DNA Replication and Repair: Mechanisms and Regulation

DNA Replication and Repair

DNA Replication

DNA replication is a fundamental process in all living cells, ensuring the accurate duplication of genetic material before cell division. The process involves several key steps and enzymes to maintain fidelity and efficiency.

A. DNA Replication is Semi-Conservative

• Semi-conservative replication means that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

• This mechanism was demonstrated by the Meselson-Stahl experiment.

B. DNA Synthesis is Bidirectional

• Replication begins at specific origins on the DNA molecule.

• In prokaryotes (e.g., E. coli), replication starts at a single origin and proceeds in both directions, forming two replication forks.

• In eukaryotes, multiple origins of replication exist on each chromosome, allowing rapid and efficient DNA synthesis.

• Bidirectional replication results in the formation of two identical daughter chromosomes.

C. Unwinding DNA Strands

• Several enzymes are involved in unwinding the double helix:

  • DNA helicase: Unwinds the DNA double helix at the replication fork.

  • Topoisomerase: Relieves supercoiling ahead of the replication fork.

  • Single-stranded binding proteins (SSB): Stabilize unwound DNA strands and prevent re-annealing.

D. DNA Synthesis Proceeds in the 5' → 3' Direction

• DNA polymerase adds nucleoside triphosphates (dATP, dGTP, dCTP, dTTP) to the 3' end of the growing DNA strand.

• Two phosphates are removed from each nucleotide to provide energy for the formation of the phosphodiester bond.

• DNA synthesis always proceeds in the 5' to 3' direction.

E. The RNA Primer

• DNA polymerases cannot initiate synthesis de novo; they require a short RNA primer synthesized by primase.

• The RNA primer provides a free 3' hydroxyl group for DNA polymerase to extend.

• After elongation, the RNA primer is removed (by DNA polymerase I in prokaryotes or RNase H in eukaryotes) and replaced with DNA.

• DNA ligase seals the remaining nicks in the sugar-phosphate backbone.

F. DNA Replication has a Leading and Lagging Strand

• DNA synthesis is continuous on the leading strand (in the direction of the replication fork).

• On the lagging strand, synthesis is discontinuous, producing short fragments called Okazaki fragments.

• Okazaki fragments are later joined together by DNA ligase.

G. Proofreading

• DNA polymerase possesses 3' to 5' exonuclease activity, allowing it to remove incorrectly paired bases and replace them with the correct nucleotide.

• This proofreading function increases the fidelity of DNA replication.

Telomeres and Telomerase

Telomeres are repetitive nucleotide sequences at the ends of eukaryotic chromosomes that protect genetic material from degradation during replication.

• During replication, the ends of linear chromosomes cannot be fully replicated, leading to progressive shortening.

• Telomerase is an enzyme with an RNA component that serves as a template to extend the 3' end of the telomere, compensating for this loss.

• Telomerase activity is high in germ cells and some stem cells, but low or absent in most somatic cells.

DNA Repair

DNA repair mechanisms are essential for correcting errors that occur during replication or as a result of environmental damage, maintaining genome stability.

A. Mutations

• Mutation: A permanent change in the DNA sequence.

• Spontaneous mutations can occur during DNA replication.

• Types of mutations:

  • Tautomer formation: Rare forms of bases can mispair, leading to mutations.

  • Deamination: Loss of an amino group from a base (e.g., cytosine to uracil).

  • Depurination: Loss of a purine base (adenine or guanine).

• Mutagens can be:

  • Chemical: Base analogues, base-modifying agents, intercalating agents.

  • Radiation: UV light can cause thymine dimers; ionizing radiation can cause double-strand breaks.

B. DNA Repair Mechanisms

• Excision repair:

  • Removes damaged bases or nucleotides and replaces them with the correct sequence.

  • Types include base excision repair and nucleotide excision repair.

  • DNA polymerase fills in the gap, and DNA ligase seals the nick.

• Mismatch repair:

  • Corrects errors missed by proofreading, such as mispaired bases or small insertion/deletion loops.

  • In E. coli, the system distinguishes the newly synthesized strand by its lack of methylation.

  • MutS, MutL, and MutH proteins are involved in recognizing and repairing mismatches.

Table: Types of DNA Damage and Repair Mechanisms

Type of Damage	Cause	Repair Mechanism
Base mispairing	Replication errors	Proofreading, Mismatch repair
Thymine dimers	UV radiation	Nucleotide excision repair
Depurination/Deamination	Spontaneous hydrolysis	Base excision repair
Double-strand breaks	Ionizing radiation	Homologous recombination, Non-homologous end joining

Key Equations

• Phosphodiester bond formation (generalized):

$\text{(dNMP)}_{n} + dNTP \rightarrow \text{(dNMP)}_{n+1} + PP_i$

• Where dNMP = deoxynucleotide monophosphate, dNTP = deoxynucleotide triphosphate, $PP_i$ = pyrophosphate.

Example: Proofreading by DNA Polymerase

• If DNA polymerase incorporates an incorrect base, its 3' to 5' exonuclease activity removes the mismatched nucleotide, and synthesis resumes.

Additional info:

• Telomerase is a ribonucleoprotein reverse transcriptase.

• Defects in DNA repair mechanisms can lead to genetic diseases and cancer.