DNA Replication

DNA Replication

Overview

• DNA replication involves copying DNA, a process that occurs during the S phase of the cell cycle.

Basic Steps

1. Double-stranded DNA to Single-stranded DNA:
   • The initial double-stranded DNA must be converted into single strands.
2. Enzyme Requirement:
   • DNA polymerase is the enzyme that synthesizes new DNA.
3. Monomers (Nucleotides):
   • Individual nucleotides are required and are available within the cell.

Detailed Process

1. Denaturation/Unzipping:
   • Double-stranded DNA is denatured (or "unzipped") into two single strands.
2. Template Strands:
   • The original DNA strands serve as templates.
   • Example:
     • Template Strand 1: 5' - ATCGCT - 3'
     • Complementary Template Strand 2: 3' - TAGCGA - 5'
3. DNA Polymerase Action:
   • DNA polymerase moves along the template strand in a 3' to 5' direction.
   • It facilitates the association of complementary nucleotides.
   • It catalyzes the formation of covalent bonds between these nucleotides.
4. Outcome:
   • Two new DNA strands are created, each consisting of an old template strand and a newly synthesized strand.

Chromosomal Perspective

• In G1 or G0 phase, a chromosome consists of one double-stranded DNA.
• After the S phase (in G2), each chromosome has two chromatids:
  • Each chromatid contains one old (template) strand and one new strand.
• This is termed semi-conservative replication because each new DNA molecule contains one original and one newly synthesized strand.

Bacterial Chromosomes

• Bacteria lack a cell cycle like eukaryotes.
• Bacterial chromosomes are circular DNA.
• The origin of replication (ORI) is a specific site where replication begins.
• After replication, bacteria have two loops of DNA.

DNA Polymerases: Detailed Characteristics

• There are multiple types of DNA polymerases. We'll discuss two.

Key Points:

1. Directionality:

   • DNA polymerase moves along the template in a 3' to 5' direction.

2. Requirement for Existing Nucleic Acid:

   • DNA polymerase requires existing nucleic acid on the five prime side to attach new nucleotides.
   • It needs something to attach nucleotides to; it can’t just start from scratch.
   • Example:
     • Template: 3' - ATG - 5'
     • If there's nothing on the 5' side, DNA polymerase can't add a complementary base (C) to the 3' end.
     • Requires a pre-existing nucleic acid for attachment.

3. Covalent Bond Formation Limitation:

   • DNA polymerase cannot form a covalent bond on the five prime side of a nucleotide.

   • If there's pre-existing DNA, DNA polymerase can't connect the new strand to it.

     • New DNA: 5' - T A - 3'
     • Existing DNA: 5' - C - 3'
     • DNA polymerase can't catalyze the formation of the covalent bond between A and G.

Summary of DNA Polymerase Idiosyncrasies:

1. Moves in a 3' to 5' direction on the template.
2. Needs existing nucleic acid on the five prime side to attach new nucleotides.
3. Cannot form a covalent bond with the five prime side of a nucleotide when encountering existing DNA.