Non-Homologous End Joining (NHEJ) | DNA Double-Strand Break Repair Animation

DNA strand breaks are categorized into single-strand breaks and double-strand breaks based on the extent of damage. Single-strand breaks occur when only one DNA strand is broken, while double-strand breaks compromise both strands simultaneously. These breaks can be caused by ionizing radiation, free radicals, and chemical reactions. Single-strand breaks are repaired through specific repair mechanisms that use the intact complementary strand as a template. Double-strand breaks are more dangerous and are repaired through homologous recombination repair and non-homologous end joining repair mechanisms.

Radiation causes two types of DNA damage with different biological consequences. Single strand breaks are generally not lethal because cells have efficient repair mechanisms (base excision repair, nucleotide excision repair, mismatch repair). One gray of radiation causes approximately 1,000 single strand breaks, which cells can repair effectively. Double strand breaks are lethal and lead to mitotic catastrophe or apoptosis. One gray causes about 40 double strand breaks on average. Cells repair double strand breaks through non-homologous end joining (error-prone) or homologous recombination (high-fidelity).

This video explains how DNA damage response mechanisms work in healthy cells versus cancer cells. In healthy cells, single-strand DNA breaks are repaired by PARP proteins, while double-strand breaks are repaired through homologous recombination using BRCA1 protein. In hereditary breast cancer patients with BRCA1 defects, double-strand breaks cannot be repaired, potentially transforming cells into cancer cells. PARP inhibitors exploit this vulnerability by blocking single-strand break repair in tumor cells, causing them to accumulate lethal double-strand breaks, while healthy cells with functional BRCA1 can still repair the damage and survive.

Alkylating agents can cause both single-strand breaks and double-strand breaks in DNA. Single-strand breaks occur when the agent modifies one strand of the DNA double helix, while double-strand breaks involve damage to both strands simultaneously. Both types of damage prevent proper DNA replication and transcription, triggering cellular repair mechanisms that may lead to apoptosis (programmed cell death) if the damage is too extensive to repair.

DNA repair is essential for maintaining genetic integrity, with two main categories of damage: double-strand breaks (requiring homologous recombination or non-homologous end joining) and single-strand damage (repaired via base excision repair, nucleotide excision repair, or direct repair). Double-strand breaks are repaired through homologous recombination (using a sister chromatid as template for accurate repair) or non-homologous end joining (direct ligation without template, which can introduce errors). Single-strand damage is addressed through three strategies: excision repair (removing damaged bases), bypass repair (continuing replication past lesions), and direct repair (reversing specific damages without removing nucleotides). Key proteins involved include Ku, XRCC4, Ligase IV, and DNA polymerase with proofreading activity.

DNA repair strategy depends critically on cell cycle phase. During G2 and S phases, sister chromatids provide templates for homologous recombination repair, which is more accurate than non-homologous end joining. A logistic switch controlled by cell cycle-dependent kinases regulates pathway choice, involving factors like BRCA1, RACK1B complex, and CTIP. These proteins undergo phosphorylation and priming during normal cell cycle progression. In response to DNA damage, additional phosphorylations occur, directing assembly of repair factors around break sites. This generates chromatin structures believed to be inaccessible for chromatin remodeling, creating a specialized environment conducive to homologous recombination. The MRN complex remains in an inactive state until activated by CTIP, which functions as both an activator and scaffolding protein to coordinate repair factor assembly.

This section covers eukaryotic DNA replication within the cell cycle and repair mechanisms. Replication occurs during S phase, with only 10-15% of cells actively replicating at any time. Origins contain conserved sequences bound by the origin recognition complex (ORC). The pre-replication complex (pre-RC) assembles with six MCM proteins during late mitosis/early G1. Activation occurs in S phase, with MCM proteins moving with the replication fork. After replication, MCM proteins are displaced and cannot be reloaded until the next cycle, ensuring each origin fires only once. Eukaryotic Okazaki fragments average 150 nucleotides. Major repair pathways include nucleotide excision repair (NER) for bulky lesions, base excision repair (BER) for damaged bases, and non-homologous end joining (NHEJ) for double-strand breaks. Defects in these pathways cause human diseases including xeroderma pigmentosum and neurodegenerative disorders.

The eukaryotic cell cycle comprises four phases: M phase (mitosis, chromosome segregation producing two daughter cells with diploid chromosomes), G1 phase (synthesis of cellular components for S phase entry), S phase (DNA replication increasing chromosome number from 23 to 46 pairs, transitioning from diploid to tetraploid state), and G2 phase (preparation for mitosis). Progression is regulated by growth factors (stimulatory signals) and checkpoints (repressive signals). The Restriction Point commits cells to completing the cycle. Four checkpoints verify DNA integrity and proper chromosome alignment. Unresolved defects trigger apoptosis, preventing damaged cells from proliferating and potentially causing cancer.

