The cell cycle governs the growth, replication, and division of cells, and is one of the cornerstones of understanding biology. It consists of interphase and mitosis. Because there is so much emphasis on mitosis in biology classes, students often imagine that cells spend a lot of time doing it. This is not the case. Most of the life of a cell is spent in interphase which is further subdivided into the following three stages: During G1, the cell grows and synthesizes proteins necessary for DNA replication. In the S phase, DNA replication occurs. Finally, G2 follows S phase, during which the cell continues to grow and prepares for division. Then and only then can mitosis occur, and it’s job is to divide all duplicated DNA and organelles into two new genetically identical daughter cells. Mitosis is separated into 4 phases: Prophase, Metaphase, Anaphase, Telophase and Cytokinesis.
Prophase is the first phase of mitosis, and the events that occur during this phase prepare the cell to divide. First, it’s important to understand the difference between chromatin and chromosomes. When a cell is in interphase it’s DNA is in a loosely packed fiber form called chromatin. A chromosome is a more condensed form of DNA that is tightly wound around small protein balls called histones For the cell to effectively sort the two sets of DNA without tangling up, DNA in its chromatin form condenses into chromosome form during prophase. Because DNA never leaves the nucleus the nuclear envelope must disintegrate during prophase so that the two sets of DNA are free to float to opposite sides of the cell. Additionally, centrosomes, which serve as anchors for the spindle fibers, move to opposite poles of the cell. Microtubules, which form the spindle fibers, start to assemble and extend from the centrosomes towards the center of the cell, forming the spindle apparatus, which will help in organizing and segregating the chromosomes later.
Before we dive into metaphase, we need to answer the question “what are sister chromatids?”. Remember that there are two identical sets of DNA in the cell during mitosis. Sister chromatids are pairs consisting of a chromosome and its copy that are held together by a structure called the centromere. During metaphase, the spindle fibers formed during prophase attach to the centromeres of the condensed chromosomes. Once they are attached, the spindle fibers align the chromosomes along what is called the metaphase plate which a line down the middle of the cell. Because the arrangement of chromosomes during this phase is crucial to their accurate segregation into new daughter cells, the cell undergoes a checkpoint during metaphase. If chromosomes are not properly aligned and attached to the spindle fibers, the cell halts progression until the issues are resolved.
During anaphase of mitosis, paired sister chromatids are separated and pulled towards opposite poles of the cell. The spindle fibers that attached to the centromeres during metaphase contract during anaphase. This exerts a force on the centromeres, causing the sister chromatids to move apart and ensures that each daughter cell will receive an identical set of chromosomes. As the chromatids reach the poles, they are now considered individual chromosomes.
Once chromatids arrive at opposite ends of the cell, the spindle fibers disassemble, and two new nuclear envelopes start to form around the separate clusters of chromosomes at each pole. These chromosomes also relax into chromatin, returning to their less tightly packed state. Telophase marks the completion of chromosome movement and the re-establishment of nuclear compartments, setting the stage for the final stage of cell division.
During cytokinesis, the cytoplasm of the cell divides, ultimately yielding two daughter cells, each containing a complete set of chromosomes. This process varies slightly between animal and plant cells.
In animal cells, cytokinesis begins with the formation of a contractile ring composed of actin filaments at the center of the cell. The contraction of the ring contracts leads to the formation of a cleavage furrow which eventually pinches the cell into two distinct daughter cells, each containing a nucleus with a complete set of chromosomes and a share of organelles.
In plant cells, instead of a contractile ring, vesicles containing cell wall material accumulate along the plane of cell division. These vesicles fuse together, forming a structure called the cell plate. The cell plate grows outward until it fuses with the cell membrane at the edges of the parent cell, effectively dividing it into two daughter cells