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Here is a complete summary of Cytology Lecture 5: Cell Cycle and Division by Giorgi Berishvili, PhD (Geomedi, 2026):

Lecture 5 - Cell Cycle and Division: Summary

1. Why Cells Divide

  • Cell division is fundamental to life - for reproduction in unicellular organisms and for growth/tissue replacement in multicellular organisms.
  • All cells arise from pre-existing cells, exclusively during the mitosis phase.
  • The core purpose of the cell cycle is to copy and transmit genetic information to the next generation.

2. The Cell Cycle - Four Phases

PhaseKey Events
G1 (Gap 1)Cell regains full size; active RNA and protein synthesis
S (Synthesis)DNA replication, histone synthesis, centrosome duplication begins
G2 (Gap 2)Accumulation of proteins required for mitosis
M (Mitosis)Separation of two genome copies; nuclear + cytoplasmic division

3. G1 Phase Crossroad (G0 and Differentiation)

  • Not all cells divide continuously.
  • Under unfavorable conditions, cells can exit to G0 phase (quiescence).
  • Arrested cells can re-enter the cycle after stimulation by mitogens or growth factors.
  • Many specialized cells reach terminal differentiation and stop dividing permanently.
  • Stem cells divide continuously to replace lost cells.

4. Cell Cycle Control - Cyclins and CDKs

  • Progression through each phase is controlled by checkpoints.
  • Key proteins: Cyclins (cytoplasmic regulatory proteins) that activate Cyclin-Dependent Kinases (CDKs).
  • CDKs trigger:
    • Manufacturing new cellular components
    • Positioning components correctly
  • Different cyclin-CDK pairs are active at different phases.
  • At the end of each phase, cyclins are degraded via the ubiquitin-proteasome pathway.

5. Interphase

  • The cell increases in size.
  • DNA is duplicated (S phase) and the centrosome is duplicated.
  • S-Cdk triggers assembly of the replication machinery and prevents re-replication by inactivating Cdc6 and ORC via phosphorylation.

6. DNA Replication (S Phase)

  • Produces two identical DNA molecules.
  • Speed: 100-1,000 nucleotides per second.
  • Both strands serve as templates (semi-conservative).
  • Key proteins:
    • ORC (Origin Recognition Complex) - recognizes origins of replication
    • Cdc6 - recruited by ORC
    • DNA helicase - unwinds the double helix, creating a replication fork
    • DNA polymerases - synthesize new strands

7. M Phase - Overview

  • Consists of two sub-processes:
    1. Mitosis - nuclear/chromosomal division
    2. Cytokinesis - cytoplasmic division
  • Mitotic spindle carries out chromosomal segregation.
  • Contractile ring carries out cytoplasmic division.
  • Triggered by M-Cdk, which acts in a positive feedback loop with Cdc25 phosphatase (G2/M checkpoint).
  • M-Cdk triggers: chromosome condensation, spindle assembly, nuclear envelope breakdown, cytoskeletal remodeling.

8. Stages of Mitosis

Prophase

  • Centrosomes migrate to opposite poles and begin organizing the mitotic spindle.
  • Chromosomes condense (via condensin protein complexes) and become visible under a microscope.
  • Sister chromatids are joined at centromeres via kinetochores and held together along their length by cohesins.

Prometaphase

  • Nuclear envelope breaks down (fragments into small vesicles).
  • Nuclear pores disassemble.
  • Spindle microtubules capture chromosomes at kinetochores.
  • Chromosomes begin moving between spindle poles.

Metaphase

  • Chromosomes align at the metaphase plate (equatorial plane).
  • The spindle assembly checkpoint halts progression until all kinetochores are properly attached to spindle fibers.

Anaphase

  • Cohesins are enzymatically degraded - sister chromatids separate.
  • Kinetochores are pulled apart; separated chromatids are now called chromosomes.
  • Anaphase A: chromatids move toward the poles.
  • Anaphase B: the two spindle poles move apart.
  • Error: Mitotic nondisjunction - failure of sister chromatids to separate, leading to aneuploidy.

