L.11 DNA Repair

Why is DNA repair important?

• have a lot of genes dividing frequently therefore a lot of room for error

• Repai maintains OG genome → lots of resources to make that happen

Causes of DNA damage

• metabolism

• UV light

• Radiation

• Chemicals

• Replication errors

Responses to DNA damage

• cell cycle checkpoint activation

• Transcriptional program activation

• DNA repair

• Apoptosis

Categories of DNA repair

• direct reversal

• Base excision repair

• Nucleotide excision repair

• Mismatch repair

• Double stranded break repair (homologous and non homologous)

Mismatch excision repair (MMR)

• fix damage from replication

• Initiated by mismatch recognition and strand discrimination

• Molecules recruited to complex

• Excision → initiate endonueclease activity, makes single strand breaks, opens EXO1, dissociate DNA Lison

• Repair with DNA Pol I and lighted

MMR fixes ____

Damage from replication

MMR initiation

• mismatch recognition and strand discrimination

• Other molecules recruited to complex

MMR excision

• endonuclease activity

• Single strand breaks near mismatch

• Open exonuclease 1 (EXO1) entry sites

• DNA lesion dissociates

MMR repair

resybthesis with DNA Pol I and located to restore fidelity

4 steps of DNA repair

• recognize and initiate

• Excision

• Repair

• Legate

Autosomal recessive mutations in CP genes result in

Xeroderma pigmentosum

UV damage repair fixes____

Damage from UVB inducced formation of T-T diner lesions

How can T-T diners be excised and repaired

• photoreactivation repair indicated by DNA photolyase, activate with blue light, break dimer ring, repair by restore base pairing

OR

• nucleotide excision prepaid by XP genes to get rid of T-T diner

• Repair with DNA Pol I ligated

Nucleotide excision repair process

• recognizes bulky lesions of DNA (groups of problem) by

1. Global genome NER

2. Trabcription coupled NER excision and repair

Base excision repair process

• recognize nonbulky lesions (oxidative agents) and repairs (1 base) with DNA glycosylase

Most common issue to repair

Single stranded DNA breaks

Serious type of DNA damage

Double stranded breaks

Why are double stranded breaks so bad?

• immediate serious consequence, if don’t attend to, chromosomes are halves and data gets lost at cell division

Types of double stranded break repair

• no homologous and joining (NHEJ)

• Homologous repair (HR)

NHEJ

• fast and effective, error prone

• Excise and ligate as quick as it can

HR

• error-free but can only occur in mitosis

• Uses sister (homologous) chromosome as template

Cell cycle check points and what happens

• G1/S cell growth check point → if cell big enough and has proteins needed to proceed

• S DNA synthesis checkpoint → jf DNA synthesized correctly

• M Mitosis checkpoint→ confers mitosis completed

Regulator in cell cycle

• positive allow to progress

• Negative hold it back

DSBR coordinated with?

The cell cycle

How is DSBR related to the cell cycle

• ATM signalling pathway leading to check point arrest in S phase, G1/S boundary, and G2/M boundary

• The key regulator is CHEK1

Which pathway regulates DBSR?

ATM DNA repair signalling cascade

Cell cycle

• G1/G0 (normal growth/ rest)

• G1/S checkpoint

• Synthesis and DNA synthesis checkpoint

• G2/Growth and development

• Mitosis checkpoint

• Mitosis

ATM DNA repair signalling cascade initiation and recognition

• initiated by damage to DNA

• Recognition complexes form at lesion

ATM DNA repair signalling cascade activation and actions

Activator ATM, master kinase coordinates repair processes necessary at each level of cascade, not just DNA

What do master kinase coordinates repair

• DNA

• cell cycle components

• Transcription factors

• DNA repair proteins

ATM DNA repair signalling cascade occurs at a specific time because

Cell cycle arrest allows time to repair

Purpose of ATM DNA repair signalling cascade?

Repair of lesion ensures correct original template DNA and cell can continue in cycle without mutation

What happens if ATMDNA repair doesn’t work?

Apoptosis

What does ATM do in the cascade

phosphorylates H2AX (histone type) to signal where DNA is broken

Signalling cascade induced by double strand breaks or replication stress

• the master regulator ATM activated cell cycle stops, regulated by CHEK2 (kinase)

• DNA repair or apoptosis (by p53)

Genes and diseases associated with DNA repair defects (5)

• P53= tumor suppressor, mutated in most cancer

• XP and skin cancer related to repair mechanism

• Breast cancer and DSBR gene (BRCA-2)

• Genome instability syndromes (neurodeveloplent, growth, premature aging)

• Rare disease inheritance (in germ cells)

Major DNA repair mechanisms

• mismatch excision repair

• Nucleotide and base excision repair

• Transcription-coupled excision repair

• Double stranded break repair

DNA damage response initiates at??

Cell Check points (G1/S, S, or M) ensures fixed before enters next phase

ATM kinase DNA repair signalling cascade coordinates _____

• recognition complexes at DNA lesion

• Regulation of cycle, transcription factors, repair proteins, repair, check points, death

Defects in DNA repair results in _____ (possibilities)

• cancer

• Neurodevelopmental/growth/premature aging disorders

• Rare disease inheritance