All living things are made up of cells. Some only have one cell, such as bacteria, archaea, and some plants, fungi and other unicellular organisms. Many living things are multicellular, including all animals and most plant species. All species, however, begin life as a single cell, even humans. Without cell division, life could not exist. Organisms use cell division in order to reproduce, as well as to grow (if the organism is made up of more than one cell). The cells in your body are frequently or preparing to divide; some divide dozens of times during their cell lifetimes. Other cells are with you all of your life, and the only time they divide is when they are first split off from another cell.

Although cells have different rates at which they divide, the carefully choreographed routine of growth and cell division is the same from cell to cell, whether it happens in a growing human embryo or in a college student waiting for a broken bone to heal, or even in recently planted seeds in the garden just beginning to sprout shoots. This continually repeating routine is called the cell cycle, and it is comprised of two main stages: interphase and mitosis. These two stages each involve several steps. Mitosis is the phase of the cell cycle in which the cell copies its genetic information and duplicates the nucleus, so that the cell can can divide in two.

People often confuse the terms mitosis and meiosis. They are closely related terms, since they both have to do with cell division, but they are also different processes, with fundamentally different outcomes. It is important to know the difference. The cell cycle is the continually renewing process by which an organism’s cells grow, prepare for division, divide and begin again. Mitosis is the phase of the cell cycle in which they divide. Cells have something called a ploidy number – this is the number of chromosomes in a cell. It is represented by the variable N. In humans, chromosomes are grouped in pairs, which makes human cells (with the exception of reproduction cells) diploid, or 2N. Mitosis results in two daughter cells that are both genetically identical to the original cell, and also both have a 2N ploidy number. In some species, mitosis might result in daughter cells that are 4N or 7N or N, for example, but they will always have the same ploidy number as the parent cell.

Meiosis is a separate process of cell division in species that engage in sexual reproduction. It is used for gametogenesis, which is how the body creates gametes, or sex cells. In humans, these cells are spermatozoa (sperm) and ova (eggs). A 2N cell undergoes a series of steps of cell division that are similar but not the same as the ones in mitosis in order to generate daughter cells. In both mitosis and meiosis, the cell division results in the parent cell being replaced by the daughter cells. Unlike mitosis, meiosis results in four daughter cells, not two, and they are not identical to each other because they recombine their genetic information. Furthermore, each of the four daughter cells has a ploidy number of N.

Since many species are not diploid the way humans are, the gamete daughter cells of other species may not have ploidy numbers of N, but they will be half, or haploid, of whatever the ploidy number of the parent cell was. The reason for this is because during sexual reproduction, one of these haploid gametes will fuse with a haploid gamete from an individual, usually of a different sex, forming a diploid zygote with a unique genome. In humans, this happens when a sperm fuses with an egg, beginning a pregnancy. The resulting zygote will grow into an embryo and then a fetus, and the resulting human that is born will have a different genetic code than anyone before, because of the genetic recombination that happens during meiosis. Find out more details about the similarities and differences between mitosis and meiosis in cell growth and sexual reproduction.

The four stages of mitosis are:

1. Prophase
   
2. Metaphase
   
3. Anaphase
   
4. Telophase
   

They are also known as the mitosis phases, or mitosis subphases. Sometimes a stage is added between the first and second, called prometaphase. Regardless of how many stages are described, the divisions are manmade ones that do not affect what happens on a cellular level. Scientists find these stages useful for understanding and communicating with each other about microbiology. In nature, however, the cell cycle is happening fluidly and continuously, without pauses to signal the end of metaphase and the beginning of anaphase. Before mitosis begins, interphase must end. Interphase is the part of the cell cycle in which the cell grows and does its job, whether that job is to be a nerve cell, a smooth muscle cell or a vascular tissue cell in a plant stem. There are three stages of interphase, and these are:

1. Gap phase 1, or G₁
   
2. Synthesis phase, or S phase
   
3. Gap phase 2, or G₂

During the gap phases, the cell grows. During S phase, the cell continues to do its daily tasks, but it also replicates its DNA. This means that it creates a copy of every single chromosome in its genome. By the end of S phase, there are twice as many chromosomes in the nucleus. Each identical copy of a chromosome is bound together by something called a centromere, and now the entire pair is called a chromosome, while each individual is called a sister chromatid. They will stay this way until partway through mitosis, which begins at the end of Gap phase 2.

