explain the role of each enzyme in the process: - Helicase: unwinds and separates the double-stranded DNA molecule into two single strands. - RNA primase: synthesizes short RNA fragments called primers, which serve as a starting point for DNA polymerase to add nucleotides. - DNA polymerase: adds new nucleotides to the growing DNA strand, following the template strand. - Ligase: joins the newly synthesized DNA fragments (Okazaki fragments in the lagging strand) to create a continuous DNA molecule.

Step 2: Identifying the correct order of enzyme activities

Based on the role of the enzymes mentioned above, we can determine the correct order of their activity in DNA replication: 1. Helicase: It must act first to unwind and separate the DNA strands. 2. RNA primase: After the strands are separated, RNA primase synthesizes the primers needed for DNA polymerase to start adding nucleotides. 3. DNA polymerase: Utilizing the primers, DNA polymerase must be the third enzyme in the sequence to synthesize the new DNA strand. 4. Ligase: Finally, ligase connects the synthesized DNA fragments to complete the replication process.

Step 3: Comparing the correct order with the given options

Now that we know the correct order of enzyme activities (Helicase, RNA primase, DNA polymerase, Ligase), we can compare this order to the options provided in the exercise: (A) Helicase, ligase, RNA primase, DNA polymerase (B) DNA polymerase, RNA primase, helicase, ligase (C) RNA primase, DNA polymerase, ligase, helicase (D) Helicase, RNA primase, DNA polymerase, ligase The correct order of enzyme activities matches option (D). Therefore, the answer to this exercise is (D) Helicase, RNA primase, DNA polymerase, ligase.

Key Concepts

Enzymes in DNA Replication

Enzymes play a crucial role in DNA replication, each performing a specialized task to ensure the accurate copying of genetic material. The process begins when helicase unwinds the DNA helix, creating two separate strands. Following this, RNA primase synthesizes RNA primers, which are necessary for DNA polymerase to start its work. DNA polymerase then adds nucleotides to build the new DNA strands. Lastly, ligase seals the gaps between the newly formed DNA fragments, particularly the Okazaki fragments on the lagging strand.

DNA Polymerase

DNA polymerase is the enzyme responsible for the synthesis of new DNA strands. It reads the existing DNA strand and assembles a complementary strand by adding the correct nucleotides in a sequence dictated by the original DNA template. This enzyme works in the 5' to 3' direction, meaning it can only add nucleotides to the 3' end of the newly forming strand. It's worth noting there are several types of DNA polymerases, each with a specialized function, such as repairing DNA or filling in gaps left after RNA primer removal.

Helicase

If DNA replication is thought of as building a new road, helicase is the machinery that first clears the path. It unwinds the double-stranded DNA, separating the two intertwined strands to create single-stranded templates for replication. Helicase breaks the hydrogen bonds between nucleotide pairs at what is known as the replication fork, setting the stage for the entire replication process to proceed.

RNA Primase

RNA primase sets the stage for DNA polymerase to do its job. Unlike DNA polymerase, RNA primase can start from scratch, synthesizing a short RNA segment called a primer on the DNA template strand. This primer provides a 3' hydroxyl group to which DNA polymerase can attach the first nucleotide, kickstarting the elongation of the new DNA strand. Without RNA primase, DNA polymerase would not be able to begin replication.

Ligase

Ligase is like the finishing crew in the DNA replication process. It connects Okazaki fragments on the lagging strand by forming phosphodiester bonds between separate pieces of DNA. In essence, ligase acts as a glue, healing the nicks in the sugar-phosphate backbone of the DNA and ensuring that the replication process results in a continuous and unbroken double helix.

Okazaki Fragments

Okazaki fragments are short sequences of DNA nucleotides which are synthesized discontinuously and later linked together to form the lagging strand during DNA replication. As replication is bidirectional and DNA polymerase can only synthesize in one direction, these fragments allow replication to proceed in the overall 3' to 5' direction on one strand. Understanding Okazaki fragments is essential because their synthesis and subsequent joining involve a coordination of multiple enzymes, showcasing the complexity and precision of DNA replication.