Cytokinesis
- Mitosis is the process of separating the duplicates of each of the cell’s chromosomes.
- It is usually followed by division of the cell.
- However, there are cases (cleavage in the insect embryo is an example) where the chromosomes undergo the mitotic process without division of the cell.
- Thus a special term, cytokinesis, for the separation of a cell into two.
- In animal cells, a belt of actin filaments forms around the perimeter of the cell, midway between the poles.
- The interaction of actin and a myosin (not the one found in skeletal muscle) tightens the belt, and the cell is pinched into two daughter cells.
- In plant cells, a cell plate forms where the metaphase plate had been.
- The cell plate is synthesized by the fusion of multiple membrane-bounded vesicles.
- Their fusion supplies new plasma membrane for each of the two daughter cells.
- Synthesis of a new cell wall between the daughter cells then occurs at the cell plate.
- Cytokinesis is the process whereby the cytoplasm of a single eukaryotic cell is divided to form two daughter cells.
- It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation to the next.
- In plant cells, a dividing structure known as the cell plate forms across the centre of the cytoplasm and a new cell wall forms between the two daughter cells.
Cell plate formation
- Due to the presence of a cell wall, cytokinesis in plant cells is significantly different from that in animal cells.
- Rather than forming a contractile ring, plant cells construct a cell plate in the middle of the cell.
- The Golgi apparatus releases vesicles containing cell wall materials.
- These vesicles fuse at the equatorial plane and form a cell plate.
- The cell plate begins as a fusion tube network, which then becomes a tubule-vesicular network (TVN) as more components are added.
- The TVN develops into a tubular network, which then becomes a fenestrated sheet which adheres to the existing plasma membrane.
- Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side:
- Phragmoplast forms and cell plate starts to assemble in the center of the cell.
- Towards the right: Phragmoplast enlarges towards the outside of the cell, leaving behind mature cell plate in the center.
- The cell plate will transform into the new cell wall once cytokinesis is complete.
- Cytokinesis in terrestrial plants occurs by cell plate formation.
- This process entails the delivery of Golgi-derived and endosomal vesicles carrying cell wall and cell membrane components to the plane of cell division and the subsequent fusion of these vesicles within this plane.
- After formation of an early tubule-vesicular network at the center of the cell, the initially labile cell plate consolidates into a tubular network and eventually a fenestrated sheet.
- The cell plate grows outward from the center of the cell to the parental plasma membrane with which it will fuse, thus completing cell division.
- Formation and growth of the cell plate is dependent upon the phragmoplast, which is required for proper targeting of Golgi-derived vesicles to the cell plate.
- As the cell plate matures in the central part of the cell, the phragmoplast disassembles in this region and new elements are added on its outside.
- This process leads to a steady expansion of the phragmoplast, and concomitantly, to a continuous retargeting of Golgi-derived vesicles to the growing edge of the cell plate.
- Once the cell plate reaches and fuses with the plasma membrane the phragmoplast disappears.
- This event not only marks the separation of the two daughter cells, but also initiates a range of biochemical modifications that transform the flexible cell plate into a cellulose-rich, stiff primary cell wall.
- The heavy dependence of cell plate formation on active Golgi stacks explains why plant cells, unlike mammalian cells, do not disassemble their secretion machinery during cell division.
- Cytokinesis refers to the division of:
A) Nucleus
B) Chromosomes
C) Cytoplasm
D) Mitochondria
Answer: C) Cytoplasm - Cytokinesis typically follows:
A) S phase
B) Mitosis
C) DNA replication
D) G1 phase
Answer: B) Mitosis - Cytokinesis ensures that:
A) DNA is synthesized
B) Proteins are degraded
C) Each daughter cell gets a nucleus and cytoplasm
D) Nuclear membrane dissolves
Answer: C) Each daughter cell gets a nucleus and cytoplasm - In which phase of mitosis does cytokinesis usually begin?
A) Metaphase
B) Prophase
C) Anaphase
D) Telophase
Answer: D) Telophase - In which type of cell is the cell plate formed during cytokinesis?
