Question

During replication of a DNA molecule, the daughter strands are not produced in exactly the same manner. One strand, the leading strand, is made toward the replication fork, while the lagging strand is made in fragments in the opposite direction. This difference in the synthesis of the two strands is the result of which of the following? a. DNA polymerase is not fast enough to make two leading strands of DNA. b. The two template strands are antiparallel, and DNA polymerase makes DNA only in the \(5^{\prime}\) to \(3^{\prime}\) direction. c. The lagging strand is the result of DNA breakage due to UV light. d. The cell does not contain enough nucleotides to make two complete strands.

Short answer

The correct answer to why the two new DNA strands are not formed in the same manner during DNA replication is option b - The two template strands are antiparallel, and DNA polymerase makes DNA only in the \(5^{\prime}\) to \(3^{\prime}\) direction.

Step-by-step solution

Understand DNA Replication

DNA replication is a biological process where a DNA molecule is copied to produce two identical DNA molecules. In this process, the two strands of the original DNA molecule are separated, serving as templates for the new strands. DNA polymerase catalyzes the synthesis of new DNA strands, always moving in the \(5^{\prime}\) to \(3^{\prime}\) direction.

Evaluate Option a

Option a suggests that DNA polymerase is not fast enough to make two leading strands. However, the speed of DNA polymerase is not the reason for the formation of the lagging strand. DNA polymerase is equally efficient in synthesizing both strands once the unwinding of DNA occurs.

Evaluate Option b

Option b states that the two template strands of DNA are antiparallel and DNA polymerase can only synthesize DNA in the \(5^{\prime}\) to \(3^{\prime}\) direction. This statement is biologically accurate. Because of the antiparallel nature of the DNA strand, the DNA polymerase can synthesize one strand continuously and the other in fragments, leading to the formation of the leading and lagging strands respectively.

Evaluate Option c

Option c is incorrect. DNA breakage due to UV light does lead to errors in DNA replication, however, it is not related to the synthesis of the lagging strand in fragments.

Evaluate Option d

Option d suggests that the cell doesn't contain enough nucleotides to form two complete strands. This is incorrect since DNA replication only proceeds when there are sufficient nucleotides present in the cell.

Key concepts

Leading and Lagging Strands

During the critical process of DNA replication, the newly formed daughter DNA strands are synthesized in distinct manners. One of these strands, known as the leading strand, is synthesized continuously towards the replication fork—a Y-shaped point where the DNA double helix is separated into two single strands during replication. In contrast, the other strand, called the lagging strand, is synthesized in short segments called Okazaki fragments, which are later joined together.

The reason behind this difference lies in the directionality of enzyme function during DNA replication. DNA polymerase, the enzyme responsible for new strand synthesis, can only add nucleotides in the 5' to 3' direction. As a result, the leading strand is assembled smoothly, as the direction of replication matches the direction of polymerase action. Conversely, the lagging strand, which runs in the 3' to 5' direction relative to the replication fork, must be synthesized in a discontinuous fashion to accommodate the directional limitation of DNA polymerase.

Understanding this concept is fundamental as it highlights the ingenious yet complex mechanisms cells use to accurately replicate their genetic material. Each newly synthesized strand is a complementary copy of the original template strand, ensuring genetic fidelity is maintained from one generation to the next.

DNA Polymerase

At the heart of DNA replication is DNA polymerase, a type of enzyme that plays a crucial role in the synthesis of new DNA strands. There are several types of DNA polymerases, each having a specific function during the replication and repair processes. DNA polymerase not only adds new nucleotide subunits to a growing DNA strand but also proofreads the newly added bases to ensure the accuracy of replication.

One key feature of DNA polymerase is its directional limitation; it can only add nucleotides to the 3' end of a growing DNA strand, hence synthesizing new DNA in the 5' to 3' direction. This function is essential for the sequential addition of nucleotides based on the original template strand. Furthermore, DNA polymerase also requires a primer—a short pre-existing nucleic acid strand or RNA segment—to begin synthesis. These primers provide a starting point with a free 3' hydroxyl group onto which the enzyme can attach the first nucleotide.

The activity of DNA polymerase is intricately coordinated within the context of the replication fork, where it ensures that each new strand is an accurate copy of the original DNA molecule. This coordination is vital for the propagation of genetic information and for maintaining the integrity of the organism's genome.

Antiparallel DNA Strands

DNA strands are inherently antiparallel, a term used to describe the orientation of the two strands of a DNA molecule relative to each other. This means that one strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction. The '5'' and '3'' ends refer to the fifth and third carbon atoms on the sugar molecule in the DNA backbone. This structure is critical for various DNA functions, including replication and transcription.

The antiparallel nature of DNA strands is central to their ability to serve as templates for replication. As the replication fork progresses, enzymes like helicases unwind the double helix, forcing the strands apart and presenting two single-stranded DNA templates running in opposite directions. DNA polymerase then adds nucleotides complementary to the templates; however, due to the unidirectional activity of the polymerase, the antiparallel structure dictates distinct replication mechanisms for each strand—the continuous leading strand synthesis and the discontinuous lagging strand synthesis.

This arrangement ensures that the DNA polymerases working on the two new strands can do so in tandem, keeping the replication process efficient and synchronized within the tight confines of the replication fork. Without the antiparallel property, the faithful replication of the entire DNA molecule would be impossible.

5' to 3' Synthesis

The 5' to 3' synthesis refers to the specific direction in which DNA polymerase adds nucleotides to synthesize a new DNA strand. Each nucleotide has three components: a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base. The phosphate group is attached to the 5' carbon of the sugar, while the 3' carbon has a hydroxyl group (-OH) that is crucial for forming the phosphodiester bond with the next nucleotide.

In DNA replication, the addition of each nucleotide occurs as DNA polymerase joins the 5' phosphate group of a new nucleotide to the 3' hydroxyl group at the end of the nascent strand. This action elongates the new strand in the 5' to 3' direction. The enzyme's ability to only work in this direction imposes a directionality constraint on the overall process of DNA replication, shaping the mechanistic differences in how leading and lagging strands are synthesized.

This directionality is a foundational aspect of DNA replication. It helps students understand the logic behind many biological phenomena related to DNA dynamics, such as the need for Okazaki fragments on the lagging strand and the role of RNA primers in initiating the synthesis of new DNA strands. Through this lens, we can appreciate the elegance and intricacy with which nature preserves and propagates genetic information.