Cell cycle progression depends on cyclin-Cdk complexes and their downstream effectors. Cyclin D-CDK4 phosphorylates pRb to release E2F, enabling cyclin E synthesis for S phase entry. Cyclin E-CDK2 activates CDC45 for DNA replication. When DNA damage occurs, ATM/ATR sensors activate p53, which induces p21 CIP to inhibit cyclin D-CDK4 in G1, or Chk1 to activate CDC25 and suppress cyclin E-CDK2 in S phase. Both responses halt replication until damage is repaired. If repair fails, p53 initiates apoptosis through Bax-mediated mitochondrial disruption and caspase activation, eliminating damaged cells.

The cell cycle encompasses all events between one cell division and the next in eukaryotic cells. Its duration varies by species, cell type, and environmental conditions. The cycle comprises two main phases: interphase (growth and preparation) and the M phase (cell division). Interphase includes G1 (cell growth and metabolic activities), S (DNA synthesis and duplication), and G2 (preparation for mitosis). The M phase involves mitosis (nuclear division) and cytokinesis (cytoplasmic division). While most cells undergo mitosis, specialized gamete-producing cells perform meiosis for sexual reproduction.

DNA metabolism involves two major classes of enzymes: polymerases that synthesize nucleic acids and nucleases that degrade them. DNA polymerase adds nucleotides to the 3' end of growing strands in the 5' to 3' direction, using parental DNA as a template. Nucleases include exonucleases, which remove nucleotides from ends (3' to 5' or 5' to 3'), and endonucleases, which cut internally at specific recognition sites. These enzymes work together to build, modify, and repair genetic material throughout the cell cycle.

Three essential enzyme categories are used in RDT: nucleases for cutting DNA, polymerases for synthesis, and ligases for joining fragments. DNA polymerase requires a template strand and primer to initiate synthesis. DNA Polymerase I from E. coli has three activities: 5'→3' polymerase, 3'→5' proofreading, and 5'→3' exonuclease. The Klenow fragment is a modified version with exonuclease activity removed, ideal for RDT applications. DNA ligase catalyzes phosphodiester bond formation between DNA fragments, with T4 DNA ligase being most effective for in vitro applications. DNA end chemistry is fundamental to cloning efficiency: blunt ends require more efficient ligation than sticky ends. Modification enzymes include Alkaline Phosphatase (removes 5' phosphate to prevent self-ligation), Polynucleotide Kinase (adds 5' phosphate using ATP), and Terminal Deoxynucleotidyl Transferase (adds nucleotides to 3' ends without template). These enzymes enable precise DNA manipulation for cloning and genetic engineering applications.

DNA manipulation (genetic engineering) involves direct alteration of an organism's genome using biotechnology to introduce, edit, or remove genes. Three key enzymes enable this process: restriction enzymes (endonucleases) act as 'genetic scissors' that cut DNA at specific recognition sites, producing either blunt ends (straight cuts) or sticky ends (staggered cuts with overhanging nucleotides); DNA ligases function as 'molecular glue' that joins DNA fragments by reforming phosphodiester bonds in the sugar-phosphate backbone; and polymerases add nucleotides to build DNA or RNA strands. These enzymes, all sharing the 'ase' suffix, work together to cut, join, and replicate DNA for scientific and medical applications.

This video explains the three major categories of DNA manipulation enzymes: nucleases (which break down nucleic acids, including exonucleases that remove nucleotides from ends and endonucleases that cut within DNA strands), restriction enzymes (specific endonucleases that recognize and cut specific DNA sequences, classified into types I, II, and III based on their structure and mechanism), and DNA ligases (which join DNA fragments by forming phosphodiester bonds). The video also covers practical applications including gel electrophoresis for analyzing DNA fragments, Southern blotting for detecting specific DNA sequences, and restriction mapping for determining DNA fragment structures. Key factors affecting enzyme activity include DNA purity, buffer conditions, temperature, and the presence of methyl groups that can block restriction enzyme activity.

This section covers three essential enzymes used in molecular biology. Restriction enzymes (endonucleases) cut DNA at specific recognition sequences (4-8 nucleotides, often palindromic). Type 1 and Type 3 enzymes have restriction and methylation activities, require ATP, and cut distant from recognition sites. Type 2 enzymes only have restriction activity, don't require ATP, and cut at or near recognition sites. DNA polymerases synthesize DNA from nucleotides, classified by template and product (DNA-dependent DNA polymerase for replication, RNA-dependent DNA polymerase for reverse transcription). Key activities include 5' to 3' polymerization, 3' to 5' proofreading exonuclease, and 5' to 3' exonuclease. DNA ligase forms phosphodiester bonds between 3' hydroxyl and 5' phosphate groups. T4 DNA ligase joins both double and single-stranded DNA using ATP, while E. coli DNA ligase primarily joins double-stranded DNA using NAD+. These enzymes are fundamental for DNA replication, repair, and molecular cloning techniques.