BABS2202 Chap.1 Eukaryotic Cell Reproduction and Cycle Control
Eukaryotic Cell Reproduction and Cycle Control
This chapter covers the first three lectures of BABS2202 and forms the opening half of the published Lectures 1–6 Theory Test range. The cell cycle is treated as a controlled sequence of state changes rather than a clock. G1 integrates growth conditions and commitment, S phase replicates DNA, G2 verifies completion, and M phase segregates duplicated chromosomes and partitions the cell.
Cyclin-dependent kinases provide catalytic activity, cyclins help supply timing and substrate choice, phosphorylation alters regulator state, and targeted proteolysis removes selected proteins so important transitions become directional.
The most valuable skill is perturbation reasoning.
If a regulator is absent, excessively active, resistant to degradation or trapped in the wrong phosphorylation state, locate the first transition that should change. Then distinguish the biochemical prediction from the population phenotype.
A G2-to-M block, for example, can produce a population enriched for replicated DNA, but DNA content alone does not identify whether mitotic CDK activation, spindle formation or another event failed. Add chromosome morphology, substrate phosphorylation, protein interactions and live timing before assigning a mechanism.
What this chapter covers
- 01
G1, S, G2 and M as distinct states with different physical jobs and evidence
- 02
Quiescence, senescence and death as different outcomes rather than interchangeable cycle exits
- 03
Checkpoint logic: a physical prerequisite controls a biochemical brake or permission step
- 04
Cyclin–CDK assembly, activating and inhibitory phosphorylation, phosphatases and kinase inhibitors
- 05
Ubiquitin ligases, proteasomal degradation and the directionality of cell-cycle transitions
- 06
Spindle attachment, sister-chromatid separation, mitotic exit and cytokinesis
- 07
Experimental distinction among abundance, complex formation, catalytic activity and phenotype
Diagnosing failure to enter mitosis
- +1Replicated DNA and duplicated centrosomes with no chromosome condensation place the dominant block at the G2-to-M transition.
- +1Persistent inhibitory phosphorylation could keep the cyclin–CDK complex inactive despite cyclin accumulation.
- +1A kinase inhibitor, defective cyclin binding or another assembly problem could also explain low substrate phosphorylation.
- +1Measure inhibitory phosphorylation, cyclin–CDK association and kinase output; a targeted rescue with active CDC25 tests the first model.
Key terms
- Cyclin-dependent kinase
- A protein kinase whose activity and substrate targeting depend on association with a suitable cyclin and on additional regulatory inputs.
- Restriction point
- A late-G1 commitment transition after which a cell is substantially committed to genome duplication and completion of the cycle.
- Checkpoint
- A control arrangement that delays or redirects progression when a required physical condition has not been satisfied.
- APC/C
- An E3 ubiquitin ligase complex that helps trigger sister-chromatid separation and mitotic exit by promoting selective regulator destruction.
- Kinetochore
- A chromosome-associated structure that binds spindle microtubules and participates in attachment, force generation and checkpoint signalling.
- Cytokinesis
- Physical division of the cytoplasm that completes production of separate daughter cells after nuclear segregation.
Eukaryotic Cell Reproduction and Cycle Control FAQ
Does phosphorylation always activate a cell-cycle protein?
No. Phosphorylation is a change, not a universal on switch. Different sites can activate, inhibit, alter localisation or create a binding or degradation signal. A good answer names the protein, site logic and consequence. Mitotic CDK can carry an activating input and an inhibitory phosphate at the same time; CDC25 promotes activation by removing the inhibitory phosphate.
How do I distinguish G2 from early mitosis?
Use several observations. Replicated DNA fits both. Duplicated centrosomes also do not settle the distinction. Chromosome condensation, nuclear-envelope state, spindle formation and phosphorylation of mitotic substrates provide more direct evidence. Time-lapse imaging can reveal whether a cell enters mitosis and stalls or never enters it.
Why is protein destruction important if phosphorylation is reversible?
Selective destruction changes which network components exist and therefore helps make transitions directional. Removing an anaphase inhibitor can release sister separation; later destruction of mitotic cyclin lowers CDK activity and permits exit. Timing remains essential, because premature removal is as dangerous as failure to remove the target.
Is every accumulated protein a direct target of the missing ubiquitin ligase?
No. Accumulation may be indirect. Direct support requires appropriate binding, ligase-dependent ubiquitin attachment and restoration of turnover when ligase function returns. Broad proteasome inhibition demonstrates dependence on proteolysis but affects many substrates and does not identify the responsible E3 ligase.
Exam move
Build a four-row state table for G1, S, G2 and M with DNA state, chromosome appearance, centrosome state, dominant regulators and next prerequisite. Reconstruct it without notes, then apply one loss-of-function and one constitutive-activity perturbation to each transition. For every result, write a biochemical readout and a cell-level readout.
This chapter sits inside the stated Lectures 1–6 Theory Test range, so practise short causal explanations as well as recognition: name the blocked transition, molecular action and predicted observation in that order.
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