Cell Cycle and Cell Division

The cell cycle is an orderly sequence of cell growth and cell division events that produce two new daughter cells. Cells on the road to cell division proceed through a sequence of correctly timed and carefully regulated growth stages, DNA replication, and division that produce two identical (clone) cells.; There are two major phases in the cell cycle: interphase, and mitotic. The cell grows during interphase, and DNA is recycled. The replicated DNA and cytoplasmic contents are divided during the mitotic phase, and the cell divides.

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Cell Cycle 

The sequence of events by which a cell duplicates its genome synthesizes the cell's other constituents and subsequently divides into two daughter cells is called the cycle of cells.

The cell cycle comprises three coordinated processes of cell division, DNA replication, and cell growth.

Cell cycle period can vary from organism to organism, and from cell type to cell type. (E.g., 90 minutes in the yeast cell cycle, 24 hours in humans.)

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Interphase

Interphase is divided into 3 additional phases: the “G1” phase, the “S” phase, and the “G2” phase.

G1 Phase (Gap 1 Phase)

• Corresponds to the time between the mitosis and DNA replication initiation.

• The cell is metabolically active during the G1 phase and grows continuously but does not replicate its DNA.

S Phase (Synthesis Phase)

• Period during which there will be synthesis or replication of DNA.

• The amount of DNA per cell doubled during this time. (You just double the amount of DNA; no chromosomes stay the same.)

• DNA replication in animal cells begins in the nucleus during the S phase, and the centriole duplicates in the cytoplasm.

G2 Phase (Gap 2 Phase)

In preparation for mitosis, proteins are synthesized whilst cell growth continues.

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• Some cells do not display division such as heart cells, nerve cells, etc. Such cells enter an inactive phase called G0 or G1-phase quiescent.

• Cells are metabolically active in this process but do not divide unless they are called upon to do so.

Cell Division

Mitosis or M phase

In animals, the division of mitotic cells is only seen in the diploid somatic cells, while mitotic divisions in both haploid and diploid cells can be seen in the plants.

It is also called an equational division, as the number of parent and progeny cell chromosomes is identical.

Mitosis is Subdivided into the Following 4 Stages:

• Prophase

• Metaphase

• Anaphase

• Telophase

Prophase

• This follows interphase phases S and G2.

• Now the centrioles start moving towards opposite cell poles.

• Chromosomal material condenses in prophase to form compact mitotic chromosomes.

• Initiation of mitotic spindle assembly with the assistance of the microtubules.

• Cell organelles, such as complexes of Golgi, endoplasmic reticulum, nucleolus, and the nuclear envelope are gone.

Metaphase

• The full disintegration of the nuclear envelope marks the start of metaphase.

• The chromosomes spread through the cell's cytoplasm.

• Chromosomal condensation is completed and they can be clearly observed under the microscope.

• This is the stage where chromosomal morphology is most easily studied.

• At this point, the metaphase chromosome consists of two sister chromatids, which the centromere keeps together.

• Centromeres serve as sites where spindle fibers are attached to the chromosomes.

• Chromosomes are moved to the center of the cell position.

• The metaphase is distinguished by all the chromosomes entering the equator with one chromatid of each chromosome connected by the kinetochore to the spindle fibers of one pole and its sister chromatid connected by the kinetochore to the spindle fibers of the opposite pole.

• The plane of metaphase chromosomal alignment is called the metaphase plate or equatorial plate.

Anaphase

• Each chromosome assembled at the metaphase plate is split simultaneously at the beginning of anaphase, and the two daughter chromatids begin to migrate towards the two opposite poles.

• When each chromosome travels away from the equatorial plate, the middle of each chromosome is towards the pole and therefore at the front edge, with the chromosome's arms trailing behind it.

Telophase

• At the start of telophase, the chromosomes decondense and form a chromatin network at their respective poles.

• Nuclear envelope assembles around a network of chromatins.

Cytokinesis

• Following karyokinesis, a separate process called cytokinesis splits the cell itself into two daughter cells.

• This is achieved in an animal cell by the appearance of a furrow inside the plasma membrane.

• The furrow slowly deepens, and eventually joins the middle separating the cytoplasm into two cells.

• Cellular cells of plants undergo cytokinesis via plate cells. Wall formation in the cell plate method starts at the center of the cell and extends towards the current lateral walls.

• The formation of the new cell wall begins with the formation of a simple precursor called the cell plate representing the middle lamella between two adjacent cells ' walls..

• Organelles such as mitochondria and plastids are distributed between the two daughter cells at the time of cytoplasmic division.

• Cytokinesis is not observed in certain organisms as a result of which multinucleate disease results in the formation of syncytium (e.g. liquid endosperm in cocoonut). 

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Meiosis

• The specialized sort of cell division that reduces the number of chromosomes by half results in haploid daughter cells being formed.

• It is responsible for the formation of haploid gametes which form diploid zygote by fusion during sexual reproduction.

• Meiosis involves two nuclear and cell division concurrent processes called meiosis I and meiosis II but only a single DNA replication process.

• Meiosis interphase is similar in nature to mitosis interphase.

Meiosis I

Prophase I

• Division meiosis prophase I is typically longer and more complex than mitosis prophase.

• It was subdivided further into the following five phases, based on chromosomal behavior.

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Metaphase I:

• The bivalent chromosomes align themselves with the equatorial plate.

• The microtubules from the opposite spindle poles belong to the pair of homologous chromosomes.

Anaphase I:

• The homologous chromosomes separate while at their centromeres sister chromatids remain linked.

Telophase I

• Reappearing of the nuclear membrane and nucleolus.

• Cytokinesis follows telophase I.

• Although the chromosomes undergo some dispersion in many cases, they do not attain the extremely extended interphase nucleus state. The stage between the two meiotic divisions is known as interkinesis and is typically short-lived.

• Prophase II is accompanied by interkinesis, much easier prophase than prophase I.

Meiosis II

Meiosis II resembles a normal mitosis.

Prophase II:

• Upon cytokinesis Meiosis II is initiated immediately.

• By the end of Prophase II, the nuclear membrane disappears.

• The chromosomes again become compact.

Metaphase II:

At this point, the chromosomes align at the equator, and the microtubules from opposite spindle poles are bound to the sister chromatid kinetochores.

Anaphase II:

• Centromere splitting for each chromosome.

• Chromosomes move towards cell poles opposite to each other.

Telophase II:

• A nuclear envelope again enshrines the two groups of chromosomes. EOLBREAK Cytokinesis follows that four haploid daughter cells develop).

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Importance of Cell Division: 

Significance of Mitosis

• Mitosis usually results in diploid daughter cells being formed with identical genetic complements.

• Multicellular organism growth is caused by mitosis.

• Cell growth has the effect of disrupting the nucleus-cytoplasm ratio. Cell divide thus to restore the nucleo-cytoplasmic ratio.

• Mitosis is important in repairing cells. The upper layer cells of the epidermis, gut lining cells, and blood cells are continually being replaced.

• Mitotic differences in the meristematic tissues -the apical and lateral shifts result in continuous plant development throughout their lives.

Significance of Meiosis

• The conservation of specific chromosome numbers of each species in sexually reproductive organisms over generations is achieved through meiosis.

• It also increases the genetic variability within the organism population from one generation to the next. Variations are very important to the evolutionary process.