DNA Repair Mechanisms in Molecular Biology

Mutations

Changes in the normal DNA sequence.

Sex cells

Mutations affect progeny negatively.

Somatic cells

Mutations can lead to tumors or cancer.

DNA damage rate

10,000-100,000 damages per day per cell.

Strand slippage

Looping out during replication causes nucleotide changes.

Point Mutations

Change in a single base pair in DNA.

STOP codons

Terminate protein synthesis prematurely.

Spontaneous deamination

Conversion of C to U or T.

Substitution

Exchanges one base for another in DNA.

Sickle cell anemia

Caused by a substitution in beta-hemoglobin gene.

Silent mutations

No change in protein despite codon change.

DNA Excision Repair System

Repairs damaged DNA by excising segments.

Base excision repairs

Selective removal of mismatched bases.

DNA glycosylase

Enzyme that removes damaged base from DNA.

Apurinic endonuclease I

Cuts DNA backbone at apurinic sites.

Deoxyribose phosphate removal

Cleared by AP lyase associated with DNA polymerase.

Mismatch Excision Repair

Corrects mismatches post-replication.

Methylation detection

Used by E. coli to identify new DNA strand.

MSH2 and MSH6

Proteins that bind to mispaired DNA segments.

Nucleotide Excision Repair

Fixes distortions caused by chemically modified bases.

T-T dimers

DNA damage caused by UV light exposure.

XP-C protein

Detects DNA distortions in nucleotide excision repair.

XP-G

Endonuclease that cuts damaged DNA strands.

XP-F

Second endonuclease involved in DNA repair.

DNA polymerase

Fills gaps in DNA during replication.

DNA ligase

Seals nicks in DNA after replication.

Nonhomologous End Joining

Repairs double-strand breaks without sister chromatids.

Ku protein

Binds to double-strand break ends in repair.

DNA-PK

Protein kinase involved in DNA repair processes.

Synapse formation

Bringing together broken DNA ends for repair.

Nucleases

Enzymes that process DNA ends by removing bases.

Ligation

Joining two double-stranded DNA molecules together.

Genetic recombination

Mechanisms altering DNA sequences for diversity.

General recombination

Exchange between homologous DNA sequences during meiosis.

Site-specific recombination

Targeted genetic rearrangements at specific DNA sites.

Holliday junction

Crossed-strand structure formed during homologous recombination.

RecA

Central protein promoting strand exchange in recombination.

RecBCD enzyme

Initiates recombination by unwinding double-stranded DNA.

RAD51

ATPase facilitating homologous DNA sequence invasion.

Strand exchange

Process where DNA strands swap segments during repair.

Collapsed replication fork

Structure needing repair during DNA replication.

Crossing over

Exchange of genetic material between homologous chromosomes.

Meiosis

Cell division process generating genetic diversity.

Nucleotide sequences

DNA segments that can be joined during repair.

Nick

Break in phosphodiester backbone of DNA.

Replication Fork

Y-shaped structure during DNA replication.

Exonucleolytic Digestion

Removal of nucleotides from DNA ends.

Single-Stranded DNA

DNA with only one strand present.

Strand Invasion

Process where single-stranded DNA invades double-stranded DNA.

RecA

Protein facilitating strand invasion in bacteria.

Rad51

Protein facilitating strand invasion in eukaryotes.

Branch Migration

Movement of hybrid DNA region during recombination.

Holliday Structure

Intermediate structure formed during homologous recombination.

Ligation

Joining of DNA ends after cleavage.

Leading Strand

DNA strand synthesized continuously during replication.

Lagging Strand

DNA strand synthesized discontinuously in fragments.

Double-Strand Break (DSB)

Severe DNA damage involving both strands.

Presynapsis

Stage where DSB ends are recognized and processed.

Synapsis

Stage where strand invasion forms D-loop.

Synthesis-Dependent Strand Annealing (SDSA)

Process leading to localized conversion without crossover.

Break-Induced Replication (BIR)

Mechanism copying distal chromosome part post-DSB.

Double Holliday Junction

Structure formed during double-strand break repair.

Resolvas

Enzymes that process Holliday junctions into products.

Non-Crossover Products

Result of Holliday junction resolution without exchange.

Loss of Heterozygosity (LOH)

Loss of different alleles at a locus.

TOPOIIIα

Enzyme involved in dissolving hemi-catenanes.