The phases of cell division occur in various stages of the cell cycle. These stages of cell division help in the production and maintenance of cells, tissues, and organs. The different types of cells that are involved in cell division will be discussed with appropriate diagrams and pictures.
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Definition of cell division
Cell division is defined as the process whereby a parent cell divides into two or more daughter cells.
What is cell division?
Cell division is the means of reproduction in unicellular organisms and the means of tissue growth and maintenance in multicellular creatures.
Normally, cell division happens as part of a wider cell cycle. In eukaryotes, there are two types of cell divisions namely, the vegetative cell division (mitosis), in which each daughter cell is genetically identical to the parent cell, and reproductive cell division (haploid gametes), in which the number of chromosomes in the daughter cells is reduced by half (meiosis).
The survival of eukaryotes is dependent on interactions between many different cell types, therefore cell division is necessary for an organism to critically maintain a balanced distribution of various cell types through the highly regulated process of cell proliferation which is responsible for this. Diverse cell populations have different ways of growing and dividing, but the essential principles are the same in all multicellular life forms.
The majority of the body’s tissues expand by multiplying their cells, although this process is tightly controlled to maintain a balance between different tissues. In adult humans, most cell division is used for tissue renewal rather than growth, with many different types of cells being replaced on a regular basis. Skin cells, for example, are regularly shed and replaced; mature differentiated cells do not divide in this situation, but their population is replenished by the division of immature stem cells. Mature cells in other tissues, such as those in the liver, can still divide to allow for growth or regeneration following harm.
Other types of cells, either cannot divide or are prevented from dividing by certain molecules produced by nearby cells. As a result, some tissues in adult humans have a greatly reduced capacity to renew damaged or diseased cells. Heart muscle, nerve cells of the central nervous system, and lens cells in mammals are examples of such tissues that cannot regenerate or have a highly reduced capacity for renewal. The cell maintenance and repair are limited to replacing intracellular components rather than entire cells.
Cell cycle and cell division
Cell division is the process by which one cell, known as the parent cell, divides into two new cells, known as daughter cells. The manner in which this occurs differs depending on whether the cell is prokaryotic or eukaryotic.
Prokaryotic cells are simpler (they have one circular chromosome, no nucleus, and only a few other organelles) than eukaryotic cells (have multiple chromosomes contained within a nucleus as well as numerous other organelles) therefore, cell division is simpler in prokaryotes.
When the cell divides, all of these cell parts (nucleus and organelles) must be duplicated and then separated and cell division is just one of several stages that a cell goes through throughout its life known as the cell cycle.
The cell cycle is a series of events that occur repeatedly in the production of new cells which includes processes like growth, DNA synthesis, and cell division. In prokaryotes, the cell cycle is straightforward, that is the cell grows, its DNA replicates, and the cell divides. This type of division and cycle is known as asexual reproduction in prokaryotes. However, the cell cycle in eukaryotes is more complicated because of the various organelles and the nucleus.
Types of cell division
- Prokaryotic cell division
- Eukaryotic cell division
Prokaryotic cell division
Prokaryotic cells divide through the process of binary fission since prokaryotes are simple organisms with only one membrane and no internal division. For instance, when a prokaryote divides, it simply replicates its DNA and divides in half. But this procedure is slightly more complicated because DNA must first be unwound by special proteins. The unwinding of the DNA is because DNA in prokaryotes typically exists in a ring, and it can become quite tangled when used by the cell during cell division. Hence, the DNA must be stretched out in order to be copied efficiently.
Another importance of the unwinding of the DNA is because it also allows the two new DNA rings to be separated after they are created. After their creation, the two strands of DNA divide the prokaryote cell into two distinct sides and then grow longer until it splits down the middle. The whole prokaryotic cell division process is depicted in the diagram below.
From the above image, the tangled line is the DNA and the remaining components are labeled as plasmids which are small rings of DNA that are also copied during binary fission and can be picked up in the environment from dead cells that disintegrate.
Types of eukaryotic cell division
- Mitosis
- Meiosis
Mitosis
Eukaryotic organisms have membrane-bound organelles and DNA that is found on chromosomes, making cell division more difficult. Before dividing, eukaryotes must replicate their DNA, organelles, and cellular mechanisms. Many organelles divide through a process that is essentially binary fission, leading scientists to believe that eukaryotes were formed by prokaryotes living inside other prokaryotes.
