Cell Cycle and Cell Division
Cell Cycle and Cell Division
The cell cycle is a series of events by which a cell duplicates its genome, synthesizes other constituents of the cell and eventually divides into two daughter cells.
In a typical eukaryotic somatic cell, cell cycle has two main phases- long non-dividing interphase (I-phase) and a short
nucleus dividing the mitotic phase (M-phase).
Interphase is a metabolically active phase and is divided into the first gap phase or G1-phase, synthetic phase or S-phase and second gap phase or G2-phase.
G1 phase: This phase is between the end of M-phase of the previous cell cycle and initiation of S-phase. It is the longest phase of interphase. In this phase, the cell grows in size and RNA and proteins are synthesized. Cell organelles also increase in number. From G1, phase, cells may enter into S-phase or sometimes in the G0 phase.
S-phase: During this phase DNA synthesis takes place. DNA content becomes double, though the ploidy level remains the same. Histone proteins are synthesized during S-phase.
G2 phase: In this phase, synthesis of DNA stops, however, the formation of RNAs and proteins continues which are required for multiplication of cell organelles, spindle formation, aster formation, and cell growth. In animal cells, centrosome begins to divide and there is an extensive synthesis of tubulin protein to form microtubules.
M-phase is divided into -karyokinesis or the division of nucleus followed by cytokinesis or the division of cytoplasm.
Karyokinesis in again divisible into - prophase, metaphase, anaphase, and telophase.
G0 phase is a period in the cell cycle in which cells exist in a quiescent state. Go phase is viewed either as an extended G, phase, where the cell is neither dividing nor preparing to divide or a distinct quiescent stage that occurs outside the cell cycle.
Cell Cycle Checkpoints
Two key classes of regulatory molecules, cyclins and cyclin-dependent protein kinases (CDKs) determine a cell's progress
through the cell cycle.
Cell cycle checkpoints are the control mechanisms that ensure the fidelity of cell division. There are two regulatory mechanisms which take decision about cell division.
The first checkpoint is called G, cyclin (C,; or G,/S) which is present in between G, and $ phase. In G,, CDK becomes active
by G, cyclin, and ATP at its activation site. It causes the transition of G, to S phase. G, cyclin is destroyed at the end of S-phase and CDK becomes inactive.
Second checkpoint is present between G) and M-phase. During the G2 phase, inactive CDK binds to mitotic cyclin (Cy
or G/M). It gets phosphorylated at its both activation and inhibitory sites but still remains inactive. On removal of PO,
from the inhibitory site, it becomes active and causes the transition from G2 to M-phase. At the end of M-phase, mitotic
cyclin (Cy) is degraded, PO, at activation site is removed and the cell enters into G1 again.
Cell division is the process of formation of new or daughter cells from pre-existing or parent cells. It is of three types-amitosis, mitosis, and meiosis.
Amitosis or Direct Cell Division
mitosis is characterized by the splitting of the nucleus followed by that of cytoplasm. It is a simple method of cell division described by Flemming (1882).
In this method, the nucleus elongates and constricts in the middle to form two daughter nuclei without the formation of spindle fiber or the appearance of chromosomes.
It does not divide the nuclear material equitably, e.g., meganucleus of Paramecium, internodal cells of Chara, etc.
Mitosis or Equational Division
Mitosis or mitotic cell division is meant for the multiplication of cells. It generally takes place in vegetative or somatic cells.
In this process, one parent cell divides into two daughter cells, but the chromosome number remains the same as in parent cell,i.e., daughter cells exactly resemble parent cell both quantitatively as well as qualitatively hence, it is called equational division. Stages of mitosis are as follows:
Prophase: It is the first stage of mitosis. During this stage viscosity of cytoplasm increases, chromatin fibers become shorter and thicker due to the coiling and get condensed into distinct thread-like chromosomes in late prophase. Each chromosome consists of two coiled sister chromatids joined by a centromere that Spears on.
Metaphase: The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, i.e., metaphase. Condensation of chromosomes is completed by this time. At this stage, the number and morphology of chromosomes can be easily studied and counted under a microscope. The chromosomes line up along the center of the cell and spindle fibers attach to each chromosome at the centromere forming the metaphase plate. Small disc-shaped structures at the surface of the centromeres called kinetochores serve at the site of attachment of spindle fibers.
Anaphase: This is the shortest stage of mitosis. Chromosomes divide at the point of centromere and thus, two sister chromatids get separated. Chromatids move to the opposite poles of the cells due to the contraction of spindle fibers and the appearance of interzonal fibers. Nucleolus, Golgi complex and ER re-form.
Telophase: The separated chromatids or newly formed chromosomes reach the opposite pole. The nuclear envelop re-form and the chromatin decondensed. In general, the events of prophase occur in reverse sequence during this phase. RNA synthesis restarts causing nucleolus to appear.
Cytokinesis: During this, mitosis ends with the division of cytoplasm by a process known as Cleavage. It starts towards the middle of anaphase and is completed with the telophase. it differs in plants and animal cells. In plants, it usually occurs by cell plate method wherein animals it takes place by cleavage.
Significance of mitosis
Growth and development: A full-grown organism is developed from a single-celled zygote by repeated mitotic divisions.
Genetic stability: Ali the daughter cells of a multicellular organism have the same number and type of chromosomes as parent cells due to equitable distribution of all the chromosomes.
Repairing and healing: The mechanism for replacing old or worn-out cells and heating of a wound by new cell production depends on mitosis.
Regeneration: Some organisms are able to regenerate their missing parts of the body or whole organism through mitosis.
Meiosis or Reductional Division
Meiosis is a kind of cell division that occurs in a diploid cell and reduces the chromosome number by half which results in the production of haploid daughter cells. The term was coined by Farmer and Moore.
It involves two successive nuclear divisions meiosis | and meiosis II but no DNA replication prior to the second division. There is a short interkinesis between meiosis.