Cell Cycle, DNA Damage, & Repair

Question 1

Stages of the Cell Cycle

Answer 1

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

Question 2

ATM and ATR

Answer 2

Protein kinases that detect damage to DNA; in response to DNA damage, they phosphorylate p53 protein

Question 3

p53

Answer 3

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

Question 4

p21

Answer 4

Inhibits the protein activity of CDK; in the presence of p21, Rb remains unphosphorylated

Question 5

Retinoblastoma protein (Rb)

Answer 5

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

Question 6

Pre-RC Cycle

Answer 6

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

Question 7

Restriction Point

Answer 7

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

Question 8

Activation of cyclin-Dependent Kinase (CDK)

Answer 8

Only active when bound with cyclin and phosphorylated by CAK (Cyclin-activating kinase)

Question 9

CDK Inhibitors (CDI)

Answer 9

Ink4 (p16) and Cip/Kip (p21) inhibit different homologues of the CDK enzyme

Cip/Kip family will inhibit any CDK/cyclin complex

Question 10

Rad17

Answer 10

DNA damage sensor protein; “sensitive to Radiation” - binds to damaged DNA; recruits transducer proteins

Question 11

ATM.ATR

Answer 11

Transducer proteins recruited by Rad 17 in response to DNA damage; ATM and ATR phosphorylate p53

Question 12

Endogenous sources of DS breaks

Answer 12

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

Question 13

Exogenous sources of DS breaks

Answer 13

X-rays, mammography, CT, angioplasty,

Question 14

Non-Homologous End Joining (NHEJ)

Answer 14

Occurs throughout the cell cycle; imperfect

Question 15

Homologous Repair (HR)

Answer 15

Requires presence of sister chromatid, limited to S and G2 phase; perfect repair

Question 16

Artemis

Answer 16

Nuclease involved in NHEJ

Question 17

LIG-4

Answer 17

Ligase involved in NHEJ

Question 18

NHEJ Pathway

Answer 18

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

Question 19

Lig 4 syndrome

Answer 19

Caused by mutation in the Lig 4 gene; NHEJ & VDJ recombination pathways are affected; leads to immune deficiency and the development of lymphoid tumors

Question 20

RS-SCID Syndrome

Answer 20

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

Question 21

SPO 11

Answer 21

Nuclease that forms DS break during meiosis, enabling recombination between homologues

Question 22

VDJ Recombination

Answer 22

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).

Question 23

Avastin

Answer 23

Chemotherapeutic agent; a monoclonal antibody that inhibits angiogenesis by inhibiting vascular endothelial growth factor A (VEGF-A)