Cell division calls for the distribution of identical genetic material, i.e., DNA to two daughter cells. This article takes you through the various stages in cell division.
Cell, the building block of life, is the basic functional unit of all living organisms. Cells can either be individual organisms, like most bacteria or amoeba which are unicellular, or can be functional units to other (higher) organisms, such as animals or plants. Human beings are multicellular, comprising approximately 100 trillion cells. Cell division stages are a series of events which occur during cell division and replication. Depending upon the type of cell, cell division can be of three different ways:
- In prokaryotic cells (cells without nucleus), cell division occurs through a process called binary fission.
- In eukaryotic cells (cells with a nucleus), cell division may occur through mitosis, or meiosis.
Further, depending upon the organism and the function of eukaryotic cell, cell division is of two types:
(1) Mitosis: Mitosis is the simpler and more common type of cell division wherein one cell divides into two identical daughter cells.
(2) Meiosis: Meiosis, on the other hand, occurs only in sexually reproducing organisms. Here, one cell undergoes two successive divisions to produce four genetically different daughter cells.
Mitosis is the most common type of cell division occurring in eukaryotes. One diploid parent cell divides into two genetically identical diploid daughter cells.
What are haploid and diploid cells?
- A haploid cell is a cell containing exactly one set of each chromosome. (e.g. A, B)
- A diploid cell is a cell containing two sets of each chromosome. (e.g. AA, BB)
Cell division by mitosis can be divided into the following 5 stages: Interphase, Prophase, Metaphase, Anaphase, and Telophase.
Gap I (G1)
- This is the primary stage where the cell prepares itself for division.
- There are no significant changes initiating the division here.
- This time frame is allocated towards the development of the cell for multiplication; thus, the duration may vary from cell to cell.
- Naturally, cell development through this stage will depend upon growth factors, such as nutrients, etc.
- This is the actual step where the cell division starts, at a very small yet important scale.
- Chromosomes start replicating inside the nucleus, so that each one has an exact identical twin.
- These two identical entities of each chromosome are called ‘chromatids.’
- Any two chromatids are joint together at a single point known as the ‘centromere.’
Gap II (G2)
- Once the chromosomes are replicated, other cell organelles begin to replicate as well.
- So, by the end of G2, the cell has two chromosomes of each organelle.
- Though G2 is a stage of Interphase, it is not an inevitable one. Some cells are known to directly proceed to cell division without G2.
- The chromosomes begin to condense and are now microscopically visible.
- Spindle fibers (made of microtubules) begin to form from the centrioles.
- At the same time, centrioles start moving towards the opposite ends of the cell.
- Lastly, the nucleolus and nuclear membrane disintegrate, marking the beginning of metaphase.
- The chromosomes are positioned along the equatorial plane by the spindle fibers.
- The spindle fibers then attach onto the centromere of every chromosome pair.
- The centromere has an exact structure for the spindle fiber to attach, known as the ‘kinetochore.’
- The centromeres split due to the spindle fibers exerting pressure from the opposite ends.
- The chromosomes (only half of the initial ones) move towards opposite poles of the cell.
- The centrioles move as farther away from one another as possible.
- Nuclear membrane starts reappearing.
- The spindle fibers begin detaching themselves from the chromosomes.
- Chromosomes begin decondensing at this stage.
- In plant cells, a line of demarcation begins to appear in the cell―the exact line of cell division, also known as the ‘telophasic bundle.’
- Mitosis is technically defined as the division of the nucleus; thus, cytokinesis or cytoplasmic division is not a stage of mitosis.
- In plant cells: In place of the telophasic bundle, an actual cell plate develops, parting the parent cell into two halves. However, the two newly formed cells don’t disjoin completely and remain stuck at the common plate.
- In animal cells: Two clefts develop in between the parent cell, burrowing further to actually divide the cytoplasm. Actin and myosin are filaments present in the cytoplasm which help in various kinds of cell movements. These filaments are lined along the cell membrane during cytokinesis and they help in deepening the cleft and completing the division.
- Observed in sexually reproducing organisms, meiosis is the process of cell division that leads to the formation of reproductive cells or gametes in higher organisms.
- A somatic cell undergoes two successive divisions to produce four gametocyte cells, each having half the number of chromosomes as the parent cell.
What are somatic cells?
Any cell other than a reproductive cell is known as a somatic cell (soma = body).
What are gametocytes?
A cell that leads to the formation of a gamete upon division is a gametocyte.
In meiosis, cell division stages can be broken down into two subcategories: Meiosis I and Meiosis II
- Meiosis I has 5 different sub-stages, known as Prophase I, Metaphase I, Anaphase I, Telophase I, and Cytokinesis I
- Meiosis II has 5 different sub-stages, known as Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis II
- Before these four stages of cell division begin, the initial process is similar to that of Interphase in mitosis, except the substage G2.
