7.14
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 role does it play in cell division?
The contractile ring is a structure composed of actin filaments and myosin II filaments that forms during cytokinesis to divide a cell into two daughter cells. It appears as a cleavage furrow that deepens around the cell's equator starting in anaphase. The contractile ring generates the force necessary to pinch off and separate the two new cells through coordinated contraction of its protein components.
Q2: How does RhoA regulate contractile ring assembly and function?
RhoA is the chief regulator of contractile ring assembly and function. It is activated locally at the cell cortex near the cell's equator by a guanine nucleotide exchange factor that exchanges GDP for GTP. RhoA activation promotes actin filament polymerization and stimulates myosin II assembly, ensuring the contractile ring forms at the correct location and time after chromosome segregation.
Q3: What proteins make up the contractile ring and how do they generate force?
The contractile ring consists of actin filaments, myosin II filaments, and septin filaments. Myosin motor activity generates contraction by moving along adjacent antiparallel actin filaments, causing them to slide past one another. This sliding action contracts the ring progressively until it pinches off two new cells. Septin filaments stabilize the ring structure during this process.
Q4: What is the role of anillin in contractile ring organization?
Anillin acts as the main organizer of the contractile ring by binding with actin, myosin II, membrane phospholipids, septin, and other structural and regulatory components. While RhoA activates contractile ring assembly, anillin coordinates the assembly of these diverse components into a functional contractile structure necessary for proper cell division.
Q5: How does the contractile ring coordinate with other cell cycle events?
The contractile ring forms after chromosome segregation during anaphase, coordinated by RhoA and anaphase spindle fibers. As the ring contracts, it progressively disassembles actomyosin filaments concomitantly, requiring fewer filaments to maintain ring thickness. During final cytokinesis stages, the contractile ring and central spindle mature to form the midbody, which completes cell separation through abscission.
Q6: What happens when contractile ring formation is disrupted?
Inefficient or absent contractile ring formation leads to abnormal cell division, impaired cell growth, and potential tumor formation. Proper contractile ring assembly and function are essential for successful cytokinesis and the production of two viable daughter cells with appropriate cytoplasmic content and genetic material.
Q7: How is RhoA activation regulated at the cell cortex?
RhoA activation is regulated by a guanine nucleotide exchange factor (Rho-GEF) located in the cortex region, the site of future cell division. Rho-GEF exchanges the GDP bound to inactive RhoA with GTP, activating the protein. This localized activation ensures the contractile ring forms specifically at the cell's equator where division will occur.