Question

Which of the following enzymes is NOT involved in DNA replication? a. Primase b. DNA ligase c. RNA polymerase I d. Telomerase

Short answer

RNA polymerase I

Step-by-step solution

Identify Function of Each Enzyme

List the role of each enzyme in DNA replication.1. Primase: Synthesizes RNA primer to start DNA replication.2. DNA ligase: Seals nicks in the DNA backbone, joining Okazaki fragments on the lagging strand.3. RNA polymerase I: Synthesizes rRNA in the nucleolus, not involved in DNA replication.4. Telomerase: Adds repetitive sequences to the ends of chromosomes, important in replicating telomeres.

Analyze Each Enzyme's Role

Consider each enzyme's role in the replication process.1. Primase is essential for starting DNA synthesis.2. DNA ligase is crucial for joining DNA fragments during replication.3. RNA polymerase I is involved in rRNA synthesis, not DNA replication.4. Telomerase extends the telomeres to protect DNA ends during replication.

Determine the Enzyme Not Involved

Based on the analysis, identify the enzyme that does not participate in DNA replication.RNA polymerase I does not play a role in the actual process of DNA replication, it is involved in synthesizing rRNA.

Key concepts

DNA replication

DNA replication is the fundamental process by which a cell duplicates its DNA. This process ensures that each daughter cell receives a complete set of genetic information. The process starts at specific locations in the genome called origins of replication, where the DNA double helix is unwound. Several enzymes work together to replicate the DNA strands.
Key players in DNA replication include:

• Helicase: Unwinds the DNA strands.
• Primase: Synthesizes RNA primers.
• DNA polymerase: Adds nucleotides to the growing DNA strand.
• DNA ligase: Joins discontinuous DNA fragments (Okazaki fragments).
• Telomerase: Extends the ends of chromosomes.

Understanding DNA replication is crucial for grasping how genetic information is passed down from one generation to the next.

RNA polymerase I

RNA polymerase I is an enzyme that synthesizes ribosomal RNA (rRNA) in the nucleolus. Unlike the enzymes involved in DNA replication, RNA polymerase I does not play a role in duplicating DNA. Instead, it focuses on transcribing rRNA, which is essential for ribosome assembly and protein synthesis in the cell.
Here's a brief overview:

• RNA polymerase I synthesizes rRNA specifically within the nucleolus.
• rRNA forms the core structure of ribosomes and assists in translating mRNA into proteins.

This enzyme is integral to protein synthesis but is not a participant in the DNA replication process. Its primary function lies in the nucleus but separate from the replication activities.

Primase

Primase plays a critical role in the DNA replication process. This enzyme synthesizes short RNA primers which are necessary for DNA polymerases to start DNA synthesis. Without primers, DNA polymerases cannot initiate the replication process.
Key points about primase:

• It creates short RNA primers on the DNA template strand.
• These primers provide a 3' hydroxyl group that DNA polymerase needs to add nucleotides.
• The RNA primers are later removed and replaced with DNA.

Primase essentially 'primes' the DNA template, making it possible for the replication machinery to build the new DNA strand.

DNA ligase

DNA ligase is essential in the DNA replication process, especially on the lagging strand. During replication, the lagging strand is synthesized in short fragments known as Okazaki fragments.
DNA ligase's role includes:

• Sealing nicks in the DNA backbone by forming phosphodiester bonds.
• Joining Okazaki fragments to create a continuous DNA strand.

By sealing these nicks, DNA ligase ensures the integrity and stability of the newly synthesized DNA. Without this enzyme, the DNA fragments would remain disconnected, leading to genomic instability.

Telomerase

Telomerase is an enzyme that helps maintain the length of telomeres, the repetitive DNA sequences at the ends of chromosomes. During DNA replication, the very ends of linear chromosomes cannot be completely replicated by standard DNA polymerases, leading to progressive shortening.
Telomerase combats this issue by:

• Adding telomeric repeats to the ends of chromosomes.
• Using an RNA template within the enzyme to extend the telomere DNA.
• Helping to protect the chromosome from degradation and fusion with neighboring chromosomes.

Telomerase activity is particularly important in stem cells and germ cells, where it helps maintain genomic integrity over many cell divisions. This enzyme plays a crucial role in ensuring that the entire genome is accurately replicated.