Question

Which of the following enzymes involved in DNA replication is unique to eukaryotes? a. helicase b. DNA polymerase c. ligase d. telomerase

Short answer

The enzyme unique to eukaryotes involved in DNA replication is telomerase.

Step-by-step solution

Understand the context

First, it's important to know that DNA replication is a essential process that happens in both prokaryotic and eukaryotic cells. However, eukaryotic cells have some unique challenges due to their linear chromosomes. This includes the end-replication problem where the very ends of chromosomes, known as telomeres, are difficult to replicate using the normal machinery.

Identify the function of each enzyme

Helicase unwinds the DNA double helix. DNA polymerase synthesizes new strands of DNA complementary to the original strands. Ligase joins fragments of DNA together. Telomerase lengthens telomeres at the ends of eukaryotic chromosomes.

Determine the unique eukaryotic enzyme

Helicase, DNA polymerase, and ligase are required and present in both eukaryotic and prokaryotic cells. Telomerase, however, is unique to eukaryotes because it extends the telomeres at the end of the linear chromosomes, solving the end-replication problem that is specific to eukaryotic cells.

Key concepts

Telomerase

One of the crucial components in DNA replication for eukaryotic cells is the enzyme telomerase. Its role is distinct because it addresses a specific challenge in these cells—the end-replication problem.

During replication, the enzymes that copy DNA cannot continue all the way to the very end of linear chromosomes. This is where telomerase steps in; it adds repetitive nucleotide sequences to the ends, known as telomeres, compensating for the inability of the usual replication machinery to fully copy the ends. Without telomerase, with each cell division, chromosomes would progressively shorten, which could lead to the loss of important genetic information over time and contribute to aging and cell death.

The existence of telomerase allows for the maintenance of telomere length, which is particularly important in rapidly dividing cells such as stem cells and in cancer cells, where telomerase can be overactive, contributing to the uncontrolled growth characteristic of tumors.

End-replication Problem

When we delve into the end-replication problem, we face a fundamental limitation of DNA replication in eukaryotic organisms. This problem arises because the enzymes responsible for DNA replication, DNA polymerases, can only synthesize DNA in one direction—from the 5' to 3' end.

At the end of linear chromosomes, the lagging strand synthesis leaves a small gap after removing the RNA primer, as there's no upstream DNA for the next primer to attach to. If not addressed, this would result in a shortening of the chromosome with each successive round of cell division. This poses a risk to genetic stability and cellular function.

However, in eukaryotic cells, telomerase action solves this problem, underscoring its crucial role in maintaining the integrity of the genome over an organism's lifespan. Without it, cells would age faster, leading to early senescence or apoptosis.

Eukaryotic Chromosome Structure

Understanding eukaryotic chromosome structure is pivotal to grasp how DNA replication occurs in more complex cells. Eukaryotic chromosomes are linear and consist of DNA wound around histone proteins, forming nucleosomes; this is further folded into a highly organized structure known as chromatin.

At each end of the chromosome is a telomere, a region of repetitive nucleotide sequences that protect the ends of the chromosome from deterioration and from fusion with neighboring chromosomes. Telomeres play an essential role in ensuring the complete replication of chromosomes and prevent the loss of essential genes. The unique structure of eukaryotic chromosomes requires the use of telomerase to maintain stability and health of the cells over time, contrasting with the circular chromosomes of prokaryotes which do not face the end-replication problem.