Editing Cell biology (section)

== Cell cycle and DNA damage repair system ==
The cell cycle is composed of a number of well-ordered, consecutive stages that result in cellular division. The fact that cells do not begin the next stage until the last one is finished, is a significant element of cell cycle regulation. Cell cycle checkpoints are characteristics that constitute an excellent monitoring strategy for accurate cell cycle and divisions. Cdks, associated cyclin counterparts, protein kinases, and phosphatases regulate cell growth and division from one stage to another.<ref>{{Cite journal|last=Nurse|first=Paul|date=2000-01-07|title=A Long Twentieth Century of the Cell Cycle and Beyond|journal=Cell|language=English|volume=100|issue=1|pages=71–78|doi=10.1016/S0092-8674(00)81684-0|issn=0092-8674|pmid=10647932|s2cid=16366539|doi-access=free}}</ref> The cell cycle is controlled by the temporal activation of Cdks, which is governed by cyclin partner interaction, phosphorylation by particular protein kinases, and de-phosphorylation by Cdc25 family phosphatases. In response to DNA damage, a cell's DNA repair reaction is a cascade of signaling pathways that leads to checkpoint engagement, regulates, the repairing mechanism in DNA, cell cycle alterations, and apoptosis. Numerous biochemical structures, as well as processes that detect damage in DNA, are ATM and ATR, which induce the DNA repair checkpoints<ref>{{Cite journal|last1=Cimprich|first1=Karlene A.|last2=Cortez|first2=David|date=August 2008|title=ATR: an essential regulator of genome integrity|journal=Nature Reviews Molecular Cell Biology|language=en|volume=9|issue=8|pages=616–627|pmid=18594563| pmc=2663384| doi=10.1038/nrm2450|issn=1471-0080}}</ref>

The cell cycle is a sequence of activities in which cell organelles are duplicated and subsequently separated into daughter cells with precision. There are major events that happen during a cell cycle. The processes that happen in the cell cycle include cell development, replication and segregation of chromosomes.  The cell cycle checkpoints are surveillance systems that keep track of the cell cycle's integrity, accuracy, and chronology. Each checkpoint serves as an alternative cell cycle endpoint, wherein the cell's parameters are examined and only when desirable characteristics are fulfilled does the cell cycle advance through the distinct steps. The cell cycle's goal is to precisely copy each organism's DNA and afterwards equally split the cell and its components between the two new cells. Four main stages occur in the eukaryotes. In G1, the cell is usually active and continues to grow rapidly, while in G2, the cell growth continues while protein molecules become ready for separation. These are not dormant times; they are when cells gain mass, integrate growth factor receptors, establish a replicated genome, and prepare for chromosome segregation. DNA replication is restricted to a separate Synthesis in eukaryotes, which is also known as the S-phase. During mitosis, which is also known as the M-phase, the segregation of the chromosomes occur.<ref name="Giacomello 204–224">{{Cite journal|last1=Giacomello|first1=Marta|last2=Pyakurel|first2=Aswin|last3=Glytsou|first3=Christina|last4=Scorrano|first4=Luca|date=2020-02-18|title=The cell biology of mitochondrial membrane dynamics|url=http://dx.doi.org/10.1038/s41580-020-0210-7|journal=Nature Reviews Molecular Cell Biology|volume=21|issue=4|pages=204–224|doi=10.1038/s41580-020-0210-7|pmid=32071438|s2cid=211170966|issn=1471-0072}}</ref>  DNA, like every other molecule, is capable of undergoing a wide range of chemical reactions. Modifications in DNA's sequence, on the other hand, have a considerably bigger impact than modifications in other cellular constituents like RNAs or proteins because DNA acts as a permanent copy of the cell genome. When erroneous nucleotides are incorporated during DNA replication, mutations can occur. The majority of DNA damage is fixed by removing the defective bases and then re-synthesizing the excised area. On the other hand, some DNA lesions can be mended by reversing the damage, which may be a more effective method of coping with common types of DNA damage. Only a few forms of DNA damage are mended in this fashion, including pyrimidine dimers caused by ultraviolet (UV) light changed by the insertion of methyl or ethyl groups at the purine ring's O6 position.<ref>{{Cite journal|last1=You|first1=Zhongsheng|last2=Bailis|first2=Julie M.|date=July 2010|title=DNA damage and decisions: CtIP coordinates DNA repair and cell cycle checkpoints|url=http://dx.doi.org/10.1016/j.tcb.2010.04.002|journal=Trends in Cell Biology|volume=20|issue=7|pages=402–409|pmid=20444606|  pmc=5640159| doi=10.1016/j.tcb.2010.04.002|issn=0962-8924}}</ref>