Question

How are Okazaki fragments synthesized? a. using the leading strand template, and synthesizing \(5^{\prime} \rightarrow 3^{\prime}\) b. using the leading strand template, and synthesizing \(3^{\prime} \rightarrow 5^{\prime}\) c. using the lagging strand template, and synthesizing \(5^{\prime} \rightarrow 3^{\prime}\) d. using the lagging strand template, and synthesizing \(3^{\prime} \rightarrow 5^{\prime}\)

Short answer

Okazaki fragments are synthesized using the lagging strand template and in the \(5^{\prime} \rightarrow 3^{\prime}\) direction, making the correct answer option c.

Step-by-step solution

Recall DNA replication basics

During DNA replication, the double-stranded DNA is unwound by enzymes, and each strand serves as a template for synthesizing a complementary new strand. DNA polymerases can only synthesize new DNA strands in the \(5^{\prime} \rightarrow 3^{\prime}\) direction. The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments, called Okazaki fragments.

Determine the template strand

First, we can eliminate options a and b because Okazaki fragments are synthesized on the lagging strand, not the leading strand. So, the correct answer should be either option c or option d.

Determine the direction of synthesis

Now, we have to determine the direction of synthesis for Okazaki fragments. As mentioned earlier, DNA polymerases can only synthesize DNA in the \(5^{\prime} \rightarrow 3^{\prime}\) direction. Therefore, Okazaki fragments must also be synthesized in the \(5^{\prime} \rightarrow 3^{\prime}\) direction.

Choose the correct answer

Based on our analysis, the correct answer is: using the lagging strand template and synthesizing \(5^{\prime} \rightarrow 3^{\prime}\). So the answer is option c.

Key concepts

DNA Replication

Understanding DNA replication is fundamental to grasping how living organisms transmit genetic information from one generation to the next. The replication process involves complex orchestration where enzymes unwind the DNA double helix to allow two new strands to form alongside the original strands.

Each of the two original strands acts as a template from which nucleotides are assembled to create two new complementary strands. This semi-conservative method ensures that each new DNA molecule is composed of one old and one new strand.

Central to this process is the directionality of synthesis. DNA polymerases, enzymes that synthesize DNA, can only add nucleotides in a 5^{prime} rightarrow 3^{prime}direction. This poses a particular challenge on one of the strands during replication, leading to the creation of Okazaki fragments - a concept we'll explore in more detail in the context of the lagging strand.

DNA Polymerases

At the heart of DNA replication lies a group of enzymes known as DNA polymerases. Functioning much like meticulous scribes, these enzymes are responsible for reading the original DNA strands and assembling the new ones by adding nucleotides one by one.

DNA polymerases work with remarkable precision and speed, and they have a built-in proofreading ability to correct errors during the replication process. However, their pivotal constraint is their inability to initiate the synthesis of a new strand from scratch. They require a short RNA primer, which is why primase, another enzyme, has to lay down a short RNA segment first.

Once the RNA primer is in place, DNA polymerases extend the DNA strand by adding nucleotides in the 5^{prime} rightarrow 3^{prime}direction. This limitation plays a crucial role in explaining why replication is continuous on one strand and discontinuous on the other.

Lagging Strand

While the leading strand in DNA replication is synthesized smoothly and continuously, the opposing strand, known as the lagging strand, faces a directionality dilemma. Because polymerases can only work in a 5^{prime} rightarrow 3^{prime}direction and the strands are antiparallel, the replication of the lagging strand is more piecemeal.

To overcome this issue, the lagging strand is synthesized in short segments, each requiring a fresh RNA primer. These segments, named Okazaki fragments after their discoverers, are gradually joined together by the enzyme DNA ligase to form a continuous strand. As a result, the lagging strand replication is often described as discontinuous.

Okazaki fragments are vital in comprehending why replication is not as straightforward on the lagging strand. Each fragment starts with an RNA primer and extends 5^{prime} rightarrow 3^{prime}until it reaches the primer of the preceding fragment, where it stops, allowing the next fragment to be initiated.