Cells produce new cells by cell division. Cell division is required for growth, repair, regeneration and reproduction. Eukaryotic cells divide by mitosis and meiosis cell division. Prokaryotic cells divide by binary fission.
Rudolf Virchow in 1855, explained for the first time that new cells are produced from the pre-existing cells, i.e. “Omnis cellula e cellula”. Cell division is a process by which the DNA of one cell is transferred to the daughter cells and from parents to offspring.
Cell division has to be precise and done in a coordinated way to ensure that progeny cells contain a functional genome and there is no defect. The sequence of events occurring in a cell division is termed as the cell cycle, which includes the duplication and synthesis of DNA and cellular contents and finally the division to form daughter cells.
The steps in the cell cycle are:
Interphase – The resting phase, where the cell prepares for the division. There are three stages:
- G1 (Gap 1) – Cell grows and is metabolically active
- S (Synthesis) – DNA synthesis or replication takes place in the nucleus and in the animal cells, centriole also duplicates in the cytoplasm.
- G2 (Gap 2) – Marked by the synthesis of proteins and cell growth.
M phase – It is the actual phase of cell division.
Also see: 4 Phases of Cell Cycle
Eukaryotic cells divide by mitosis and meiosis.
Mitosis is the process used for growth, repair and regeneration. A cell divides mitotically to give rise to two daughter cells, which are genetically identical to the parent cell. It is required for maintaining genetic continuity in an organism.
The DNA is first duplicated and then segregated into two nuclei by karyokinesis, i.e. division of the nucleus. Karyokinesis is followed by cytokinesis, which is a division of cytoplasm.
The main steps of mitosis are:
Karyokinesis (Nuclear Division)
- Prophase – Condensation of chromosomes and two attached centromeres are visible.
- Metaphase – Chromosomes get attached to spindle fibres and align at the equator.
- Anaphase – Chromatids separate and move to the opposite poles as the centromere splits.
- Telophase – Chromosomes form clusters at the poles. Formation of nuclear envelopes around the chromosomes at each pole leads to the formation of two daughter nuclei.
Cytokinesis – Cytoplasm of the cell also divides and two daughter cells are formed at the end of the division.
Meiosis is also known as a reduction division because the chromosome number gets halved in the daughter cells. Meiosis gives rise to four daughter cells having half the number of chromosomes as daughter cells. Meiosis occurs during the formation of gametes in sexual reproduction. Meiosis is responsible for genetic variation and recombination. Due to meiosis, the number of chromosomes across generations remains the same in a species.
In meiosis, the first stage is the reduction division, i.e. meiosis I. At the end of meiosis I two daughter nuclei are formed having half the number of chromosomes as the parent cell. Meiosis I is followed by meiosis II, which is similar to a mitotic division. At the end of meiosis, one parent cell divides and forms four daughter cells. DNA synthesis takes place only once during the ‘S’ phase. There is a short-lived interkinesis phase between meiosis I and meiosis II.
The main steps of meiosis are:
- Prophase I – It is the extended phase of meiosis and is further divided into various stages.
- Leptotene – chromosomes start compacting.
- Zygotene – pairing of homologous chromosomes, synapsis.
- Pachytene – recombination and crossing over.
- Diplotene – dissolution of synaptonemal complex and formation of chiasmata.
- Diakinesis – terminalisation of chiasmata.
- Metaphase I – Bivalent chromosomes align at the equatorial plate.
- Anaphase I – Homologous chromosomes separate and move to opposite poles. The stage is different from the mitotic anaphase as the sister chromatids remain intact.
- Telophase I – Formation of nuclear membrane and nucleolus reappears.
- Cytokinesis – Nuclear division is followed by cytoplasmic division forming dyad or two daughter cells having haploid chromosomes.
Meiosis II is similar to mitosis.
- Prophase II – Nuclear membrane disappears.
- Metaphase II – Alignment of chromosomes at the equator attached to spindles radiating from opposite poles at the kinetochore.
- Anaphase II – Splitting of centromere, separation and movement of sister chromatids to the opposite poles.
- Telophase II – Chromosomes get enclosed within the nuclear membrane.
- Cytokinesis – Formation of four haploid cells, i.e. tetrad.
Prokaryotic cells divide by binary fission. Binary fission is similar to mitosis but less complex. Here the parent cell divides into two daughter cells. It is a common mode of asexual reproduction in prokaryotes. The DNA is replicated and then the segregation of DNA and cytoplasm takes place to give rise to two identical cells.
The cell division is under strict genetic control and regulated at every step of the cell cycle. Programmed cell death or apoptosis process controls the cell growth, multiplication and also removal of unnecessary cells, e.g. during embryogenesis, cells tissue between two digits are removed otherwise we would have got webbed feet. Cells containing defective genes are also removed.
When the genes that regulate the cell division become defective, it results in the rapid proliferation of the cell. The uncontrolled cell division results in the formation of tumors and leads to cancer.
This was in brief about the process of cell division. Get access to all the NEET Questions with explanations, only at BYJU’S.