A) Animal cell
B) Plant cell
C) Fungal cell
D) Bacterial cell
Answer: B) Plant cell
- What structure is responsible for cytokinesis in animal cells?
A) Cell plate
B) Mitotic spindle
C) Contractile ring
D) Phragmoplast
Answer: C) Contractile ring - The contractile ring in animal cells is primarily composed of:
A) Microtubules
B) Actin and myosin
C) Cellulose
D) DNA and proteins
Answer: B) Actin and myosin - The contractile ring forms around the cell:
A) At one end
B) At the poles
C) Midway between the poles
D) At the nucleus
Answer: C) Midway between the poles - What happens when the actin and myosin filaments tighten the contractile ring?
A) Nuclear envelope dissolves
B) Cell elongates
C) The cell is pinched into two
D) Chromosomes replicate
Answer: C) The cell is pinched into two - Which myosin is involved in cytokinesis, not found in skeletal muscle?
A) Myosin I
B) Myosin II
C) Unconventional myosin
D) Smooth muscle myosin
Answer: C) Unconventional myosin
- What replaces the contractile ring in plant cell cytokinesis?
A) Equatorial furrow
B) Cell plate
C) Mitotic spindle
D) Centrosome
Answer: B) Cell plate - Cell plate formation begins at:
A) Nuclear envelope
B) Plasma membrane
C) Equatorial plane
D) Centrosome
Answer: C) Equatorial plane - The vesicles that form the cell plate are derived from:
A) Endoplasmic reticulum
B) Nucleus
C) Golgi apparatus
D) Lysosomes
Answer: C) Golgi apparatus - The cell plate eventually forms:
A) Plasma membrane only
B) Nuclear envelope
C) New cell wall
D) Contractile ring
Answer: C) New cell wall - What is the role of the phragmoplast?
A) Contracts the cell membrane
B) Forms spindle fibers
C) Guides vesicles to the cell plate
D) Activates cell wall enzymes
Answer: C) Guides vesicles to the cell plate
- Which network is formed initially during cell plate formation?
A) Tubular-vesicular network (TVN)
B) Fenestrated sheet
C) Chromatin mesh
D) Protein scaffold
Answer: A) Tubular-vesicular network (TVN) - The fenestrated sheet is the precursor to the:
A) Nuclear membrane
B) Cytoplasmic membrane
C) Cell wall
D) Endoplasmic reticulum
Answer: C) Cell wall - What happens to the phragmoplast as the cell plate matures?
A) It expands and disassembles centrally
B) It becomes rigid
C) It forms microvilli
D) It migrates to the nucleus
Answer: A) It expands and disassembles centrally - Fusion of vesicles at the midline of a plant cell helps in forming:
A) Mitotic spindle
B) Cell plate
C) Plasmodesmata
D) Flagella
Answer: B) Cell plate - What material strengthens the mature cell plate into a cell wall?
A) DNA
B) Actin
C) Cellulose
D) Protein kinase
Answer: C) Cellulose
- Cytokinesis in plants is heavily dependent on:
A) Ribosomes
B) Golgi-derived vesicles
C) Mitochondria
D) Lysosomes
Answer: B) Golgi-derived vesicles - The phragmoplast is primarily composed of:
A) Lipids and enzymes
B) Actin and tubulin
C) Microtubules and actin
D) Microtubules
Answer: D) Microtubules - TVN stands for:
A) Tubulin vesicular network
B) Tubule-vesicular network
C) Temporary vacuole nucleus
D) Terminal vesicle node
Answer: B) Tubule-vesicular network - Which organelle remains active during plant cytokinesis, unlike in animal cells?
A) Nucleus
B) ER
C) Golgi apparatus
D) Chloroplast
Answer: C) Golgi apparatus - Why do plant cells not use contractile rings?
A) Lack of myosin
B) Due to the rigid cell wall
C) Lack of actin
D) No nucleus
Answer: B) Due to the rigid cell wall
- Which of the following is true for animal cytokinesis but not plant?