Prophase is the first and longest of the four stages of mitosis. Prophase takes about 36 minutes to complete in human cells. Centrioles, which are structures made of microtubules that are located near the cell’s nucleus, move to opposite sides of the cell. Centrioles are part of larger structures called centrosomes. Later, these will be play an important role in dividing the the nucleus. The nuclear envelope dissolves, leaving the chromosomes floating freely. The DNA condenses very tightly around strands of chromatin, making the chromosomes bulky enough to be visible under microscopes. At other times during the cell cycle, they are not visible. This condensation simplifies nuclear division once chromosomes begin to move around within the cell, in later stages.

Metaphase is a short stage, lasting only three minutes. During metaphase, microtubules that are growing (replicating) from the centrioles at the cell poles reach the chromosomes. They begin to attach to the chromosomes. They attach to protein bundles on the centromeres called kinetochores. The microtubules are also called spindle fibers. There are other spindle fibers growing from the centrioles that do not attach to the chromosomes, but reach the spindle fibers growing from the opposite side and attach to each other. The spindle fibers that attach to the chromosomes are called kinetochore microtubules, while the ones that attach to each other are called interpolar microtubules. The kinetochore microtubules align the chromosomes along a middle plane of the cell called a metaphase plate. This is an imaginary line that is halfway between each of the centrioles at the cell poles. The chromosomes line up along this plate to prepare for the next step. Some scientists note an intermediate phase before metaphase called prometaphase, which takes some features of prophase and some features of metaphase, while many scientists do not.

The third stage of mitosis is called anaphase. Like metaphase, it lasts only three minutes. Anaphase begins only when certain conditions have been met during metaphase. Each chromosome has a centromere on it, binding the sister chromatids together. During metaphase, one spindle fiber emanating from each centrosome – the axes at opposite poles of the cell – must attach to the chromosome’s centromere. The cell does not move forward to anaphase until each chromosome has two spindle fibers attached to it. If both of the spindles on any of the chromosomes are from the same centrosome, that also will prevent the cell from moving forward to anaphase. The cell cycle has many checkpoints to make sure errors do not happen, because errors cause genetic mutations.

During metaphase, each of the spindle fibers attached to the centromere in such a way that it was fastened to one sister chromatid or the other. During anaphase, the spindle fibers shorten, which causes the sister chromatids to separate and move away from each other toward opposite sides of the cell. When they separate, the centromere splits apart as well, one half going with each sister chromatid. The ploidy number is always a count of how many chromosomes are in the cell, and the count of chromosomes is always a count of how many centromeres are in the cell. When the centromeres split in two, they each became their own centromere, and that means that each sister chromatid became its own chromosome. That in turn means that the ploidy number has doubled, for the time being. In a human somatic (non-reproductive) cell, where there were 2N or 46 chromosomes before, there are now 4N or 92 chromosomes. Forty-six move to one end of the cell, and forty-six to the other end. During anaphase, the interpolar microtubules also work to push and pull the cell so that it stretches and becomes oblong. This widens the distance between the two centrosomes.

Telophase is the final of the four stages of mitosis, and lasts for 18 minutes in human cells. The chromosomes finish their migration toward the two poles of the cell. In a human cell, this means that there are now 46 chromosomes at each pole. The spindle fibers that pulled the chromosomes there dissipate. The chromosomes uncoil again, while at the same time, a nuclear membrane forms around each of the two groups. This forms two new nuclei. Simultaneously, a process called cytokinesis occurs, which divides the rest of the cell into two separate daughter cells, and returns the ploidy number from 4N to 2N, since each new cell will once again have the same number of chromosomes as the original parent cell (46 for a human cell).

In animal cells, cytokinesis happens when a filament ring forms in the same spot where the metaphase plate was before, at the midpoint between the two poles. It constricts the cell, pinching it inward in the center, until a cleavage furrow forms. This looks like an hourglass whose connecting passage becomes increasingly narrow until the two globes break off into two separate spheres. In plant cells and other cells with cells walls, the Golgi apparatus synthesizes vesicles that form a cell plate along the cell’s equator, which is in the same place as the metaphase plate and where the filament ring constricts the cell in animal cells. Over time, the cell plate becomes bound by a cell membrane that is continuous with the cell wall; it functionally becomes a cell wall itself, dividing one new daughter cell from the other, both of which are surrounded by the original cell walls. Regardless of the type of cell, at the end of telophase, the cell returns to the beginning of the cell cycle: interphase.