35.14
The Contractile Ring
Contractile rings are composed of microfilaments and are responsible for separating the daughter cells during cytokinesis. Contractile ring assembly p…
The final step of the cell cycle that divides a cell into two daughter cells is called cytokinesis. Cytokinesis begins after chromosome separation in mitosis and ends when the cell divides.
The beginning of cytokinesis is marked by the appearance of a crease, called the cleavage furrow. Starting in anaphase, the furrow deepens and spreads to form a ring around the cell. This compression, which ultimately divides the cell into two, is generated by the contractile ring.
A protein called RhoA is the chief regulator of contractile ring assembly and function. To ensure the contractile ring is formed in the right place, RhoA is activated locally at the cell cortex, near the equator of the cell. RhoA, along with anaphase spindle fibers, also ensures that the contractile ring is formed at the right time, after chromosome segregation.
The contractile ring is made up of structural proteins, including actin filaments and myosin II filaments. RhoA activity results in the assembly of myosin II and anti-parallel actin filaments into the structure of the contractile ring. RhoA promotes localized actin filament polymerization which is necessary for the contractile ring formation.
The contractile ring components generate the force necessary to divide the cell. One mechanism of contraction involves myosin motor activity. Here, myosin filaments move toward the plus end of adjacent antiparallel actin filaments. This activity pulls the anti-parallel actin, causing them to slide past one another, contracting the ring.
The contraction of the ring continues until it pinches off two new cells. The ring is degraded once the new cells are formed.
Inefficient or absence of contractile ring formation can lead to abnormal cell division, impaired growth, and the potential for tumor formation.
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Q1: What is the contractile ring and what is its role in cell division?
The contractile ring is a dynamic structure composed of actin filaments and myosin II proteins that forms at the cell's equator during cell division. It functions as the primary mechanism for dividing the cytoplasm, contracting to pinch the cell into two daughter cells. This structure is essential during mitosis and cytokinesis for completing cell division.
Q2: How do actin filaments and myosin II work together in the contractile ring?
Actin filaments form the structural backbone of the contractile ring, while myosin II proteins act as molecular motors that pull these filaments. The interaction between myosin II heads and actin filaments generates the force needed to contract the ring, creating the mechanical tension that drives cell division.
Q3: What is a cleavage furrow and how does it form?
A cleavage furrow is the visible indentation that appears on the cell surface during cytokinesis, marking where the contractile ring is actively constricting. It forms as the contractile ring tightens, progressively deepening until it completely separates the parent cell into two distinct daughter cells.
Q4: When does the contractile ring assemble during the cell cycle?
The contractile ring assembles during late mitosis, specifically during anaphase and telophase when chromosomes are separating and moving toward opposite poles. Its formation is precisely timed to ensure that cytokinesis occurs after nuclear division is complete, preventing damage to genetic material.
Q5: What triggers the contraction of the contractile ring?
Contractile ring contraction is triggered by signaling pathways activated during late mitosis, which regulate myosin II activity and actin filament dynamics. These signals ensure the ring contracts at the correct time and location, coordinating the mechanical forces needed to complete cell division successfully.
Q6: How is the contractile ring positioned at the cell equator?
The contractile ring is positioned at the cell equator through signals from the mitotic spindle, which marks the division plane between separating chromosomes. This precise positioning ensures the ring forms perpendicular to the spindle axis, allowing equal distribution of cytoplasm and organelles to daughter cells.