Cell Cycle, DNA Damage, & Repair Flashcards
Stages of the Cell Cycle
G1 - Cell growth; Building and loading of Pre-Rc onto origin
S - Activation of Pre-Rc, DNA Synthesis
G2 - Protein synthesis and cell growth
M - chromosome segregation
ATM and ATR
Protein kinases that detect damage to DNA; in response to DNA damage, they phosphorylate p53 protein
p53
Phosphorylated by ATM/ATR in response to cellular damage; it acts as a TF that stimulates the synthesis of protein p21, which is a CDK inhibitor
p21
Inhibits the protein activity of CDK; in the presence of p21, Rb remains unphosphorylated
Retinoblastoma protein (Rb)
When unphosphorylated, Rb binds transcription factor E2F; while bound to Rb, E2F cannot promote transcription of a group of genes necessary for DNA synthesis and the cell cannot progress from G1 to S phase
The Rb-E2F blocking mechanism is relieved when R is phosphorylated by CDK, which occurs in response to a signal for cell division to proceed
Pre-RC Cycle
Pre-RC is built and loaded onto replication origins in G1 in response to ubiquitin-dependent degradation of cyclins at the end of M phase
Restriction Point
The point in G1 in which growth and division are coordinated; cells can exit the cell cycle here to differentiate; this is the commitment point of the cell cycle
Activation of cyclin-Dependent Kinase (CDK)
Only active when bound with cyclin and phosphorylated by CAK (Cyclin-activating kinase)
CDK Inhibitors (CDI)
Ink4 (p16) and Cip/Kip (p21) inhibit different homologues of the CDK enzyme
Cip/Kip family will inhibit any CDK/cyclin complex
Rad17
DNA damage sensor protein; “sensitive to Radiation” - binds to damaged DNA; recruits transducer proteins
ATM.ATR
Transducer proteins recruited by Rad 17 in response to DNA damage; ATM and ATR phosphorylate p53
Endogenous sources of DS breaks
VDJ Recombination: Double strand breaks necessarily occur during rearrangement of the immunoglobulin heavy and light chain DNA strands; failure in the DS break repair machinery can therefore lead to severe immunodeficiency; normal VDJ recombination occurs via NHEJ
Chromosomal recombination during meiosis
Exogenous sources of DS breaks
X-rays, mammography, CT, angioplasty,
Non-Homologous End Joining (NHEJ)
Occurs throughout the cell cycle; imperfect
Homologous Repair (HR)
Requires presence of sister chromatid, limited to S and G2 phase; perfect repair
Artemis
Nuclease involved in NHEJ
LIG-4
Ligase involved in NHEJ
NHEJ Pathway
Ku protein is recruited to the site of DNA ds break, which then functions to recruit DNA-PK, which binds DNA; Artemis (a nuclease) is recruited, and LIG-4
Lig 4 syndrome
Caused by mutation in the Lig 4 gene; NHEJ & VDJ recombination pathways are affected; leads to immune deficiency and the development of lymphoid tumors
RS-SCID Syndrome
Radio-sensitive severe combined immunodeficiency; results from mutation in the artemis gene; ATM-dependent NHEJ and VDJ recombination pathways are affected, leading to immune deficiency
SPO 11
Nuclease that forms DS break during meiosis, enabling recombination between homologues
VDJ Recombination
The process by which diversity is generated in immunoglobulin chains; hairpin-capped double strand breaks are created by the RAG1/2 nuclease, which cleaves the DNA at recombination signal sequences; these hairpins are then opened by the Artemis nuclease and joined by NHEJ.
Patients with mutations in NHEJ genes are unable to produce functional B cells and T cells and suffer from severe combined immunodeficiency (SCID).
Avastin
Chemotherapeutic agent; a monoclonal antibody that inhibits angiogenesis by inhibiting vascular endothelial growth factor A (VEGF-A)
