NHEJ Pathway Overview

NHEJ Pathway

• NHEJ pathway is responsible for joining broken DNA ends, regardless of the sequence.
• It is not lymphocyte-specific.
• Plays a crucial role in V(D)J recombination for generating antibody diversity.
• Responsible for joining ends of protein-coding segments to create composite coding sequences for heavy and light chains in antibodies.
• Also involved in T-cell receptor production.

Importance of NHEJ

• Inherited syndromes causing hypersensitivity to radiation and immunodeficiency (SCID) are linked to mutations in NHEJ pathway genes.
• This highlights the importance of NHEJ in maintaining genetic stability and immune function.

NHEJ in Bacteria

• Found in bacteria, though less frequent than in eukaryotes.
• Bacillus subtilis spores use NHEJ for repairing DNA breaks during germination.
• This system involves a Ku-like protein and DNA ligase packaged within the spore.
• NHEJ in spores helps them survive extreme environmental conditions.

Homologous Recombination (HR)

• Error-free mechanism for repairing double-strand breaks after DNA replication.
• Utilizes sister chromatids as templates ensuring accurate repair.

Synthesis-Dependent Strand Annealing (SDSA)

• A key step in HR.
• Broken ends are bound and trimmed by specialized proteins.
• Exposed single-stranded regions are coated by proteins including RecA (E.coli) or Rad51 (eukaryotes).
• Rad51 forms filaments that search for complementary sequences on the undamaged sister chromatid.
• Strand invasion and D-loop formation allows priming of DNA synthesis from the 3' end of the invading strand.
• Newly synthesized DNA replaces the damaged region, ensuring accurate repair.

Rad51 in HR

• Facilitates search and alignment of homologous sequences during SDSA.
• Forms filaments on single-stranded DNA that bind to the undamaged sister chromatid.

Proteins involved in HR

• Rad51 plays a crucial role in searching and aligning homologous sequences.
• Other proteins involved in trimming, binding, and strand invasion contribute to the precise repair process.

Meiotic Recombination

• Similar to SDSA, but instead of searching the sister chromatid, Rad51-Dmc1 filament searches a nonsister chromatid from the homologous chromosome.
• This leads to strand invasion, D-loop formation, and chiasma formation in meiosis I.
• This recombination event connects the homologous chromosomes during meiosis.

Translesion DNA Synthesis (TLS)

• Error-prone mechanism used to bypass replication blocks caused by DNA damage.
• In E.coli, the SOS system is activated in response to damage, allowing replication to continue despite the presence of lesions.

Steps in TLS in E.coli

• UV light induces RecA protein synthesis.
• Stalled replication polymerase allows for unwinding ahead of the polymerase, exposing single-stranded DNA.
• Single-strand binding (SSB) proteins bind to the exposed DNA.
• RecA proteins join the SSB proteins to form a protein-DNA filament, activating the SOS response.
• The filament induces genes encoding bypass polymerases, which can bypass replication blocks.

Bypass Polymerases

• Tolerate damaged bases that stall regular replicative polymerases.
• Possess larger pockets that can accommodate damaged bases.
• Have higher error rates than replicative polymerases due to lack of proofreading activity.
• Can only add a few nucleotides before detaching, minimizing the accumulation of errors.

Eukaryotic Bypass Polymerases

• Include Pol ζ (zita), Rev1, Pol κ (kappa), Pol η (ita), and Pol ι (iota).
• Access to DNA is regulated to ensure their use only when needed.

Monoubiquitination of PCNA

• Regulates binding of bypass polymerases in eukaryotes.
• Adding a single ubiquitin molecule to PCNA changes its conformation, allowing it to bind bypass polymerases.
• This process facilitates TLS.

Xeroderma Pigmentosum (XP)

• Autosomal recessive genetic disease.
• Characterized by abnormal skin pigmentation and extreme sensitivity to sunlight.
• Caused by defects in nucleotide-excision repair (NER).
• Leads to accumulation of pyrimidine dimers in DNA, increasing the risk of skin cancer.

Explore the NHEJ pathway, its crucial role in DNA repair, and its significance in V(D)J recombination for antibody diversity. Learn about its impact on genetic stability, immune function, and its presence in bacteria. This quiz covers essential information related to genetic processes and immune responses.