A) Forms new membrane
B) Forms a cell plate
C) Uses actin ring
D) Involves Golgi vesicles
Answer: C) Uses actin ring - Which phase sees initiation of cytokinesis?
A) Prophase
B) Metaphase
C) Telophase
D) Interphase
Answer: C) Telophase - In insect embryos, mitosis may occur without:
A) Chromosome replication
B) Cell division
C) Spindle formation
D) Cell growth
Answer: B) Cell division - Cell plate grows in which direction?
A) Outward from center
B) From edges to center
C) From one pole to another
D) Along spindle
Answer: A) Outward from center - What is formed after cytokinesis is completed in plant cells?
A) Middle lamella
B) Primary cell wall
C) Endoplasmic reticulum
D) Golgi stack
Answer: B) Primary cell wall
- The flexible cell plate is transformed into a stiff wall due to:
A) Myosin
B) Actin polymerization
C) Cellulose deposition
D) DNA synthesis
Answer: C) Cellulose deposition - Which cytoskeletal element is not involved in plant cytokinesis?
A) Microtubules
B) Actin
C) Intermediate filaments
D) Myosin
Answer: C) Intermediate filaments - Which of the following phases is absent in cytokinesis?
A) Prophase
B) Telophase
C) S phase
D) Metaphase
Answer: C) S phase - Cytokinesis ensures:
A) Equal DNA content only
B) Separation of organelles
C) Division of cytoplasm
D) Formation of centrosome
Answer: C) Division of cytoplasm - Fusion of vesicles during cell plate formation occurs at the:
A) Nuclear envelope
B) Center of the cell
C) Golgi body
D) Mitochondrial membrane
Answer: B) Center of the cell
- What structure disassembles after cell plate fusion is complete?
A) Mitotic spindle
B) Phragmoplast
C) Cell wall
D) Centrosome
Answer: B) Phragmoplast - Why do plant cells need a new cell wall during cytokinesis?
A) To trap DNA
B) To replace broken cell membrane
C) To separate daughter cells structurally
D) To facilitate ATP production
Answer: C) To separate daughter cells structurally - Vesicles used in plant cytokinesis are rich in:
A) Cell wall precursors
B) DNA
C) ATP
D) Histones
Answer: A) Cell wall precursors - What disappears in the center after the cell plate is formed?
A) Microfilaments
B) Phragmoplast
C) Cell membrane
D) Chromosomes
Answer: B) Phragmoplast - The role of actin and myosin in animal cells is similar to which in plant cells?
A) Cell wall
B) Cell plate
C) Phragmoplast
D) Vacuole
Answer: C) Phragmoplast
- Which term best describes plant cytokinesis?
A) Cleavage
B) Budding
C) Cell plate formation
D) Binary fission
Answer: C) Cell plate formation - What is the main function of cytokinesis?
A) Formation of nucleus
B) Production of spindle
C) Separation of cytoplasm
D) DNA condensation
Answer: C) Separation of cytoplasm - What prevents the reuse of DNA origins after S phase?
A) DNA polymerase
B) CDK-cyclin
C) Vesicle fusion
D) Cytokinesis
Answer: B) CDK-cyclin - Final step in cytokinesis in animal cells is called:
A) Telophase
B) Cleavage furrow formation
C) Nuclear envelope formation
D) Membrane stiffening
Answer: B) Cleavage furrow formation - After cytokinesis, the cell returns to which phase?
A) G0
B) G1
C) G2
D) M
Answer: B) G1
- Cytokinesis can occur during meiosis.
A) True
B) False
Answer: A) True - Golgi disassembles during plant cytokinesis.
A) True
B) False
Answer: B) False - Actin forms a ring during cytokinesis in plant cells.
A) True
B) False
Answer: B) False - In plants, cytokinesis always requires the phragmoplast.
A) True
B) False
Answer: A) True - Cytokinesis is required for nuclear division.
A) True
B) False
Answer: B) False