Telophase

  • Chromosomes arrive at opposite poles.
  • Spindle microtubules depolymerize.
  • Nuclear envelope reassembles around each set of chromosomes; nuclear pores are re-incorporated.
  • Chromosomes decondense (condensins dephosphorylated and released).
  • Gene transcription resumes. Mitosis is complete.

9. Cytokinesis

  • A cleavage furrow appears on the cell surface.
  • Driven by contraction of an F-actin + myosin bundle (contractile ring) in the cell cortex.
  • The contractile ring:
    • Assembles during anaphase beneath the plasma membrane.
    • Progressively constricts and is disassembled after the two daughter cells are sealed off.
  • Result: two daughter cells, each with one diploid nucleus and identical DNA copies.
  • When daughter cells are equal in size: symmetric cell division.

10. Centromeres (Key Detail)

  • Human centromeres span up to 1 million base pairs.
  • Composed of alpha satellite DNA (~171 bp repeats).
  • Contain a special histone variant: CENP-A (Centromere Protein-A), replacing H3 in centromeric nucleosomes.
  • Defined by protein assembly, not a specific DNA sequence.
  • Each chromosome must contain one centromere for stable inheritance.

11. Mitotic Spindle

  • Composed of microtubules and associated motor proteins.
  • Microtubules grow from centrosomes (MTOCs), later called spindle poles.
  • Minus ends at spindle poles; plus ends radiate outward.
  • Three types of spindle microtubules:
    1. Astral - anchor the spindle to the cell cortex
    2. Kinetochore (chromosomal) - connect to chromosomes
    3. Interpolar - overlap at the midzone and push poles apart

12. Plant Cell Division

  • Plant cells have a rigid cell wall, so no cleavage furrow is possible.
  • Instead, interpolar microtubules at telophase form the phragmoplast, which guides construction of a new cell wall between the two daughter cells.

Key Exam Points to Remember

  • Cyclin-CDK pairs control each cell cycle phase; cyclins are degraded by ubiquitin-proteasome system.
  • G2/M checkpoint is controlled by M-Cdk + Cdc25 positive feedback loop.
  • Spindle assembly checkpoint at metaphase ensures all kinetochores are attached before anaphase.
  • Cohesins hold sister chromatids together; their degradation triggers anaphase.
  • Cytokinesis uses an actin-myosin contractile ring; plants use a phragmoplast instead.
  • CENP-A is the specific histone variant at centromeres.
  • Nondisjunction = failure of chromatid separation in anaphase.

Give me MCQ type questions from this pdf

Based on the full content I already extracted from the PDF, here are exam-style MCQs covering every major topic in the lecture:

MCQ Practice Questions - Cytology L5: Cell Cycle & Mitosis

Section 1: Cell Cycle Basics

1. Which phase of the cell cycle involves DNA replication?
  • A) G1
  • B) G2
  • C) M
  • D) S
✅ Answer: D) S (Synthesis) phase
2. During which phase do cells accumulate proteins required for mitosis?
  • A) G1
  • B) S
  • C) G2
  • D) M
✅ Answer: C) G2 phase
3. A cell that has exited the cell cycle due to unfavorable conditions is said to be in:
  • A) S phase
  • B) G0 phase
  • C) G2 phase
  • D) Telophase
✅ Answer: B) G0 phase
4. Which of the following cell types divides continuously to replace lost cells?
  • A) Neurons
  • B) Skeletal muscle cells
  • C) Stem cells
  • D) Red blood cells
✅ Answer: C) Stem cells
5. The universal feature of the cell cycle is:
  • A) Protein synthesis
  • B) Cytoplasmic growth
  • C) Copying and passing genetic information to the next generation
  • D) Organelle duplication
✅ Answer: C) Copying and passing genetic information to the next generation

Section 2: Cyclins and CDKs

6. Which proteins activate Cyclin-Dependent Kinases (CDKs)?
  • A) Cohesins
  • B) Condensins
  • C) Cyclins
  • D) Histones
✅ Answer: C) Cyclins
7. At the end of each cell cycle phase, cyclins are degraded via:
  • A) Lysosomal pathway
  • B) Ubiquitin-proteasome pathway
  • C) Autophagy
  • D) Caspase activation
✅ Answer: B) Ubiquitin-proteasome pathway
8. The G2/M checkpoint is triggered by which kinase complex?
  • A) S-Cdk
  • B) G1-Cdk
  • C) M-Cdk
  • D) CDK4/Cyclin D
✅ Answer: C) M-Cdk
9. M-Cdk acts in a positive feedback loop with which phosphatase?
  • A) PP2A
  • B) Cdc25
  • C) Cdc6
  • D) ORC
✅ Answer: B) Cdc25 phosphatase