- Cell growth and development are still the key factors initially, and once the cell is prepared for division, chromosome replication begins. Each chromosome, thus, replicates its genetic material, forming an identical twin.
- Microscopically, chromosomes now appear in the form of two sister chromatids, joint together at the centromere.
At this stage, the cell has replicated its DNA material and organelles. The physical division begins with many changes occurring at the same time. Thus, for better understanding, Prophase I is demarcated into the following five sub-stages:
- The chromosomes begin to condense and are now microscopically visible.
- It is important to note that they are still condensing and not condensed at this point.
- At this stage, each chromosome (consisting its identical chromatids) starts looking for its homogeneous pair-mate.
*Note: When two chromosomes are physically same in all aspects and have similar genetic coding, then one of them is said to be the homologue of the other. In other words, they are a homologous chromosome pair.
- In this stage, each chromosome finds its homologue and begins ‘rough pairing.’
- Rough pairing is when these chromosomes align next to each other.
- The chromosomes continue condensing throughout this stage.
- Thereafter, the chromosomes actually begin pairing; this process is known as ‘synapsis.’
- The two chromosomes are joint throughout their length by a ‘synaptonemal complex.’ The point where pairing begins is not specific.
- The sister chromatids disassociate from one another in this stage, while the homologous chromosomes remain connected.
- Chromatids from homologous chromosomes may exchange genetic information by physically swapping their parts. This process is known as ‘crossing over’ of chromosomes, and the exact point of crossing over is called ‘chiasmata.’
- It is the most important step because the genetic coding changes here, leading to a variation in the genetic information passed along to the daughter cells.
- The synaptonemal complex that holds any two chromosomes together begins to dissolve at this stage.
- Eventually, all the chromosome sets are completely detached from one another, other than the crossover section.
- In short, this common portion of the chiasmata is holding all the four chromatids together.
- Meanwhile, the chromosomes continue to condense, making them shorter.
- At this stage, the chiasma begins to shift towards the end of the chromatid, as though trying to separate the bond. This continues until they reach the very end of the chromatid.
- Now spindle fibers (made of microtubules) begin to form from the centrioles of the cell.
- Lastly, the nucleolus and the nuclear membrane disintegration marks the beginning of the next step―Metaphase I.
- The spindle fibers attach themselves to homologous chromosome pairs at the centromeres.
- With the nuclear membrane disintegrated, the chromosomes are free to move.
- The spindle fibers then align the chromosomes along the equatorial plane of the cell.
- With the spindle fibers being pulled from opposite ends of the cells, the homologous pairs split apart. This is also known as ‘disjunction.’
Nondisjunction is when a chromosome pair does not separate, resulting in abnormal number of chromosomes and serious defects in the offspring so produced.
- With chromosomes pulled to either sides of the cell, cytokinesis takes place.
- The cell divides into two halves, each one having half the number of chromosomes as the parent cell.
- The nuclear membrane may or may not reform, and the chromosomes decondense.
- As the name suggests, Cytokinesis literally means division of the cytoplasm or the cell body.
- Once the genetic material is replicated and segregated into different parts of the cell, the cell membrane begins pinching towards the inner side.
- A cleft is formed which gradually deepens and separates the two newly formed daughter cells.
As discussed earlier, plant and animal cells have a slightly different phenomenon of the cell membrane dividing the cell body.
- If the nuclear membrane had reformed during Telophase I, it once again starts disintegrating for cell division.
- The chromosomes start recondensing, and the centrioles start moving towards the opposite ends of the cell.
- Once again, spindle fibers arise from the centrioles and attach themselves to the centromeres at the location of the kinetochores.
- They start aligning the chromosomes along the equatorial plane of the cell by creating tension from opposite ends.
- With the spindle fibers being pulled, the sister chromatids finally break and start moving in the opposite direction.
- Once the chromatids are separated from one another, they are considered as individual chromosomes.
- With the chromosomes (earlier chromatids) concentrated in different ends of the cell, cytokinesis begins.
- Nuclear membrane reappears for both the to-be-daughter cells at this stage.
- The cells undergo cytokinesis to form four haploid daughter cells, each having half of the original parent cell’s chromosomes.
- As the illustration shows, the daughter cell’s chromosomes carry crossovers from other chromosomes, thus making each of them unique.
- Once again, the cytoplasm divides post nuclear division to physically separate the new cells.
- The cell wall again pinches inward to create a cleavage, which in turn, deepens and forms two different cell walls for each of the daughter cells.