M03.02.002 Comparison of DNA and RNA synthesis

Written by

Learning Objective: Explain the roles, mechanisms, and differences between DNA and RNA polymerases and describe the functions of key enzymes involved in DNA replication with emphasis on high-yield molecular principles relevant to medical practice.

DNA Polymerases, RNA Polymerases, and Replication Enzymes

DNA replication requires coordinated synthesis of both DNA and RNA, using DNA polymerases and RNA polymerases, respectively. Although both synthesize nucleic acids in the 5′→3′ direction and read templates 3′→5′, they differ in substrate requirements, proofreading capacity, and the ability to initiate synthesis.

DNA vs RNA Polymerases

DNA Polymerase

Synthesizes DNA (5′→3′).
Reads DNA template (3′→5′).
Substrates: dATP, dGTP, dCTP, dTTP.
Requires a primer (cannot initiate synthesis).
Has 3′→5′ exonuclease proofreading → high fidelity.

RNA Polymerase

Synthesizes RNA (5′→3′).
Reads DNA template (3′→5′).
Substrates: ATP, GTP, CTP, UTP.
No primer required (can initiate synthesis).
No proofreading → more errors tolerated.

Activity

Key Replication Enzymes (Detailed)

Helicase

Unwinds dsDNA at the replication fork.
Breaks hydrogen bonds between base pairs.

Single-Stranded Binding Proteins (SSB / RPA)

Prevent re-annealing of separated strands.
Protect ssDNA from nucleases.

Topoisomerases

Relieve torsional stress from unwinding.
Topoisomerase I: Cuts 1 strand; relaxes supercoils.
Target of: irinotecan, topotecan.
Topoisomerase II (DNA gyrase in bacteria): Cuts both strands; manages supercoiling.
Target of: fluoroquinolones, etoposide, and doxorubicin.