The cell cycle is a process in which the cell duplicates its DNA, synthesises other cell components, and divides into two new cells. However, while DNA synthesis occurs at a particular stage of cell division, the distribution of chromosomes in cells occurs during a complex series of activities in cell division. This article discusses cell cycle and cell division in detail.
Phases in Cell Cycle
A human cell divides once every 24 hours approximately. However, this can vary depending on the organism. For example, cell division takes about 90 minutes in yeasts.
The cell cycle consists of two fundamental phases: Interphase (Resting Phase) and M Phase (Mitosis phase).
Interphase (Resting Phase)
The M phase is the phase during which cells divide or undergo mitosis, while the interphase is the phase that occurs between two successive M phases.
Interphase is a transitional phase that occurs between two consecutive M phases. It lasts about 95% of the total duration of a cell cycle. It involves cell proliferation and DNA synthesis. The interphase is further broken into three phases – the G1 phase (Gap 1), S phase (Synthesis) and G2 phase (Gap 2).
- G1 (Gap 1 or Antephase) Phase:
It is the first stage of development.
It is the period between mitosis and the replication of DNA.
- The cell continues to grow.
- The cell undergoes metabolic activation.
- The cell prepares the structure necessary for DNA replication.
- The cell produces RNA and proteins.
- S (Synthetic) phase
- DNA replication occurs during this process.
- The amount of DNA present in each cell doubles. However, the number of chromosomes does not increase.
- In animal cells, the nucleus replicates first, followed by the cytoplasm.
- G2 (Gap 2) phase:
- It is the second phase of growth.
- Cell growth continues to progress in this phase.
- RNA and protein synthesis continues.
- The cell prepares for mitosis.
M Phase (Mitosis phase)
Cell division, also known as mitosis, begins with karyokinesis (the nuclear division) and concludes with cytokinesis (the cell matrix). Specific cells do not divide, for instance, heart cells. Numerous additional cells infrequently divide to replace dead cells. Cells that do not divide anymore exit the G1 phase and undergo an inactive state known as the quiescent phase (G0). These cells continue to function metabolically but do not grow.
In animals, mitotic cell division occurs exclusively in diploid somatic cells. In plants, mitotic cell division occurs in both haploid and diploid cells.
Mitosis is a kind of cell division in which a single cell (the mother cell) divides into two genetically identical daughter cells (the daughters). Mitosis is the stage of the cell cycle during which the DNA in the nucleus of the cell is divided into two identical sets of chromosomes.
Phases of mitotic cell division
The four primary phases of mitosis are prophase, metaphase, anaphase, and telophase.
Prophase is the initial phase of mitosis, followed by the second phase, G2. It includes the following series of events:
- Beginning of chromosomal material condensation.
- Centrioles migrate towards the cell’s opposite poles.
- The endoplasmic reticulum (ER), nuclear membrane, and Golgi complex disappear after prophase.
It begins with the complete disappearance of the nuclear membrane. It is the best stage to study chromosomal morphology.
- It is a process wherein chromosomes are condensed into compact structures with two sister chromatids connected with spindle fibres and kinetochores.
- Chromosomes are arranged in a pattern near the centre of the cell called the metaphase plate.
It consists of the following steps:
- At the centromere, the chromosome is divided into two chromatids.
- Two chromatids begin to move in opposite directions towards the poles.
It is the final stage of mitosis. The following events occur in this phase:
- At opposite poles, chromosomes lose their identity as a separate unit.
- A reassembled nuclear membrane surrounds the chromosomal clusters.
- The nucleolus, Golgi complex, and endoplasmic reticulum (ER) reappear.
In this phase, a cell’s cytoplasm is divided into two new daughter cells, following karyokinesis (chromosome division). Furrows form in the plasma membrane of animal cells that gradually deepen and unite to split the cytoplasm into two halves. In plant cells, the central wall extends outward to meet the lateral walls. Cell wall development occurs with cell plate formation.
Meiosis is how cells divide in half and produce haploid offspring cells. It contributes to forming the haploid phase of sexually reproducing organisms’ life cycles. Meiosis I and meiosis II are two successive cell cycles and cell division.
Meiosis I is the first cycle of nuclear division that happens during the production of gametes. It is also referred to as reduction division, as it produces half of the chromosome set as the original (parent cell.) Prophase I, metaphase I, anaphase I, and telophase I are the four phases of Meiosis I.
After a short period of relaxation, the cell prepares to undergo its second meiotic division. Finally, a period known as interkinesis occurs when the nuclear membranes of the two cells are re-established around the chromosomes in each cell.
In Meiosis II, two cells have to go through four division phases as they did during meiosis I. Meiosis II is occasionally considered an equational division since it does not reduce the number of chromosomes in the daughter cells. The daughter cells resulting from meiosis II have the same number of chromosomes as the parent cells.
Cells must divide once they reach a particular size since their ability to carry nutrients and waste to all areas of the cell declines with the increase in surface area to volume ratio. Single-celled creatures reproduce by cell division. Our bodies must make new cells while allowing old cells to die to grow and develop. Cell division is a critical part of the healing process after an injury.