Cell cycle

Definitions & Key takeaways

The cell cycle is a process that somatic cells go through that involves the duplication of DNA, growth, and division of the cell. The cell cycle can be divided into four phases: G1, S, G2, and M. G1 is the growth phase, where the cell performs all of its functions, and S is the synthesis phase, where DNA replication occurs. G2 is the growth phase, where the cell grows in size and prepares for Mitotic division, and M is the mitotic cell division phase, dividing the cell into two identical daughter cells.

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The cell cycle refers to the events that somatic cells - which includes all of the cells in our bodies except the reproductive cells - go through from the moment they’re formed until the moment they divide in two identical daughter cells.
This cycle varies in length depending on the type of cell - for rapidly dividing cells, like skin cells, it takes less than a day, whereas for other cells, like liver cells, the cell cycle can last years.
The cell cycle has two phases: interphase, and mitosis. Interphase the longest part of the cell cycle, and it’s a state of preparation, during which the cell carries out its cell functions, grows and replicates its DNA to prepare for mitosis - or cellular division.
After a parent cell divides, each of the two daughter cells enter interphase again. Now, interphase can further be broken down in three subphases: G1, S, and G2.
G1 stands for “gap” or “growth” 1, and it’s the longest phase of the cell cycle. During G1, the cell mostly grows and the organelles take care of regular cellular business - like the synthesizing proteins and producing energy.
Inside the cell nucleus, there’s our DNA, organized as chromosomes - and during G1, each chromosome is made up of a single, thin spaghetti of DNA, called a chromatid.
At the end of G1, there’s a cell cycle control point called the G1 checkpoint - where the cell checks to see if the DNA is not damaged, and it synthesized the right proteins in the correct amount.
If it turns out that there is any reason for the cell not to divide - such as DNA damage, things can go one of two ways: the cell can either enter a non-dividing state, called the G0 phase, where the DNA repair mechanisms try to fix the problem, or the cell can self-destruct in a process called apoptosis.
Now, if the cell does get the go-ahead at the G1 checkpoint, it enters the S phase. S stands for “synthesis”, because during this phase, DNA is replicated, so that each daughter cell receives identical copies of the genetic material.
So for each chromosome from G1, an identical copy is created. This happens with the help of a number of proteins, both structural proteins and enzymes, as well as energy.
Now, just to be clear - this doesn’t mean that the number of chromosomes increases - human somatic cells have 46 chromosomes throughout the cell cycle.
However, the amount of DNA they have - and, in turn, their aspect - changes throughout the cell cycle. So each chromosome enters the S phase with a single copy of the genetic information, called a chromatid.
During replication, each chromatid is copied and pasted, so the amount of DNA doubles up. The two resulting chromatids are identical to each other and to the original genetic template, and they join together in the center in a region called the centromere - - but they still make up a single chromosome.
So while the amount of genetic information has doubled, there are still 46 chromosomes that contain that genetic information.
The cell can now enter the G2 phase. G2 stands for “gap” or “growth” 2.
Even after synthesizing copies of the DNA, the cell still has to duplicate organelles so that there are enough for both daughter cells.
In fact, by the end of G2, the cell looks like a big balloon of cytoplasm and organelles, just waiting to split. However, before it can do that, it must pass the final G2 checkpoint- and if there is no DNA damage after replication, the cell can enter mitosis.
Now, mitosis is when the actually divides in two daughter cells, and it involves separating the replicated DNA in two distinct nuclei - a process called karyokinesis - and the actual separation of the daughter cells into two distinct cells - a process called cytokinesis.
Mitosis can also be broken down in four subphases. In order, these are the prophase, the metaphase, the anaphase and the telophase, and they are immediately followed by cytokinesis.
You can remember this with the mnemonic “Please Make Another Two Cells”. During prophase, the membrane around the nucleus disintegrates, and the chromosomes condense and become visible under a microscope like 46 little “X” shapes - the condensed sister chromatids make even the tiny Y chromosome in a male look like an X!
Inside the cytoplasm, two organelles called the centrosomes migrate to opposite poles of the cell. During metaphase, chromosomes move towards the middle of the cell, on a line called the metaphase plate - so think metaphase, middle.
When the chromosomes are in place, each centrosome sends out thread-like projections called spindle fibers, that attach to the centromere of each chromosome.
Next, during anaphase, the centrosomes start pulling on the spindle fibers to pull the sisters chromatids apart - like a game of tug of war.
Since both centrosomes pull with equal force in opposite directions, eventually the sister chromatids separate and reach the opposite poles of the cell.
Finally, there’s the telophase - during which a nuclear membrane forms around each new set of 46 single-chromatid chromosomes.
After that, during cytokinesis, the cell membrane pinches in until the two daughter cells separate. Now, both those daughter cells can enter the cell cycle once more if they need to.
If they don’t, they enter the non-dividing G0 phase. G0 is considered to be a phase outside the cell cycle - because while the cells are living, they are neither dividing, nor preparing to divide.
An example of this would be hepatocytes - which rest in G0 until there’s liver damage, at which point they enter G1 again, so that they can divide and replace damaged tissue.
Another example would be neurons, which spend their entire life in G0. Alright, as a quick recap.
The cell cycle can be divided in two phases: interphase, and mitosis. Interphase comprises of the G1 phase, during which the cell grows and performs its cell functions, the S phase, during which DNA is replicated, and the G2 phase, during which the cell grows again before entering mitosis.
Mitosis can be broken down into prophase, metaphase, anaphase, and telophase, during which the replicated DNA divides equally for the two daughter cells, and ends with cytokinesis, which is when the cell membrane actually divides to form the two new cells.