The Cell Cycle and Checkpoints in Cell Division

Flashcards reviewing key concepts of the eukaryotic cell cycle, cyclins and CDKs, cell cycle checkpoints, tumor suppressor pathways (p53, pRb, BRCA1), apoptosis, and cell cycle-targeted therapeutics based on Midwestern University lecture notes.

What are the four main physiological reasons for cell division in an organism?

1. Developmental growth of the organism
2. Repair of injury
3. Replacement of older cells (e.g., skin cells)
4. Building up tissue (e.g., fat or muscle cells, mostly in young people)

What three conditions do cells monitor during the cell cycle to maintain genomic integrity?

1. DNA integrity
2. Proper chromosome duplication
3. Proper spindle fiber assembly

What molecular roles do BRCA1 and BRCA2 perform, and what risk is associated with their mutation?

BRCA1 and BRCA2 are involved in recognizing and repairing double-stranded DNA breaks. Mutations in BRCA1/2 lead to a $45-85\%$ risk of developing breast and ovarian cancer.

What are the four discrete phases of the dividing eukaryotic cell cycle, and what is the non-dividing phase called?

The active dividing phases are $G_1$, S, $G_2$, and M. The non-dividing, resting phase is called $G_0$.

What specific cellular processes occur during the $G_1$, S, and $G_2$ phases of the cell cycle?

• $G_1$ phase: Protein synthesis and replication of non-chromosomal contents. • S phase: DNA synthesis where each of the $46$ chromosomes is duplicated. • $G_2$ phase: Proofreading and repair, where the cell double-checks duplicated chromosomes for errors.

What characterizes the quiescent $G_0$ phase of the cell cycle?

In $G_0$, the cell is quiescent—it metabolizes nutrients and performs its physiological functions but is not preparing for cell division.

What is a mitogen and how does its signal lead to the initiation of the cell cycle?

A mitogen is an extracellular growth factor that binds to cell surface receptors to trigger a phosphorylation cascade (often through Ras and the MAP kinase cascade). MAP kinase increases the synthesis and activity of the transcription factor Myc, which increases expression of the Cyclin D gene.

What is the first active cyclin-CDK complex formed at the beginning of the cell cycle?

The Cyclin D-CDK4/6 complex (Cyclin D bound to CDK4 or CDK6).

How do cyclin-dependent kinases (CDKs) and cyclins differ in their activity and protein level stability throughout the cell cycle?

CDK protein levels remain constant throughout the cell cycle, and CDKs possess kinase activity only when bound to a cyclin. Cyclins are regulatory subunits without intrinsic kinase activity whose levels fluctuate (synthesized and degraded cyclically), conferring substrate specificity to the CDK.

Which specific cyclin and CDK partners function during each phase of the cell cycle?

• Early $G_1$: Cyclin D with CDK4 or CDK6 • Late $G_1$: Cyclin E with CDK2 • $G_1$/S: Cyclin A with CDK2 • S/$G_2$: Cyclin A with CDK1 (Cdc2) • $G_2$/M: Cyclin B with CDK1 (Cdc2)

What are the three major scheduled cell cycle checkpoints and what conditions govern progression through each?

1. Restriction [R] point (in $G_1$): Dependent on external signaling by mitogens.
2. $G_2$-M checkpoint: Dependent on successful completion of DNA replication and error repair.
3. M-phase checkpoint (Spindle Assembly Checkpoint): Dependent on proper alignment of mitotic chromosomes during metaphase.

Which cyclin-CDK complex regulates both the $G_2$-M and M-phase checkpoints, and what event is required for progression through the M-phase checkpoint?

Both checkpoints depend on the Cyclin B-CDK1 complex. Progression through the M-phase checkpoint requires the de-activation of CDK1 via the destruction of Cyclin B.

How do Cyclin D-CDK4/6 and Cyclin E-CDK2 sequentially act on pRb to allow passage through the Restriction Point?

1. Cyclin D-CDK4/6 hypophosphorylates pRb, partially activating E2F to express Cyclin E.
2. Cyclin E-CDK2 hyperphosphorylates pRb, fully inactivating it.
3. Fully inactivated pRb releases the remaining E2F, promoting transcription of S-phase genes needed for DNA synthesis.

Who first described retinoblastoma in $1809$, and in what year was the Rb tumor suppressor gene cloned?

Retinoblastoma was first described in $1809$ by Scottish ophthalmologist Dr. James Wardrop in his book 'Observations on Fungus Hæmatodes or Soft Cancer'. The Rb gene was cloned in $1987$.

What are the two categories of cyclin-dependent kinase inhibitors (CDKIs), and which specific proteins belong to each group?

1. INK4 group: Includes $p16$; specifically inhibits CDK4 or CDK6 (induced by environmental stress like starvation or illness).
2. WAF1/CIP1 group: Includes $p21$; inhibits all cyclin-CDKs except CDK4 and CDK6 (induced by $p53$ following DNA damage).

How does DNA double-strand break (DSB) detection activate the p53 and p21 pathway to cause cell cycle arrest?

ATM kinase detects DSBs and, along with Chk2, phosphorylates and stabilizes $p53$. Active $p53$ binds the promoter of the $p21$ gene to increase $p21$ mRNA and protein expression. The $p21$ protein then inhibits cyclin-CDK complexes, arresting the cell cycle for DNA repair.

What condition characterized by joined digits occurs when apoptosis fails during embryonic hand/foot development?

Syndactyly.

How do the intrinsic and extrinsic apoptosis pathways differ in their initiation and key initiator caspases?

• Intrinsic pathway: Triggered by internal cellular stress/DNA damage; $p53$ facilitates cytochrome c release from mitochondria, forming the apoptosome to activate initiator Caspase-9. • Extrinsic pathway: Triggered by external death ligands (e.g., TNF-alpha) binding death receptors (e.g., TNFR1), forming the DISC complex to activate initiator Caspase-8. Both pathways converge on activating effector caspases (Caspase-3, -6, -7).

What is the mechanism of action and clinical indication for Palbociclib (Ibrance)?

Palbociclib is a CDK4/6 inhibitor that prevents phosphorylation of the retinoblastoma (RB) protein, causing cell-cycle arrest and reducing tumor growth. It is widely used in hormone receptor-positive breast cancer.

What is the supportive care function of Trilaciclib (Cosela) in chemotherapy treatment?

Trilaciclib inhibits CDK4/6 and temporarily arrests hematopoietic cells, protecting bone marrow stem cells that produce red blood cells from chemotherapy-induced death during small-cell lung cancer treatment.

What are the cellular outcomes of loss-of-function mutations in p53, p21, p16, and pRb in cancer?

• Loss of $p53$: Decreased $p21$ expression and loss of cell cycle inhibition. • Loss of $p21$: Inability to bind cyclin/CDKs and loss of cell cycle arrest. • Loss of $p16$: Failure to inhibit CDK4/6 in $G_1$ phase. • Loss of $pRb$: Loss of E2F inhibition, causing unrestrained S-phase gene expression.

What is the cellular consequence of a gain-of-function mutation in Ras?

It leads to mitogen-independent synthesis of Cyclin D, enabling cells to pass the Restriction Point without proper growth signals and causing loss of cell cycle regulation.