DNA Replication: Mechanisms and Requirements

Semi-Conservative Nature of DNA Replication

DNA replication is described as semi-conservative because each daughter DNA molecule consists of one parental (original) strand and one newly synthesized strand. This mechanism ensures the faithful transmission of genetic information across generations.

• Retention of original strands: Each parental strand serves as a template for a new complementary strand.

• Experimental evidence: The Meselson-Stahl experiment demonstrated that after replication, DNA molecules contain one heavy (parental) and one light (new) strand.

• Genetic stability: This process preserves genetic information and minimizes errors due to complementary base pairing (A-T, G-C).

Requirements for DNA Replication

DNA replication requires several key components to ensure accuracy and efficiency:

• Single-stranded DNA template: Provides the sequence to be copied.

• Raw materials (dNTPs): Deoxynucleoside triphosphates serve as building blocks and energy sources.

• Enzymes and proteins: Unwind DNA, synthesize primers, assemble nucleotides, and seal nicks.

Prokaryotic DNA Replication

Mode of Replication

• Bidirectional (Theta) Replication: Replication begins at a single origin (OriC) and proceeds in both directions, forming two replication forks.

• Rolling-Circle Replication: Used by plasmids and some viruses, involving strand displacement and synthesis of a new strand.

Initiation

• Origin of replication (OriC): Contains AT-rich sequences recognized by DnaA protein, which bends and unwinds DNA.

• Helicase (DnaB): Unwinds DNA further, recruited by DnaC.

• Single-stranded binding proteins (SSBs): Stabilize unwound DNA & prevents re-annealing or secondary structure formation.

• DNA gyrase (topoisomerase II): Relieves tortional strain & supercoiling ahead of the fork caused by unwinding.

Elongation

• Primase: Synthesizes short RNA primers which provide a 3' -OH group for DNA polymerase to extend.

• DNA polymerase III: Main enzyme for DNA elongation, highly processive due to the β-clamp that secures DNA polymerase to the template strand. adds dNTPs to the 3'-OH of the RNA primer

• Leading strand: Synthesized continuously toward the fork. (5'-3')

• Lagging strand: Synthesized discontinuously as Okazaki fragments, each initiated by a new primer. (5'-3')

• DNA polymerase I: Removes RNA primers from okazaki fragments and replaces them with DNA.

• DNA ligase: Joins Okazaki fragments by forming phosphodiester bonds

the replisome is a protein complex at the replication fork that coordinates helicase, primase, DNAA polymerase and other factors

Termination

• Ter sites bound by Tus proteins: Terminate replication by blocking helicase activity.

• Topoisomerase IV: separates interlinked daughter chromosomes.

• Proofreading by DNA polymerase III and mismatch repair : Ensure high fidelity (error rate ~1 in 10⁹ bp).

Eukaryotic DNA Replication

Mode of Replication

• Bidirectional replication from multiple origins: Ensures timely duplication of large, linear chromosomes.

Initiation

• Replication licensing: Ensures replication only occurs once per cell cycle. Key factors include Cdc6, Cdt1, and the MCM2-7 helicase complex loaded onto origins during G1 phase of cell cycle .

• Activation: Cyclin-dependent kinases (CDKs) phosphorylate licensing factors and helicase to initiate replication.

Unwinding

• helicase (MCM complex): separates two strands creating a replication bubble

• Replication protein A (RPA): binds to single stranded DNA to stabilize it and prevent re-annealing or formation of secondary structures

• Topoisomerase: relieves supercoiling and tortional strain caused by unwinding ahead of the replication fork.

Elongation

• DNA polymerase α-primase: Synthesizes RNA primer and short DNA stretch. This primer proves a 3'-OH group for further DNA synthesis.

• DNA polymerase ε: Synthesizes leading strand continuously towards the replication fork (5'-3').

• DNA polymerase δ: Synthesizes lagging strand (Okazaki fragments) away from replication fork (5'-3').

• PCNA (sliding clamp): Ensures processivity of polymerases.

• RFC (clamp loader): Loads PCNA onto DNA Ensuring processivity of polymerases..

