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Aug 3, 2023 · Learn about the cell cycle, the sequence of events that results in cell growth and division. Explore the phases, regulation, and checkpoints of the cell cycle with diagrams and examples.
- Overview
- Introduction
- Cyclins
- Cyclin-dependent kinases
- Maturation-promoting factor (MPF)
- The anaphase-promoting complex/cyclosome (APC/C)
- Checkpoints and regulators
The core control system of the cell cycle. Cyclins, cyclin-dependent kinases (Cdks), and the APC/C.
In the article on cell cycle checkpoints, we looked at the why of cell cycle transitions: the factors that a cell considers when deciding whether or not to move forward through the cell cycle. These include both external cues (like molecular signals) and internal cues (like DNA damage).
Cues like these act by changing the activity of core cell cycle regulators inside the cell. These core cell cycle regulators can cause key events, such as DNA replication or chromosome separation, to take place. They also make sure that cell cycle events take place in the right order and that one phase (such as G1 ) triggers the onset of the next phase (such as S).
Cyclins are among the most important core cell cycle regulators. Cyclins are a group of related proteins, and there are four basic types found in humans and most other eukaryotes: G1 cyclins, G1 /S cyclins, S cyclins, and M cyclins.
As the names suggest, each cyclin is associated with a particular phase, transition, or set of phases in the cell cycle and helps drive the events of that phase or period. For instance, M cyclin promotes the events of M phase, such as nuclear envelope breakdown and chromosome condensation1,2 .
In order to drive the cell cycle forward, a cyclin must activate or inactivate many target proteins inside of the cell. Cyclins drive the events of the cell cycle by partnering with a family of enzymes called the cyclin-dependent kinases (Cdks). A lone Cdk is inactive, but the binding of a cyclin activates it, making it a functional enzyme and allowing it to modify target proteins.
How does this work? Cdks are kinases, enzymes that phosphorylate (attach phosphate groups to) specific target proteins. The attached phosphate group acts like a switch, making the target protein more or less active. When a cyclin attaches to a Cdk, it has two important effects: it activates the Cdk as a kinase, but it also directs the Cdk to a specific set of target proteins, ones appropriate to the cell cycle period controlled by the cyclin. For instance, G1 /S cyclins send Cdks to S phase targets (e.g., promoting DNA replication), while M cyclins send Cdks to M phase targets (e.g., making the nuclear membrane break down).
In general, Cdk levels remain relatively constant across the cell cycle, but Cdk activity and target proteins change as levels of the various cyclins rise and fall. In addition to needing a cyclin partner, Cdks must also be phosphorylated on a particular site in order to be active (not shown in the diagrams in this article), and may also be negatively regulated by phosphorylation of other sites3,4 .
Cyclins and Cdks are very evolutionarily conserved, meaning that they are found in many different types of species, from yeast to frogs to humans. The details of the system vary a little: for instance, yeast has just one Cdk, while humans and other mammals have multiple Cdks that are used at different stages of the cell cycle. (Yes, this kind of an exception to the "Cdks don't change in levels" rule!) But the basic principles are quite similar, so that Cdks and the different types of cyclins can be found in each species5 .
A famous example of how cyclins and Cdks work together to control cell cycle transitions is that of maturation-promoting factor (MPF). The name dates back to the 1970s, when researchers found that cells in M phase contained an unknown factor that could force frog egg cells (stuck in G2 phase) to enter M phase. This mystery molecule, called MPF, was discovered in the 1980s to be a Cdk bound to its M cyclin partner6 .
MPF provides a good example of how cyclins and Cdks can work together to drive a cell cycle transition. Like a typical cyclin, M cyclin stays at low levels for much of the cell cycle, but builds up as the cell approaches the G2 /M transition. As M cyclin accumulates, it binds to Cdks already present in the cell, forming complexes that are poised to trigger M phase. Once these complexes receive an additional signal (essentially, an all-clear confirming that the cell’s DNA is intact), they become active and set the events of M phase in motion7 .
