Cell Cycle: Definition, Phases, Examples, Regulation | biology dictionary (2023)

Cell cycle definition

The cell cycle is a cycle of stages that cells go through to enable them to divide and make new cells. For this reason, it is also referred to as the “cell division cycle.”

New cells are born by dividing their 'parent cell', resulting in two 'daughter cells' from a single 'parent cell'.

Daughter cells start life small, containing only half the cytoplasm of the parent cell and only one copy of the DNA that is the cell's "blueprint" or "source code" for its survival. In order to divide and produce their own "daughter cells," newborn cells must grow and produce more copies of vital cellular machinery, including their DNA.

The two main parts of the cell cycle are mitosis and interphase.

Mitosis is the phase of cell division, in which a 'mother cell' divides to create two 'daughter cells'.

The longest part of the cell cycle is called "interphase," the phase of growth and DNA replication between mitotic cell divisions.

Both mitosis and interphase are divided into smaller subphases that are necessary for cell division, growth, and development to proceed smoothly. Here we will focus on interphase as the phases of mitosis were covered in our "Mitosis" article.

Interphase consists of at least three different phases during which the cell grows, produces new organelles, replicates its DNA and finally divides.

Only after the cell has grown by absorbing nutrients and copying its DNA and other essential cellular machinery can this "daughter cell" divide and become the "parent" of two "daughter cells" of its own.

The chart below shows a visual representation of the cell cycle. The small portion labeled "M" represents mitosis, while interphase is shown broken down into its major components: the G₁, S y G₂stadia.

This cell cycle is used by all eukaryotic cells to produce new cells. Prokaryotic cells, like bacteria, use a process called "binary fission."

For some unicellular eukaryotes, the cell cycle is the same as the reproductive cycle. Their "daughter cells" are independent organisms that will reproduce through mitosis.

In other organisms, the cell cycle is used for the growth and development of a single organism, while other methods are used to reproduce the organism.

For example, animals and some plants create new offspring through a process of sexual reproduction where special sex cells are created and combined.

But animals and plants still use the cell cycle to make new cells in their tissues. This allows these multicellular organisms to grow and heal throughout their lives.

cell cycle function

Because cells reproduce by dividing, the new "daughter cells" are smaller than their parent cells and can inherit the minimal cellular machinery they need to survive.

Before these daughter cells can divide to make more cells, they must grow and reproduce their cellular machinery.

The importance of the cell cycle can be understood by doing simple calculations about cell division. If cells did not grow between divisions, each generation of "daughter" cells would be only half the size of the parent generation. This would become unsustainable very quickly!

To achieve this growth and prepare for cell division, cells divide their metabolic activities into different phases of Gap 1, Synthesis and Gap 2 between cell divisions.

The complete cycle of cell division will be discussed below.

Phases of the cell cycle

Mitosis

Let's start this cell cycle with "birth".

During mitosis, the "parent cell" goes through a complex series of steps to ensure that each "daughter cell" receives the materials it needs to survive, including a copy of each chromosome. Once the materials are properly sorted, the "parent cell" splits in two, pinching the membrane in half.

You can read more about the detailed steps of mitosis and how a parent cell ensures that its daughter cells inherit what they need to survive in our Mitosis article (https://biologydictionary.net/mitosis/).

Each of the new "daughters" are now cells that live independently. But they are small and have only one copy of their genetic material.

This means they cannot split to immediately produce their own "daughters". First, they have to go through the "interphase", the phase between divisions, which consists of three different phases.

GRAM₁Phase

in gr₁In this phase, the newly formed daughter cell grows. The "G" is often said to stand for "gap" because to an outside observer with a light microscope, these phases appear to be relatively inactive "gaps" in the cell's activity.

However, given what we know today, it might be more accurate to say that 'G' stands for 'growth', as 'G' phases are bursts of protein and organelle production, as well as a literal increase in cell size.

