4 removesthempriorto replication 5

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Unformatted text preview: Error­free repair mechanism. 2. Recognizes distortions in the double helix. 3. Resulting from mis­ paired bases. 4. Removes them prior to replication. 5. Using undamaged strand as a template to guide repair 10 2A. Mismatch repair: how we repair 2A. Mismatch repair: how we repair replication mistakes 11 2A. Mismatch repair in E.coli 2A. Mismatch repair in 1. DNA is methylated at A residues in the sequence GATC. 2. Methylation occurs after replication. 3. Newly replicated DNA is hemi­methylated. 4. Mismatch repair system uses methylated strand (old strand) as template to repair error. 12 Insertions / Deletions (indels) Insertions / Deletions (indels) by Replication Slippage 13 Fragile­X Syndrome: A Trinucleotide repeat expansion resulting from replication slippage leading to mental retardation 14 Replication Slippage: Replication Slippage: Examples of neurodegenerative diseases involving triplet repeat expansions • Fragile X syndrome – X­linked dominant, Severe mental retardation and ataxia – CGG repeat in the 5’ untranslated region. • Huntingtons chorea – Dementia – CAG repeat in the coding region (a stretch of polyglutamines) • Friedreich’s ataxia – Autosomal recessive – GAA repeat in an intron 15 2B. Mismatch Repair in Humans 2B. Mismatch Repair in Humans 16 3. Double Stranded Break Repair: 3. Double Stranded Break Repair:...
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