Editing Cell cycle (section)

====G<sub>1</sub> phase (First growth phase or Post mitotic gap phase)====
[[File:Human karyotype with bands and sub-bands.png|thumb|250px|Schematic [[karyogram]] of the human chromosomes, showing their usual state in the G<sub>0</sub> and G<sub>1</sub> phase of the cell cycle. At top center it also shows the chromosome 3 pair in [[metaphase]] (annotated as "Meta."), which takes place after having undergone [[DNA synthesis]] which occurs in the [[S phase]] (annotated as S) of the cell cycle.<br>{{further|Karyotype}}]]
{{main|G1 phase}}
The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis, is called [[G1 phase|G<sub>1</sub>]] (G indicating ''gap''). It is also called the growth phase. During this phase, the biosynthetic activities of the cell, which are considerably slowed down during M phase, resume at a high rate. The duration of G<sub>1</sub> is highly variable, even among different cells of the same species.<ref name="pmid4515625">{{cite journal | vauthors = Smith JA, Martin L | title = Do cells cycle? | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 70 | issue = 4 | pages = 1263–1267 | date = April 1973 | pmid = 4515625 | pmc = 433472 | doi = 10.1073/pnas.70.4.1263 | doi-access = free | bibcode = 1973PNAS...70.1263S }}</ref> In this phase, the cell increases its supply of proteins, increases the number of organelles (such as mitochondria, ribosomes), and grows in size. In G<sub>1</sub> phase, a cell has three options. 
*To continue cell cycle and enter [[S phase]]
*Stop cell cycle and enter [[G0 phase|G<sub>0</sub>]] phase for undergoing [[Cellular differentiation|differentiation]].
*Become arrested in G<sub>1</sub> phase hence it may enter G<sub>0</sub> phase or re-enter cell cycle. 
The deciding point is called [[Cell cycle checkpoint#G1 .28Restriction.29 Checkpoint|check point]] ([[Restriction point]]). This [[Cell cycle checkpoint#G1 .28Restriction.29 Checkpoint|check point]] is called the restriction point or START and is regulated by G<sub>1</sub>/S cyclins, which cause transition from G<sub>1</sub> to S phase. Passage through the G<sub>1</sub> check point commits the cell to division.