Cell division is important for an organism to growth and survival. In humans, starting as a single cell in the embryo, divisions lead to over 30 trillion cells in adulthood. This continuous process, known as the cell cycle, includes cytokinesis—the final step.
Cytokinesis begins with cleavage furrow formation during chromosome separation, dividing cytoplasm between daughter cells. It ends when the bridge connecting them is cut and this process ensures successful cell division.
Buckle up and let’s dive right into the article to understand cytokinesis and its impact on growth.
What is Cytokinesis?
Cytokinesis definition states that cytokinesis is the last step in the cell cycle. Cytokinesis divides the parent cell’s cytoplasm into two daughter cells through a process known as cytoplasmic division or cell cleavage.
In animals, it starts during anaphase, while in plants, it begins in prophase, and concludes in telophase. This results in two daughter cells following mitosis, each with an identical chromosomal set.
After cytoplasmic division, each new cell is enveloped by a cell membrane, and organelles replicate or synthesize within the cytoplasm. Unlike chromosome replication, cytoplasmic material isn’t doubled, leading to daughter cells being roughly half the size of the parent cell. The daughter cell nuclei are similar in size due to chromosome replication before mitosis.
Process of Cytokinesis
The cytokinesis process starts alongside mitosis, where a division plane forms at the cell’s center. Microtubules that also create spindle fibers, separating chromosomes in mitosis, shape this plane.
As cytokinesis continues, a contractile ring forms, contracting to make a cleavage furrow. The furrow deepens until the initial cell splits into two. With this, cytokinesis concludes, and the new cells start their cell cycle afresh.
Cytokinesis is also seen in creating egg and sperm cells, in a process called meiosis. The procedure in meiosis mirrors mitosis, yet with a distinction: cytokinesis occurs twice, forming four new cells, each with just one chromosome copy.
Stages of Cytokinesis
Cytokinesis typically follows the final stages of nuclear division in mitosis and meiosis. It’s the last step before cells fully develop and split. In basic terms, a contractile ring made of actin filaments tightens around the cell’s middle, creating a cleavage furrow. This furrow deepens until the cell membranes separate and closes off two individual cells, marking the end of the process.
Cytokinesis unfolds in four stages: initiation, contraction, membrane insertion, and completion, each with variations in animal and plant cells.
Initiation
In the first step “initiation”, the goal is to pinpoint where the cleavage furrow forms, where actin filaments tighten like a “noose” around the cell. The spindle plays a key role here – it ensures chromosomes split evenly between nuclei. The spindle links nuclei and the membrane. It also has small structures, astral microtubules, which interact with the membrane and guide actin filament alignment for the upcoming furrow.
Contraction
After identifying the spot for the cleavage furrow, it’s time for the actin filaments to come together. Other proteins, like myosin, also gather in this area, helping pull the actin filaments. This process helps in the formation of the contractile ring.
Membrane Insertion
The protein we mentioned above, myosin, is like a tiny motor called a motor protein. While it’s crucial for muscle movement in the body, within individual cells, myosin uses energy (ATP) to contract the contractile ring. This makes the cleavage furrow deeper and the cell narrower, forming two daughter cells.
Myosin essentially makes actin strands move against each other, much like flexing a muscle. It also releases specific sections, or subunits, which causes the contractile ring to shrink and the cleavage furrow to pinch.
Completion
The process concludes as the contractile ring fully divides the cell into two new ones, each with an intact membrane and a copy of the original genetic material. The final step involves breaking the cell membrane at its smallest point. The membrane then seals at this break, resulting in two independent, identical daughter cells, each capable of functioning on its own.
Cytokinesis in an Animal Cell
In animals, the cytoplasm tightens until two daughter cells form. This begins with a cell furrow forming, a kind of pucker in the cell membrane enclosing the genetic material. This furrow is caused by a contractile ring just below the cell surface. It starts at a point around the cell’s middle and then wraps around the cell’s circumference, creating a small furrow. As the contractile ring contracts, this furrow deepens.
