Cell Cycle (UH) Subscribe
Date: 10. April 2018
Phases of Cell Cycle
Sqadia video is the demonstration of Cell Cycle. cell growth and cell division that produces two new daughter cells. In proliferating cells, G1 is the period between “birth” of a cell following mitosis and the initiation of DNA synthesis, which marks the beginning of the S phase. At the end of the S phase, a replicated chromosome consists of two daughter DNA molecules and associated chromosomal proteins. The end of G2 is marked by the onset of mitosis, during which the mitotic spindle forms and pulls apart sister chromatids. The G1, S, and G2 phases are collectively referred to as interphase. Most nonproliferating cells in vertebrates leave the cell cycle in G1, entering the G0 state. The five stages of mitosis are prophase, prometaphase, metaphase, anaphase and telophase. An abrupt change in the biochemical state of the cell occurs at the transition from metaphase to anaphase. A cell can pause in metaphase before this transition point, but once the point has been passed, the cell carries on to the end of mitosis and through cytokinesis into interphase. The part of interphase where DNA is replicated is called S phase. A comparison of the cell cycles of fission yeasts and budding yeasts illustrates that the fission yeast has a typical eukaryotic cell cycle with G1, S, G2, and M phases whereas the budding yeast has normal G1 and S phases but does not have a normal G2 phase. In contrast with a fission yeast cell, the cell in budding yeasts divides by budding.
Cell Cycle Checkpoints
The essential processes of the cell cycle such as DNA replication, mitosis, and cytokinesis are triggered by a cell-cycle control system. A complex of cyclin with Cdk acts as a protein kinase to trigger specific cell cycle events. Without cyclin, Cdk is inactive. Cdk associates successively with different cyclins to trigger the different events of the cycle. Cdk activity is usually terminated by cyclin degradation. Two key components of the cell-cycle control system are cyclins and cyclin dependant kinase. The active cyclin-Cdk complex is turned off when the kinase Wee1 phosphorylates two closely spaced sites above the active site. Removal of these phosphates by the phosphatase Cdc25 results in activation of the cyclin Cdk complex. Cyclin-Cdk complex is inhibited by a CKI. The p27 binds to both the cyclin and Cdk in the complex, distorting the active site of the Cdk. It also inserts into the ATP-binding site, further inhibiting the enzyme activity.
Role of Cyclins in Cell Cycle
The phosphorylation of a target protein, such as the CKI, allows the protein to be recognized by SCF, which is constitutively active. With the help of two additional proteins called E1 and E2, SCF serves as a ubiquitin ligase that transfers multiple ubiquitin molecules onto the CKI protein. M-cyclin ubiquitylation is performed by APC, which is activated in late mitosis by the addition of an activating subunit to the complex. Both SCF and APC contain binding sites that recognize specific amino acid sequences of the target protein. There are three D cyclins in mammals (cyclins D1, D2, and D3). The original name of Cdk1 was Cdc2 in both vertebrates and fission yeast, and Cdc28 in budding yeast. ORC remains associated with a replication origin throughout the cell cycle. Mcm ring complexes then assemble on the adjacent DNA, resulting in the formation of the pre-replicative complex.
DNA Replication Checkpoints
Cdk1 associates with M-cyclin as the levels of M-cyclin gradually rise. The resulting M-Cdk complex is phosphorylated on an activating site by the Cdk activating kinase (CAK) and on a pair of inhibitory sites by the Wee1 kinase. In the experiments, mammalian cells in culture were treated with caffeine and hydroxyurea, either alone or in combination. Hydroxyurea blocks DNA synthesis. This block activates a checkpoint mechanism that arrests the cells in S phase, delaying mitosis. But if caffeine is added as well as hydroxyurea, the checkpoint mechanism fails, and the cells proceed into mitosis according to their normal schedule, with incompletely replicated DNA. As a result, the cells die. The activation of APC by Cdc20 leads to the ubiquitylation and destruction of securin, which normally holds separase in an inactive state. The destruction of securin allows separase to cleave a subunit of the cohesin complex holding the sister chromatids together. The pulling forces of the mitotic spindle then pull the sister chromatids apart. The loss of M-cyclin leads to APC inactivation after mitosis, which allows M-cyclins to begin accumulating again.
DNA Damage Arrest
In mechanisms that control S phase initiation, G1-Cdk activity (cyclin D-Cdk4) initiates Rb phosphorylation. This inactivates Rb, freeing E2F to activate the transcription of S-phase genes, including the genes for a G1/S-cyclin (cyclin E) and S-cyclin (cyclin A). The resulting appearance of G1/S-Cdk and S Cdk activities further enhances Rb phosphorylation, forming a positive feedback loop. E2F acts back to stimulate the transcription of its own gene, forming another positive feedback loop. If cell division continued at an unchanged rate when cells were starved and stopped growing, the daughter cells produced at each division would become progressively smaller. Yeast cells respond to some forms of nutritional deprivation by slowing the rate of progress through the cell cycle so that the cells have more time to grow. As a result, cell size remains unchanged or is reduced slightly. When DNA is damaged, protein kinases that phosphorylate p53 are activated. Mdm2 normally binds to p53 and promotes its ubiquitylation and destruction in proteasomes. The p21 binds and inactivates G1/S-Cdk and S-Cdk complexes, arresting the cell in G1.