During the first 'growth' or 'gap' phase, the cell produces many essential materials such as proteins and ribosomes. Cells that rely on specialized organelles such as chloroplasts and mitochondria produce many more of those organelles during G₁Also. The cell size can increase as it absorbs more material from its environment into its machinery for life.

This allows the cell to increase its energy production and overall metabolism, preparing it for…

Fase S

During S phase, the cell replicates its DNA. The "S" stands for "synthesis", referring to the synthesis of new chromosomes from raw materials.

This is a very energy-consuming operation, because many nucleotides have to be synthesized. Many eukaryotic cells have dozens of chromosomes (massive amounts of DNA) that need to be copied.

The production of other substances and organelles slows down significantly during this time as the cell focuses on replicating the entire genome.

When S phase is complete, the cell has two complete sets of genetic material. This is crucial for cell division, as it ensures that both daughter cells can get a copy of the "blueprint" they need to survive and reproduce.

However, replicating your DNA can leave the cell a bit exhausted. That's why it has to happen...

GRAM₂Phase

Like the first gap phase of the cell cycle, the G₂The phase is characterized by high protein production.

During g₂, many cells also check that both copies of their DNA are correct and intact. If a cell's DNA is found to be damaged, its "G" function may fail.₂/M checkpoint” – so named because this “checkpoint” is at the end of the G₂phase, just between G₂in "M-phase" of "mitose".

This G₂/M checkpoint” is a very important security measure for multicellular organisms such as animals. Cancers, which can cause the death of the entire organism, can appear when cells with damaged DNA reproduce. By checking whether a cell's DNA is damaged just before replication, animals and some other organisms reduce the risk of cancer.

Interestingly, some organisms G₂complete and proceed directly to mitosis after DNA is synthesized during S phase. However, most organisms consider it safer to use G₂and the corresponding checkpoint!

if the g₂The /M checkpoint is passed and the cell cycle begins again. The cell divides through mitosis and the new daughter cells begin the cycle that will take them through G.₁, S y G₂stages to produce new daughter cells themselves.

Unless, of course, they are meant for…

An alternative hearth: G₀Phase

After being born through mitosis, some cells are not destined to divide to produce daughter cells.

Neurons, for example the nerve cells of animals, do not divide. The 'mother cells' are stem cells, and the 'daughter' neuronal cells are programmed not to go through the cell cycle on their own, because uncontrolled neuronal growth and cell division can be very dangerous for the organism.

So instead of G₁After being "born," neurons enter a stage that scientists call "G₀phase." This is a metabolic state intended solely to maintain the daughter cell, not to prepare it for cell division.

Neurons and other types of cells that do not divide can spend their entire lives in G₀stage, during which it fulfills its function for the organism in general, without ever dividing or reproducing.

Regulation of the cell cycle

It is very important for the survival of cells and organisms that the cell cycle is regulated.

Organisms must be able to stop cell division if the cell in question is damaged or if there is not enough food to support new growth; they must also be able to initiate cell division when growth or wound healing is needed.

To achieve this, cells use a variety of chemical "signaling cascades" where multiple links in a chain create complex effects based on simple signals.

In these regulatory cascades, a single protein can alter the function of many other proteins, causing widespread changes in the cell's function or even structure.

This allows these proteins, such as cyclins and cyclin-dependent kinases, to act as 'stopping points'. If cyclins or cyclin-dependent kinases do not give a green light, the cell cannot progress to later stages of the cell cycle.

Some examples of cell cycle regulation are given below.

examples of cell cycles

Here we will discuss general examples of how cells regulate their cell cycles, using a complex cascade of signaling molecules, protein-activating enzymes and signal-killing molecules.

p53

p53 is a protein known to scientists for its role in stopping the reproduction of cells with severe DNA damage.