• Histone chaperones (CAF-1, ASF1): Assemble nucleosomes on new DNA.

Primer Removal and Gap Filling

• RNase H and FEN1: Remove RNA primers from Okazaki fragments.

• DNA polymerase δ: Extends DNA to fill gaps left by primer removal

• DNA ligase I: Joins Okazaki fragments by forming phosphodiester bonds.

Termination

• Telomeres: Repetitive sequences at chromosome ends that prevent loss of genetic information, maintained by telomerase in germ, stem, and cancer cells.

• Proofreading and mismatch repair: DNA polymerases ε and δ have 3'→5' exonuclease activity to remove mismatched nucleotides; mismatch repair corrects errors missed by polymerase proofreading post-replication.

Comparison Table: Prokaryotic vs. Eukaryotic DNA Replication

DNA Replication Errors and Mutations

Sources of Replication Errors

• Mismatched base pairing: Incorrect nucleotide incorporation.

• Slippage in repetitive sequences: Causes insertions or deletions (indels).

• Chemical modifications: Oxidative stress, alkylation, deamination.

• External mutagens: UV radiation, chemicals, viruses.

Types of Base Mutations

• Point mutations: Single nucleotide changes.

• Transition mutations: Purine ↔ Purine or Pyrimidine ↔ Pyrimidine (A↔G, C↔T).

• Transversion mutations: Purine ↔ Pyrimidine (A↔C, G↔T). likely to cause structural changes in DNA and functional changes in the resulting protein

• Insertions/Deletions (Indels): Addition or removal of nucleotides, may cause frameshifts resulting in different amino acid sequences

• Expanding nucleotide repeats: Increase in repeat number (e.g., CAG in Huntington's disease). disrupts normal gene function.

• Missense mutation: Base substitution changes amino acid. can alter protein function

• Nonsense mutation: Base substitution creates a stop codon leading to a non-functional protein.

• Silent mutation: Codon change does not alter amino acid.

• Neutral mutation: Amino acid change does not affect protein function.

• Loss-of-function mutation: Reduces/eliminates protein activity. can result in nonsense mutations, frameshifts or missense mutations that impair protein structure

• Gain-of-function mutation: Protein acquires new/enhanced function. often caused my missense mutations or gene duplications

Summary Table: Types of Mutations

DNA Repair Mechanisms

Mismatch Repair (MMR)

Corrects errors that escape proofreading, such as mismatched bases and small indels.

• Detection: MutS (prokaryotes) or MSH (eukaryotes) recognize mismatches.

• Strand discrimination: Newly synthesized strand identified by lack of methylation (prokaryotes) or presence of nicks (eukaryotes).

• Excision and resynthesis: Exonuclease removes error; DNA polymerase and ligase fill and seal the gap.

Direct Repair

Reverses specific DNA damage without removing bases.

• Photoreactivation: Photolyase repairs UV-induced pyrimidine dimers using light energy.

• Alkyl transferase: Removes alkyl groups from guanine (e.g., O6-methylguanine).

Base Excision Repair (BER)

Repairs small, non-helix-distorting lesions (e.g., deaminated, oxidized, or alkylated bases).

• DNA glycosylase: Removes damaged base, creating an abasic site.

• AP endonuclease: Cleaves backbone at abasic site.

• DNA polymerase and ligase: Fill and seal the gap.

Nucleotide Excision Repair (NER)

Removes bulky, helix-distorting lesions (e.g., thymine dimers, chemical adducts).

• Damage recognition: GG-NER (global genome) or TC-NER (transcription-coupled).

• Strand incision: Endonucleases cut on both sides of lesion.

• Excision and resynthesis: DNA polymerase fills gap; ligase seals nick.

Repair of Double-Strand Breaks (DSBs)

• Homology-Directed Repair (HDR): Uses homologous sequence as template for accurate repair. High fidelity.

• Non-Homologous End Joining (NHEJ): Directly joins DNA ends, error-prone due to possible nucleotide loss.

Additional info:

• Replication factories in eukaryotes are discrete nuclear sites where replication proteins are concentrated.

• Proper chromatin assembly after replication is essential for gene regulation and genome stability.