In addition to driving the events of M phase, MPF also triggers its own destruction by activating the anaphase-promoting complex/cyclosome (APC/C), a protein complex that causes M cyclins to be destroyed starting in anaphase. The destruction of M cyclins pushes the cell out of mitosis, allowing the new daughter cells to enter G1 . The APC/C also causes destruction of the proteins that hold the sister chromatids together, allowing them to separate in anaphase and move to opposite poles of the cell.
How does the APC/C do its job? Like a Cdk, the APC/C is an enzyme, but it has different type of function than a Cdk. Rather than attaching a phosphate group to its targets, it adds a small protein tag called ubiquitin (Ub). When a target is tagged with ubiquitin, it is sent to the proteasome, which can be thought of as the recycle bin of the cell, and destroyed. For example, the APC/C attaches a ubiquitin tag to M cyclins, causing them to be chopped up by the proteasome and allowing the newly forming daughter cells to enter G1 phase8 .
The APC/C also uses ubiquitin tagging to trigger the separation of sister chromatids during mitosis. If the APC/C gets the right signals at metaphase, it sets off a chain of events that destroys cohesin, the protein glue that holds sister chromatids together8,9 .
•The APC/C first adds a ubiquitin tag to a protein called securin, sending it for recycling. Securin normally binds to, and inactivates, a protein called separase.
Cdks, cyclins, and the APC/C are direct regulators of cell cycle transitions, but they aren’t always in the driver’s seat. Instead, they respond to cues from inside and outside the cell. These cues influence activity of the core regulators to determine whether the cell moves forward in the cell cycle. Positive cues, like growth factors, typically increase activity of Cdks and cyclins, while negative ones, like DNA damage, typically decrease or block activity.
As an example, let's examine how DNA damage halts the cell cycle in G1 . DNA damage can, and will, happen in many cells of the body during a person’s lifetime (for example, due to UV rays from the sun). Cells must be able to deal with this damage, fixing it if possible and preventing cell division if not. Key to the DNA damage response is a protein called p53, a famous tumor suppressor often described as “the guardian of the genome.” 10
p53 works on multiple levels to ensure that cells do not pass on their damaged DNA through cell division3 . First, it stops the cell cycle at the G1 checkpoint by triggering production of Cdk inhibitor (CKI) proteins. The CKI proteins bind to Cdk-cyclin complexes and block their activity (see diagram below), buying time for DNA repair. p53's second job is to activate DNA repair enzymes. If DNA damage is not fixable, p53 will play its third and final role: triggering programmed cell death so damaged DNA is not passed on.
By ensuring that cells don't divide when their DNA is damaged, p53 prevents mutations (changes in DNA) from being passed on to daughter cells. When p53 is defective or missing, mutations can accumulate quickly, potentially leading to cancer. Indeed, out of all the entire human genome, p53 is the single gene most often mutated in cancers.11 p53 and cell cycle regulation are key topics of study for researchers working on new treatments for cancer.
May 30, 2017 · The cell cycle is a cycle of stages that cells pass through to allow them to divide and produce new cells. It is sometimes referred to as the “cell division cycle” for that reason. New cells are born through the division of their “parent” cell, producing two “daughter” cells from one single “parent” cell.
Progress through the cell cycle is regulated. The cycle can be controlled or put on ‘pause’ at any one of several phase transitions. Such checkpoints monitor whether the cell is on track to complete a successful cell division event. Superimposed on these controls are signals that promote cell differentiation.
Our current understanding of cell cycle regulation has emerged from a convergence of results obtained through experiments on organisms as diverse as yeasts, sea urchins, frogs, and mammals. These studies have revealed that the cell cycle of all eukaryotes is controlled by a conserved set of protein kinases, which are responsible for triggering ...
- Geoffrey M Cooper
- 2000
- 2000
Growth in an ' organism ' is carefully controlled by regulating the cell cycle. Cell cycle in Deinococcus radiodurans. The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell that causes it to divide into two daughter cells.
Jul 31, 2022 · Understand how the cell cycle is controlled by mechanisms both internal and external to the cell. Explain how the three internal control checkpoints occur at the end of G 1, at the G 2 /M transition, and during metaphase. Describe the molecules that control the cell cycle through positive and negative regulation.