Section 3: DNA Replication (S Phase)

10. Which protein complex recognizes origins of replication?
  • A) DNA helicase
  • B) DNA polymerase
  • C) ORC (Origin Recognition Complex)
  • D) Condensin
✅ Answer: C) ORC (Origin Recognition Complex)
11. The speed of DNA replication is:
  • A) 1-10 nucleotides per second
  • B) 10-50 nucleotides per second
  • C) 100-1000 nucleotides per second
  • D) 10,000 nucleotides per second
✅ Answer: C) 100-1000 nucleotides per second
12. S-Cdk prevents re-replication by inactivating which proteins?
  • A) Cohesins and condensins
  • B) Cdc25 and M-cyclin
  • C) Cdc6 and ORC
  • D) Kinetochore and CENP-A
✅ Answer: C) Cdc6 and ORC
13. DNA helicase at the replication fork functions to:
  • A) Synthesize new DNA strands
  • B) Unwind the double helix
  • C) Ligate Okazaki fragments
  • D) Recruit histones to DNA
✅ Answer: B) Unwind the double helix

Section 4: Mitosis Stages

14. Chromosomes first become visible under a light microscope during:
  • A) Prometaphase
  • B) Metaphase
  • C) Prophase
  • D) Anaphase
✅ Answer: C) Prophase
15. Which protein complexes compact chromosomes during prophase?
  • A) Cohesins
  • B) Condensins
  • C) Kinetochores
  • D) Motor proteins
✅ Answer: B) Condensins
16. During prometaphase, which structure disappears?
  • A) Cell membrane
  • B) Centrosomes
  • C) Nuclear envelope
  • D) Mitotic spindle
✅ Answer: C) Nuclear envelope
17. The spindle assembly checkpoint ensures that:
  • A) DNA is fully replicated before G2
  • B) All kinetochores are attached to spindle fibers before anaphase
  • C) The contractile ring forms correctly
  • D) Cohesins are degraded
✅ Answer: B) All kinetochores are attached to spindle fibers before anaphase
18. Chromosomes align at the equatorial plane during:
  • A) Prophase
  • B) Prometaphase
  • C) Metaphase
  • D) Anaphase
✅ Answer: C) Metaphase
19. Which event marks the beginning of anaphase?
  • A) Nuclear envelope breakdown
  • B) Chromosome condensation
  • C) Enzymatic degradation of cohesins
  • D) Cleavage furrow formation
✅ Answer: C) Enzymatic degradation of cohesins
20. In Anaphase B, which of the following occurs?
  • A) Sister chromatids separate
  • B) Chromosomes decondense
  • C) The two spindle poles move apart
  • D) Nuclear envelope reforms
✅ Answer: C) The two spindle poles move apart
21. Failure of sister chromatids to separate during anaphase is called:
  • A) Apoptosis
  • B) Mitotic nondisjunction
  • C) Terminal differentiation
  • D) Cytokinesis arrest
✅ Answer: B) Mitotic nondisjunction
22. Which of the following events occurs during telophase?
  • A) Chromosome condensation
  • B) Nuclear envelope breakdown
  • C) Chromosomes decondense and nuclear envelope reforms
  • D) Cohesin degradation
✅ Answer: C) Chromosomes decondense and nuclear envelope reforms
23. Condensins are released during telophase because they are:
  • A) Ubiquitinated
  • B) Dephosphorylated
  • C) Proteolytically cleaved
  • D) Transported out of the nucleus
✅ Answer: B) Dephosphorylated