When DNA is damaged, p53 interacts with cyclin-dependent protein kinases and other proteins to initiate repair and protective functions, and can also prevent the cell from entering mitosis, preventing cells with DNA damage from reproducing.

cyclines

Cyclins are a group of proteins produced at different points in the cell cycle. There are cyclins that occur exclusively in most phases of the cell cycle – G₁cyclinen, G₁/S-cyclins that regulate the G transition₁on cyclins S, S and M that regulate progression through the stages of mitosis.

Most cyclins are found in the cell at very low concentrations during other phases of the cell cycle, but then suddenly rise when they are needed to give the green light for the next phase of the cell cycle. Certain types of DNA damage can prevent these cyclins from advancing the cell cycle, or can prevent them from activating their cyclin-dependent protein kinases.

Some others, such as G.₁cyclins, remain high as a constant "go" signal from G₁to mitosis.

Cyclin-dependent protein kinases

Ultimately, the cell's cyclins do their job by interacting with cyclin-dependent protein kinases, that is, kinases that activate certain enzymes and proteins when bound to a cyclin. This allows cyclins to function as the "on" signal for many changes in cellular activity that occur throughout the cell cycle.

Protein kinases are a special set of enzymes that "turn on" other enzymes and proteins by attaching phosphate groups to them. When an enzyme or other protein is "activated" by a kinase, its behavior changes until it returns to its inactivated form.

The system by which one protein kinase can alter the activities of many other proteins allows simple signals, such as cyclins, to cause complex changes in cellular activity. Signal-dependent protein kinases are used to coordinate many complex cellular activities.

maturation promoting factor

An example of a functioning protein kinase is maturation-promoting factor or MPF. MPF is a protein kinase that is activated by a cyclin M, meaning it is activated during mitosis.

When MPF ​​is activated, it in turn activates several proteins in the nuclear envelope of its host cell. Changes in these proteins result in the breakdown of the nuclear envelope.

This is something that would be very dangerous at other points in the cell cycle, but is necessary during mitosis so that the chromosomes can be arranged and ensured that each daughter cell receives one copy of each chromosome.

If the M-cyclins do not appear, the MPF is not activated and mitosis cannot continue. This is a good example of how cyclins and cyclin-dependent kinases work together to coordinate (or arrest) the cell cycle.

Anaphase-promoting complex/cyclosome

Ingeniously, the MPF protein kinase not only causes the nuclear envelope to be broken during mitosis, but also causes MPF levels to fall after the nuclear envelope is broken. It does this by activating the anaphase-promoting complex/cyclosome, or “APC/C” for short.

As its name suggests, APC/C promotes anaphase, and one of the ways it does this is by breaking down MPF, an earlier-stage messenger. So MPF actually activates the proteins that destroy it.

The destruction of the MPF by the APC/C ensures that the actions that the MPF promotes, such as nuclear envelope disintegration, do not happen again until the daughter cell produces more MPF after passing through G.₁phase, phase S in G₂phase.

By activating APC/C the MPF controls itself!

Evidence

1. Which of the following reasons is NOT a reason why the interface is needed?
A.Daughter cells start life with a single copy of their DNA.
B.Daughter cells start life small, without enough cellular machinery to grow into daughter cells.
C.If cells repeatedly went through mitosis without going through interphase, each generation of daughter cells would get smaller and smaller.
D.All previous.

2. Which of the following organisms would you NOT expect to use the cell cycle described here?
A.A margarite
B.A kitten
C.an archaebacteria
D.none of the above

3. Which of the following statements is true for the G2 phase?
A.It is when the DNA of the cell is copied.
B.It is the first phase of the cell cycle after mitosis.
C.Contains the important G₂Checkpoint /M that checks the cell for DNA damage before it can reproduce.
D.None of the above.

References

• Cooper, G. M. (1997). The cell: a molecular approach. Washington, DC: ASM Press.
• Taylor, W.R. and Stark, G.R. (2001). Regulation of the G2/M transition by p53. Oncogene, 20(15), 1803-1815. doi:10.1038/sj.onc.1204252