M03.02.02 Comparison of DNA and RNA synthesis

Understanding the synthesis of DNA and RNA is crucial for comprehending genetic transmission and expression. This section outlines the processes, similarities, and differences between DNA and RNA synthesis, which are essential for various cellular functions and medical applications.

Key Concepts

• DNA Synthesis: Also known as DNA replication, involves the creation of a new DNA strand from an existing template.
• RNA Synthesis: Also known as transcription, involves creating an RNA strand from a DNA template.

Table I-2-1: Comparison of DNA and RNA Polymerases

*Certain DNA and RNA polymerases require RNA templates, commonly associated with viruses.

Similarities Between DNA and RNA Synthesis

1. Directionality:
   • Both processes synthesize the new strand in the 5′→3′ direction.
   • The template strand is scanned in the 3′→5′ direction.
2. Strand Characteristics:
   • The newly synthesized strand is complementary and antiparallel to the template strand.
3. Mechanism:
   • Each new nucleotide is added when the 3′ hydroxyl group of the growing strand reacts with a nucleoside triphosphate (NTP), base-paired with the template strand.
   • Pyrophosphate (PPi) is released during the addition of each nucleotide.

Differences Between DNA and RNA Synthesis

1. Substrates:
   • DNA synthesis uses deoxynucleoside triphosphates (dNTPs): dATP, dGTP, dCTP, dTTP.
   • RNA synthesis uses nucleoside triphosphates (NTPs): ATP, GTP, CTP, and UTP.
2. Bases:
   • DNA contains thymine (T), whereas RNA contains uracil (U).
3. Primer Requirement:
   • DNA polymerases require a primer (RNA or DNA) to initiate synthesis.
   • RNA polymerases do not require a primer and can initiate synthesis de novo.
4. Proofreading:
   • DNA polymerases have proofreading activity (3′→5′ Exonucleases) to correct mistakes.
   • RNA polymerases lack proofreading ability, leading to lower fidelity in RNA synthesis.

Figures and Charts

Figure I-2-1: DNA Replication by a Semi-Conservative, Bidirectional Mechanism

Figure I-2-2: Eukaryotic Chromosome Replication During S-Phase

Points to Remember

• DNA Replication: High-fidelity process requiring primers and proofreading.
• RNA Synthesis: Lower-fidelity process that does not require primers and lacks proofreading.
• Template Scanning: Both processes involve scanning the template strand in the 3′→5′ direction to synthesize a new strand in the 5′→3′ direction.

Bibliography

1. Molecular Biology of the Cell by Alberts B, Johnson A, Lewis J, et al. – Detailed mechanisms of DNA and RNA synthesis.
2. Lehninger Principles of Biochemistry by Nelson DL, Cox MM – Biochemical principles of nucleic acid synthesis.
3. Medical Genetics by Jorde LB, Carey JC, Bamshad MJ – Implications of nucleic acid synthesis in genetics and inherited diseases.