A eukaryote can begin mitosis after the DNA and organelles have been replicated during the interphase of the cell cycle. The first phase is the condensing of the chromosomes during prophase to avoid entanglement and eventual breaking of the DNA during the later stages of mitosis. Following the condensing, the chromosomes are then lined up in the middle of the cell as mitosis progresses to metaphase.
As mitosis progresses, each half of a chromosome, known as sister chromatids because they are replicated copies of each other, is separated into each half of the cell.
After mitosis, a process known as cytokinesis takes place which divides the cell into two new daughter cells.
Mitosis is the process by which all eukaryotic organisms divide their cells. But it is only used as a mode of reproduction by single-celled organisms. Because most multicellular organisms reproduce sexually by combining their DNA with that of another organism and organisms in these situations require a different method of cell division known as meiosis.
Meiosis
In meiosis, DNA and organelles are replicated just as they were before mitosis (i.e. during interphase of the cell cycle). Meiosis is characterized by two distinct cell divisions that occur back-to-back. Meiosis I is the first meiosis, and it separates homologous chromosomes. That is, the two alleles of each gene in an organism and these alleles are recombined and separated, resulting in daughter cells with only one allele for each gene and no homologous chromosomal pairs. While meiosis II, the second division, separates the two copies of DNA in the same way as mitosis.
Finally, meiosis in one cell produces four cells, each with only one copy of the genome, which is half the normal number as shown in the chart below.
Cell division phases
- Phases of Mitosis
- Phases of Meiosis
The stages of cell division can differ slightly depending on the type of cell division used by an organism. The different stages of each type of cell division would be discussed below.
Phases of Mitosis in Cell division
Mitosis begins with prophase when the chromosomes are condensed leading towards the prometaphase. The cell then enters metaphase, when the chromosomes align on the metaphase plate. After that, the chromosomes are then separated during anaphase, and the cytoplasm of the cell is pinched apart during telophase. The final process that breaks the cell membrane and divides the cell into two is cytokinesis.
Phases of Meiosis in Cell division
Even though the stages of meiosis are similar to those of mitosis, the chromosomes behave differently. Meiosis is divided into two phases, each of which includes two separate cell divisions with no DNA replication in between. Prophase, metaphase, anaphase, and telophase are the four stages of meiosis I and meiosis II, which are the same as mitosis. Cytokinesis marks the end of both rounds of meiosis.
Meiosis I begins when the chromosomes are condensed during prophase I and are lined up across from their homologous pairs during metaphase I. After that, there is only one form of each gene in each cell when they are separated in anaphase I and telophase I, which is known as a reduction division. Meiosis II occurs in the same way that mitosis does, with sister chromatids dividing on the metaphase plate. By telophase II, there are four cells, each with half the alleles of the parent cell and only one copy of the genome. Finally, after all these phases, cells can now differentiate into gametes and fuse to form new organisms.
FAQ on cell division
What is the purpose of cell division?
Cell division is necessary for an organism to perform the following.
(1) reproduce of an entire unicellular organism,
(2) grow and repair tissues in multicellular animals and
(3) form gametes (eggs and sperm) for sexual reproduction in multicellular animals
How many rounds of cell division occur in meiosis?
Meiosis requires 2 (two) rounds of cell division for it to occur.
What type of cells undergoes cell division?
All types of cells undergo cell division either prokaryotic cells or eukaryotic cells.
What is differentiation in cell division?
This is the process by which dividing cells change their functional or phenotypical type.
Where does cell division occur?
Cell division occurs in all parts of the cell in the body.
What are the cell division phases in order?
These phases are prophase, prometaphase, metaphase, anaphase, and telophase. They can also be referred to as the steps of cell division.
How does cell division differ between animal and plant cells?
The primary distinction between plant and animal cell division is that plant cells form the cell plate between the two daughter cells in mitosis, whereas animal cells form the cleavage furrow between the two daughter cells.
What controls cell division?
Cyclin-dependent protein kinases (Cdks) are key components of the cell-cycle control system, and their activity is dependent on their association with regulatory subunits known as cyclins. Oscillations in the activities of various cyclin-Cdk complexes initiate a variety of cell-cycle events.