CBNS 101 Lecture 12 cell division

How does the purpose of cell division differ between single-celled and multi-cellular organisms?

Single-celled: Divide as rapidly as possible depending on nutrient availability and cell size.

Multi-cellular: Division rate and frequency are tightly controlled (loss of this control causes cancer).

What are the main differences in yield between Mitosis and Meiosis?

Mitosis: Yields 2 identical diploid cells (used for growth and tissue repair).

Meiosis: Yields 4 unique haploid cells (used for sexual reproduction).

What is the longest phase of the cell cycle?

Interphase

Name the stages of interphase in order:

G1, S, G2

What occurs during the G1 Phase?

It is a gap phase focused on cell growth and the doubling of membranous organelles.

What occurs during the S Phase?

DNA replication

What occurs during the G2 Phase of Interphase?

It is a gap phase for final cell growth and preparation for mitosis.

What is the G0 Phase?

A resting state where cells temporarily or permanently withdraw from the cell cycle.

What is the M Phase and what two major processes does it include?

The division phase. Includes mitosis and cytokinesis

What happens during Early Prophase?

Chromosomes begin to condense, and the single centrosome splits into two centriole pairs.

What happens during Late Prophase?

Centriole pairs separate and move apart as microtubule asters.

What happens during Prometaphase?

The nuclear envelope breaks down, and spindle microtubules bind to condensed chromosomes.

What happens during Metaphase?

A bipolar spindle structure forms, and all chromosomes align perfectly at the cell equator (metaphase plate).

What happens during Early Anaphase vs. Late Anaphase?

Early Anaphase: Sister chromatids synchronously separate and move toward opposite poles via microtubules.

Late Anaphase: Spindle poles move further apart to lengthen the cell.

What happens during Telophase and Late Telophase?

Telophase: Daughter nuclei re-form around the separated genetic material.

Late Telophase: Cytokinesis finishes splitting the cell.

What is the G1 Checkpoint (START / Restriction Point)?

A negative regulation checkpoint that commits the cell to the cycle if nutrients, cell size, and growth factors are sufficient.

What is the G2/M Transition Checkpoint?

A checkpoint that halts entry into mitosis if DNA is damaged or replication is incomplete.

What is the Metaphase/Anaphase Transition Checkpoint?

A checkpoint that verifies all chromosomes are properly attached to the spindle before allowing sister chromatids to separate.

What did Cell Fusion Experiments reveal about mitotic regulation?

Fusing an M-phase cell with a G1 cell forced the G1 chromosomes to condense, proving that a soluble mitotic inducer exists in the cytoplasm.

What does MPF (Maturation Promoting Factor) contain?

p32 (a kinase) and p45.

What are cyclins?

proteins that cyclically accumulate and degrade during the cell cycle.

What is the function of CDKs (Cyclin-Dependent Kinases)?

Master regulators of the cell cycle.

Do CDK levels fluctuate during the cell cycle?

No

Under what circumstances will CDK inactivate?

without cyclin

What are the 4 main ways CDKs are regulated?

1. Cyclin Abundance: Binding uncovers the ATP active site (partial activation).

2. Phosphorylation/Dephosphorylation: CAK (Cdk Activating Kinase) fully activates CDK; other phosphorylations can inhibit it.

3. CKIs (CDK Inhibitors): Proteins that bind directly to cyclin-CDK complexes to turn them off.

4. Termination: Cyclins are targeted for degradation to abruptly stop CDK activity.

What are Kinetochores

Large protein structures on centromeres that bind to spindle microtubules.

The requirement for sister chromatids to attach to opposite spindle poles.

Bi-orientation

How does tension-sensing regulate spindle attachment and cohesive links?

Correct attachments create high physical tension across cohesin links. Low tension sends an inhibitory signal to detach and retry. Once all attachments are secure, cohesin is cleaved to allow anaphase separation.

What structure drives cytoplasmic division in animal cells during cytokinesis?

The cleavage furrow

What is the structure of the cleavage furrow?

a contractile ring made of overlapping actin and myosin II filaments right below the plasma membrane.

Describe midbody formation and abscission

The contractile ring narrows into a tight protein bridge called the midbody, which is ultimately severed during abscission.