Section 5: Mitotic Spindle

24. Microtubules of the mitotic spindle originate from:
  • A) Kinetochores
  • B) Centromeres
  • C) Centrosomes (MTOCs)
  • D) The nuclear envelope
✅ Answer: C) Centrosomes (MTOCs)
25. Which type of spindle microtubules connect directly to chromosomes?
  • A) Astral microtubules
  • B) Interpolar microtubules
  • C) Kinetochore (chromosomal) microtubules
  • D) Polar microtubules
✅ Answer: C) Kinetochore (chromosomal) microtubules
26. In the mitotic spindle, the minus ends of microtubules are located at:
  • A) Kinetochores
  • B) Spindle poles
  • C) The metaphase plate
  • D) The cell cortex
✅ Answer: B) Spindle poles

Section 6: Centromeres

27. The special histone variant found at centromeres is:
  • A) H2A
  • B) H3
  • C) H4
  • D) CENP-A
✅ Answer: D) CENP-A (Centromere Protein-A)
28. Human centromeres are composed of which repetitive DNA sequence?
  • A) Telomeric repeats (TTAGGG)
  • B) Alpha satellite DNA (~171 bp repeats)
  • C) Alu elements
  • D) LINE-1 sequences
✅ Answer: B) Alpha satellite DNA (~171 bp repeats)
29. Human centromeres can span up to how many base pairs?
  • A) 10,000 bp
  • B) 100,000 bp
  • C) 1,000,000 bp (1 million bp)
  • D) 10,000,000 bp
✅ Answer: C) 1 million bp
30. For stable chromosome inheritance, each chromosome must contain:
  • A) Two centromeres
  • B) Multiple origins of replication only
  • C) Exactly one centromere
  • D) No centromere
✅ Answer: C) Exactly one centromere

Section 7: Cytokinesis

31. Cytokinesis in animal cells is driven by contraction of:
  • A) Microtubule bundles
  • B) F-actin and myosin contractile ring
  • C) Condensin complexes
  • D) Interpolar spindle fibers
✅ Answer: B) F-actin and myosin contractile ring
32. The contractile ring begins to assemble during which stage of mitosis?
  • A) Prophase
  • B) Metaphase
  • C) Anaphase
  • D) Telophase
✅ Answer: C) Anaphase
33. When two daughter cells are equal in size after division, the process is called:
  • A) Asymmetric division
  • B) Meiotic division
  • C) Symmetric division
  • D) Amitosis
✅ Answer: C) Symmetric division
34. In plant cells, cytoplasmic division occurs via:
  • A) Cleavage furrow formation
  • B) Phragmoplast and new cell wall construction
  • C) Contractile ring contraction
  • D) Exocytosis of membrane vesicles only
✅ Answer: B) Phragmoplast and new cell wall construction
35. The phragmoplast in plant cells is formed from:
  • A) Astral microtubules
  • B) Kinetochore microtubules
  • C) Interpolar microtubules of the mitotic spindle at telophase
  • D) Actin filaments
✅ Answer: C) Interpolar microtubules of the mitotic spindle at telophase

Bonus - Integration Questions

36. Which sequence correctly represents the order of mitotic stages?
  • A) Metaphase → Prophase → Prometaphase → Anaphase → Telophase
  • B) Prophase → Prometaphase → Metaphase → Anaphase → Telophase
  • C) Prophase → Metaphase → Prometaphase → Telophase → Anaphase
  • D) Prometaphase → Prophase → Metaphase → Anaphase → Telophase
✅ Answer: B) Prophase → Prometaphase → Metaphase → Anaphase → Telophase
37. Which of the following is TRUE about M phase?
  • A) It consists only of mitosis
  • B) The contractile ring carries out chromosomal segregation
  • C) Mitosis handles nuclear division; cytokinesis handles cytoplasmic division
  • D) M-Cdk is inhibited by M-cyclin accumulation
✅ Answer: C) Mitosis handles nuclear division; cytokinesis handles cytoplasmic division
38. Sister chromatids are held together along their entire length by:
  • A) Condensins
  • B) Cohesins
  • C) Kinetochores
  • D) Centromere DNA
✅ Answer: B) Cohesins
Good luck on your final examination! 🎯 These 38 questions cover every key concept from the lecture. Focus especially on the cyclin-CDK regulation, stages of mitosis, checkpoint mechanisms, and the differences between animal and plant cell cytokinesis - these are classic exam topics.
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