DNA Synthesis

Overview of DNA Synthesis

DNA synthesis is the process by which new DNA strands are created, essential for cell division and genetic inheritance. The process relies on specific enzymes and follows a defined directionality and set of requirements.

• DNA Polymerases are the primary enzymes responsible for building new DNA strands.

• Organisms contain different types of DNA polymerases, each with slightly different functions.

• New DNA strands are always built in the 5' to 3' direction (elongation occurs from the 5' phosphate group to the 3' hydroxyl group).

• Template strand: The old DNA strand serves as a template for the synthesis of the new strand.

Example: During DNA replication, the enzyme DNA polymerase adds nucleotides to the 3' end of the growing DNA strand, using the template strand to ensure correct base pairing.

DNA Polymerase Requirements

DNA polymerases require specific conditions and molecules to initiate and elongate new DNA strands.

• In prokaryotes, DNA Polymerase III is the primary enzyme for elongating/building new DNA strands.

• All DNA polymerases have two central requirements:

  1. A template strand (parent DNA) that acts as a guide for building new strands.

  2. A primer: a small RNA molecule that acts as a starting point for DNA polymerase.

• Primase is the enzyme that builds RNA primers.

• Ultimately, the RNA primer is converted to DNA to be part of the newly built DNA strand.

Example: Primase synthesizes a short RNA primer, which DNA polymerase III then extends by adding DNA nucleotides.

Directionality of DNA Synthesis

DNA synthesis always proceeds in the 5' to 3' direction. This is due to the chemical structure of DNA and the mechanism of nucleotide addition.

• The 3' hydroxyl group of the growing strand attacks the 5' phosphate of the incoming nucleotide.

• This ensures that DNA polymerase can only add nucleotides to the 3' end.

Equation:

Example: DNA polymerase cannot initiate synthesis without a primer because it requires a free 3' hydroxyl group.

Key Enzymes in DNA Synthesis

• DNA Polymerase III: Main enzyme for DNA strand elongation in prokaryotes.

• Primase: Synthesizes RNA primers required for DNA polymerase to begin synthesis.

• RNA Primer: Short segment of RNA that provides a starting point for DNA synthesis.

Table: Comparison of Key Enzymes in DNA Synthesis

Additional info: In eukaryotes, multiple DNA polymerases are involved in replication, including DNA polymerase α, δ, and ε. The RNA primer is later removed and replaced with DNA by other enzymes such as DNA polymerase I and DNA ligase.