Cell Division Machinery and Cell Cycle Regulation

Key cell division machinery components:
- Spindle: Controls chromosome movement in prophase, metaphase, and anaphase.
- Centrioles: Structure involved in spindle organization.
- Kinetochore: Protein unit where chromosomes align together.
Regulatory control system components:
- Cyclins: Regulatory proteins with cyclically fluctuating concentrations.
- Cyclin-Dependent Kinases (CDKs): Kinases involved in phosphorylation events (the addition or removal of phosphate groups).
- Regulatory Destruction Mechanism: Process carried out by ubiquitin ligase that disassembles cyclins and protein complexes.

Mitochondria and Chloroplasts: Divide via binary fission (fission and fusion) due to their prokaryotic origin.
Ribosomes and Peroxisomes: Highly abundant in the cytoplasm; divided into daughter cells through simple planar division.
Endoplasmic Reticulum (ER) and Golgi Bodies: Composed of membrane networks that cut and fuse back together into daughter cells.
Vacuoles: Essential organelle partition in plant cells.
Centrioles (Exception): Organelle duplication mechanism is uniquely unknown, except that a pre-existing centriole must exist in a cell to form a new one.

$G_0$ Phase: Resting phase where content cells stay until recalled for tissue repair.
$G_1$ Phase: First executive decision-making phase; variable in duration based on genome size. Checks for proper nutrients, required proteins, and cell attachment anchors.
$G_1/S$ Checkpoint: Confirms nutrient, protein, and anchor availability before entering $S$ phase. Attempting to skip or bypass this checkpoint leads to cellular failure.
$S$ Phase: Phase where DNA replication occurs; parental strands serve as templates for daughter strands (A pairs with T, G pairs with C).
$G_2$ Phase: Verification phase post-replication utilizing DNA polymerase proofreading and excision repair nucleotide substitution to fix errors.
$G_2/M$ Checkpoint: Verifies accurate nucleotide alignment, full DNA copying, and absence of non-coding intron interruptions (ensuring only exons/coding regions remain).
$M$ Phase: Mitotic phase where chromosome rearrangement occurs.

S-Phase Promoting Factor (SPF):
- Formed when $G_1$ cyclin / Cyclin A binds to CDK and E2F (a class of transcription factor capable of coding for over a thousand proteins).
- Moves into the nucleus to prepare chromosomes and trigger DNA duplication.
M-Phase Promoting Factor (MPF):
- Formed when Cyclin B binds to CDK.
- Triggers three prophase events: chromosome condensation, nuclear envelope breakdown, and mitotic spindle initiation.
Anaphase Promoting Complex (APC):
- Acts as a ubiquitin ligase rather than a protein kinase.
- Drives the metaphase-to-anaphase transition by destroying securin via the regulatory destruction mechanism, causing cyclin levels to drop and sister chromatids to separate.

Question: How do mitochondria and chloroplasts divide into daughter cells?
- Response: They divide simply by fission and fusion (binary fission), functioning like their prokaryotic ancestors.
Question: What major membrane-bound organelles can be cut and fused during cytoplasmic division?
- Response: ER and Golgi bodies form a network of membranes that cut and fuse into daughter cells; vacuoles perform this function in plant cells.
Question: Which organelle is an exception in terms of understanding its division?
- Response: The centriole is an exception because its exact formation process is unknown, other than requiring an existing centriole to form a new one.
Question: What are the specific functions of cyclins, CDKs, and APC in moving through checkpoints?
- Response: Cyclin A binds CDK and E2F to form SPF, which enters the nucleus for DNA duplication in $S$ phase. Cyclin B binds CDK to form MPF, driving prophase events. APC acts as a ubiquitin ligase to degrade securin and drop cyclin levels, executing the metaphase-to-anaphase transition.