To accommodate the expanding surface area, the cell generates more membrane material by fusing vesicles. This new material is added near the contractile ring. The contractile ring continues to contract until it separates the two daughter cells with a narrow portion containing the remains of the mitotic spindle, called the midbody. Eventually, it breaks off, forming two daughter cells each enclosed in its membrane. After cytokinesis, organelles reassemble in the daughter cells. Some, like mitochondria and chloroplasts, replicate from existing ones, while others like the endoplasmic reticulum and Golgi apparatus regenerate from fragments when the parent cell’s nuclear envelope breaks down.
How does the Contractile Ring function?
The contractile ring’s action and position are governed by the spindle fibers or mitotic spindle, a mechanism also regulating chromosome movement in mitosis. The mitotic spindle aligns perpendicular to the equatorial plane, stretching between the cell’s poles where two sets of chromosomes sit. This safeguards proper chromosome separation during cytokinesis.
Fueling the contractile ring’s contraction are actin and myosin II, akin to the process in smooth muscle. As the cell furrow deepens, microtubules stabilize it. Once cleavage is done, the contractile ring vanishes.
Cytokinesis in a Plant Cell
In plants, cytokinesis starts by forming a cell plate in the middle, which eventually becomes the middle lamella between plant cells. The primary and secondary cell walls develop on each side of the cell plate, creating the basis for daughter cell separation.
In prophase, cytokinesis begins with the assembly of an actin filament and microtubule cytoskeleton around the cell, called the preprophase band. This band determines the cell plate’s position and disappears before mitosis reaches metaphase.
During anaphase, the cell plate forms under the control of the phragmoplast, containing mitotic spindle remains. Microtubules in the spindle transport vesicles with polysaccharides and glycoproteins to the phragmoplast center, forming the early cell plate. This plate grows until it fuses with the parent cell membrane and wall.
The preprophase band’s location determines the fusion point. Cellulose is later added to create a cell wall, dividing the parent cell into two equal-volume daughter cells, each with a diploid chromosome set. These cells can expand or grow in size afterward.
Proteins Involved in Cytokinesis Process
The contractile ring is a substantial structure containing actin, myosin, septins, and other proteins organizing these cytoskeletal parts. It’s believed that membrane-associated proteins link the contractile ring to the cell surface, similar to other cellular structures involving actin, like focal adhesions and accompanying stress fibers.
Actinomycin D
Actin’s role in cytokinesis has been recognized for a while and is backed by different discoveries. First, actin forms a ring during division in nearly all cell types. Second, anti-actin drugs like cytochalasin hinder cytokinesis. Third, mutations in actin-associated proteins cause problems in cytokinesis, seen in yeast mutants and various species.
Myosin
Myosin plays a significant role in cytokinesis. Cytoplasmic myosin II, a version of the long-tailed myosin found in muscles, is believed to be the motor-driving cytokinesis. Its activity is crucial as it’s strictly regulated through phosphorylation of specific amino acids, controlling movement and formation of multimers. A cytoplasmic myosin II molecule is a hexamer formed by two of three polypeptide pairs: the 200 kDa heavy chain (MHC), the 18 kDa essential light chain (ELC), and the 20 kDa regulatory light chain (RLC).
Septins
Septins, present in eukaryotic animals, aid in plasma membrane-cytoskeleton interactions during events like cytokinesis. These proteins have specific sequences suggesting nucleotide binding.
Additional proteins
There are various other cytoskeletal proteins located near the cleavage furrow, but their exact role in cytokinesis remains uncertain. These include proteins that cross-link or bundle actin filaments, such as a-actinin, filamin, and the recently identified anillin.
There are also actin-membrane connectors like talin and ERM proteins, as well as those that influence F-actin assembly or organization differently, like acidic calponin which hinders the ATPase activity of phosphorylated myosin. Additionally, transmembrane glycoproteins like CD43 are present. It’s important to mention that there isn’t strong evidence that most of these proteins are part of the contractile ring.
Conclusion
From the intricate orchestration of contractile rings in animals to the unique cell plate mechanism in plants, cytokinesis ensures the creation of new generations of cells. The significance of cytokinesis should be crystal clear. It is the phrase in which both animal and plant cells duplicate. The cytokinesis process helps organisms to grow in size and intricacy. Our life as intricate and enduring beings hinges on the innumerous cell divisions throughout our lives.
