3. DNA Damage And Repair Flashcards
Mutation
Describes both a process that produces altered DNA sequences and the outcome of that change
Can have diff consequences:
Normal phenotype (height) Disease phenotype No obvious effect on phenotype Very rare, beneficial effect
Originat as a result of changes in our DNA that are not corrected by cellular DNA repair systems
Changes are occasionally are induced by radiation and chemicals in our environment - MOSTLY arise from endogenous sources
Scale of Genetic variation and its consequences
Change in copy number of whole nuclear DNA molecules are almost always harmful (trisomy, deletion)
Most common are SNP on small scale (point mutations) and have no obvious effect on phenotype - neutral mutations
Single nucleotide variants / polymorphism
Most common type of genetic variation in human genome
Account for 75% of DNA changes
1 SNP per 1000 base pairs - 3 million SNPs per individual
Functional genetic variation
ABO blood group - different surface antigens on RBCs
In a normal person a number of genes are inactivated
Most genetic variation has a neutral effect on phenotype but a small fraction are harmful
Examples of functional genetic variation
ABO blood group
Immune system:
Genes of immune system are polymorphic - undergo somatic rearrangements to produce different variants
Genes involved in identifying microbial pathogens - constant positive selection to maximize diversity in proteins involved in antigen recognition
Origins of DNA sequence variation
Recombination - hot spot regions where recombination is more likely such as in subtelomeric regions
Independent assortment of paternal and maternal homologs
Various mutational elements
Endogenous chemical damage to DNA
Chemical damage to DNA caused by external mutagens
DNA replication errors
Chromosome segregation and recombination errors
Endogenous chemical damage to DNA
Hydrologic damage -
Disrupt covalent bonds that hold bases to sugars, cleaving that base from sugar to produce abasic site (depurination)
Oxidative damage -
from normal cellular metabolism, most significant are superoxide anions (O2-) hydrogen peroxide (H2O2) and hydroxyl radicals (OH-)
Aberrant DNA methylation
Inappropriate methylation by SAM to methylated DNA to produce harmful bases
Chemical damage to DNA by external mutagens
UV radiation (sunlight) -
covalent bonding between pyrimidines
High energy irradiation (x-rays) -
generate ROS - breaking DNA strands
Mutagenic chemicals (cigarette smoke, automobile fumes) -
bulky DNA adducts - distortion of the double helix
DNA repair single strand
minor DNA damage - an altered base
DNA cross linking - may block DNA replication or transcription
Base excision repair (BER)
DNA mismatch repair (MMR)
Nucleotide excision repair (NER)
Base excision repair (BER)
Repair on single strand
Lesions where single base has either been modified or excised by hydrolysis to leave an abasic site
DNA glycosolyse cleaves sugar base bond to delete the base to create abasic site
For abasic sites the residual sugar-phosphate residue is removed by endonuclease and phosphodiesterase
Gap is filled by a DNA polymerase and DNA ligase
Available throughout the cell cycle
DNA mismatch Repair (MMR)
Repair on a single strand - colorectal non-polyp
Repairs erroneous insertion, deletion, mis-incorporation of bases that occurs during DNA replication
Mismatch on the daughter DNA strand is recognized and fully excised along with the surrounding nucleotides
Generates a gap that is repaired by DNA polymerase and DNA ligase
Nucleotide excision repair (NER)
Repairs of single strand damage - Xeroderma
Allows the repair of bulky, helix- distorting DNA lesions (UV induced diners)
After lesion is detected damage site is opened and out
DNA is cleaved some distance away on either side of the lesion generating an olionucelotide of about 30 nucleotides containing the damaged site- which is discarded
More important in G1, but not restricted to only G1
Repair of DNA lesions affecting both DNA strands
Homologous recombination (HR)
Non-homologous end joining (NHEJ)
Homologous recombination (HR) - mediated DNA repair
Repair affecting both strands
Highly accurate repair mechanism - required homologous intact DNA strand to be available to act as template strand
Operated in S and G2 (before mitosis) using a DNA strand from undamaged sister chromatid as a template to guide repair
Non-homologous end joining (NHEJ)
Repair affecting both strands -SCID
No template strand is needed broken ends are fused together
Specific proteins bind to the DNA ends are recruit a special DNA ligase (DNA ligase IV) to rejoin broken ends
NHEJ is always available to cells - most important for repair in G1 phase before DNA has replicated
Xeroderma Pigmentosum
Autosomal recessive disorder
Inability to repair damage caused by UV light (NER is defective)
Malfunction of excision repair - thymine diners much remain and block production
Predisposition to skin cancer - Leads to basal cell carcinomas and other skin malignancies - by 10 years old
Signs appear in infancy or early adult hood - dry skin, freckles, loss of hearing, movement disorder, decrease life spoon
Hereditary Non-polyps is Colorectal cancer
Autosomal dominant - caused by mutations in genes involved in DNA mismatch and repair (MMR proteins)
Most common form of hereditary colorectal cancer - 50-70% risk of developing colorectal and other cancers
Propensity to develop predominantly right-sided, flat adenomas at a young age
Develop adenomas at the same rate as the general population BUT more likely to progress to cancer
Severe combined immunodeficiency (SCID)
X-linked autosomal recessive inheritance
Deficiency in both B and T lymphocytes functions with remarkeably low IgG, IgA, IgE levels - have neither cell mediated or antibody mediated immunity
Most cases- mutation in gene encoding common gamma chain (yc) a protein that is shared by the receptors for interluekins
Most severe form - defects in the pathway for double-strand break repair called non-homologous DNA end joining or NHEJ
If given bone marrow transplant by 3 months after birth = good prognosis
Bloom syndrome
Autosomal recessive pattern of inheritance
Characterized by short stature, butterfly task, high pitched voice and distinctive facial features: long narrow face, small lower jaw and prominent nose and ears
High risk of developing cancer
Mutation in BML gene
Defective RecQ helicases
Defective unwinding of DNA
Defective DNA repair
• Identify the type of DNA repair available to the cell during each phase of the cell cycle.
HR - S G2 NHEJ - most important in G1 always available NER - G1 but not resticted BER- throughout cell cycle MMR - ??
Base mismatches caused by replication errors
MMR
Small insertions/deletions due to replication slippage
MMR
Small scale single base modification
BER
Single base deletion - an abasic site resulting from hydrolysis
BER
