لخّصلي

Definition of DNA Repair DNA repair refers to a collection of processes by which a cell identifies and corrects damage to its DNA molecules.It involves the alignment of the broken DNA ends with the homologous sequence, followed by strand invasion and synthesis.DNA can be damaged by various factors including environmental agents (like UV light, chemicals), normal metabolic processes, and errors that occur during DNA replication.o Repair of Damage Caused by UV Light UV light can induce the formation of pyrimidine dimers (most commonly thymine dimers), which distort the DNA helix.Repair mechanisms include:

• Nucleotide Excision Repair (NER): As mentioned, NER is the primary mechanism for repairing UV-induced damage.Homologous Recombination (HR): This is a more accurate repair mechanism that requires a homologous sequence (usually a sister chromatid) as a template for repair.Types of DNA Repair o Nucleotide Excision Repair (NER) Nucleotide excision repair is a mechanism that removes a wide range of DNA lesions, including those caused by UV light and chemical exposure.o Base Excision Repair (BER) Base excision repair is a repair pathway that specifically corrects small, non-helix-distorting base lesions.The process involves: 1.2.3.4.2.3.4.2.

Definition of DNA Repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to its DNA molecules. These processes are crucial for maintaining the integrity of the genetic information, preventing mutations, and ensuring proper cellular function. DNA can be damaged by various factors including environmental agents (like UV light, chemicals), normal metabolic processes, and errors that occur during DNA replication.

Types of DNA Repair
• Nucleotide Excision Repair (NER)
Nucleotide excision repair is a mechanism that removes a wide range of DNA lesions, including those caused by UV light and chemical exposure. The process involves:

1. Recognition of the damaged DNA segment.


2. Excision of a short single-stranded DNA segment containing the damage.


3. Synthesis of new DNA to fill the gap, using the complementary strand as a template.


4. Ligation to restore the DNA strand continuity.
   • Base Excision Repair (BER)
   Base excision repair is a repair pathway that specifically corrects small, non-helix-distorting base lesions. The steps involved include:


5. Recognition and removal of the damaged or incorrect base by a DNA glycosylase enzyme.


6. Cleavage of the DNA backbone at the site of the missing base.


7. Insertion of the correct base by a DNA polymerase.


8. Ligation of the DNA strand to restore its integrity.
   • Repair of Damage Caused by UV Light
   UV light can induce the formation of pyrimidine dimers (most commonly thymine dimers), which distort the DNA helix. Repair mechanisms include:

• 
  Nucleotide Excision Repair (NER): As mentioned, NER is the primary mechanism for repairing UV-induced damage.


• 
  Photoreactivation: Some organisms possess a photolyase enzyme that can directly reverse thymine dimers upon exposure to light.
  • Repair of Double-Strand Breaks (DSBs)
  Double-strand breaks are severe forms of DNA damage that can lead to genomic instability. There are two main repair mechanisms:

1. 
   Non-Homologous End Joining (NHEJ): This is a quick repair process that directly ligates the broken ends of DNA together. It does not require a template and is more error-prone.


2. 
   Homologous Recombination (HR): This is a more accurate repair mechanism that requires a homologous sequence (usually a sister chromatid) as a template for repair. It involves the alignment of the broken DNA ends with the homologous sequence, followed by strand invasion and synthesis.
   These DNA repair mechanisms are vital for the maintenance of genetic stability and prevention of diseases such as cancer.