Cancer and the Cell Cycle

The Cell Cycle

The cell cycle is like a journey that all cells go through, and it has different phases, just like a trip has different parts. The cell's life is broken into two parts, interphase and mitosis.

Interphase: This is like the preparation phase before the journey. The cell gets ready by growing and making more parts (like proteins and organelles) it will need for the next steps. It also copies its DNA, which is like a recipe book with instructions for making new cells. Some cells stay in interphase for a really long time. While other cells move through the cycle much quicker.
Mitosis: This is the main part of the journey but it actually goes by pretty quickly! It can take months or even years to plan the perfect vacation even though the vacation might only last a week. At the end of interphase, the cell makes sure its DNA is all set and organized properly. Mitosis is the process of splitting the cell into two new cells, each with a full set of DNA. ***Cytokinesis***: After the end of the trip it's time to pack up and head home. Cytokinesis splits the parts of the cell in two, making sure both new cells have all the proteins, organelles, and DNA needed to be fully functioning cells. In the same way that you might start packing before the last day of your trip, cytokinesis can begin before the end of mitosis.

Interphase

Step 1 of the cell cycle is called "Interphase." It's like the preparation phase before the cell goes on its main journey. Interphase has three sub-phases: G1, S, and G2. Each of these sub-phases has its own specific tasks that are crucial for the cell's success.

G1 Phase (Gap 1 Phase): This is like the starting point of the cell's journey. During G1, the cell grows in size and produces more proteins and organelles, which are like tiny machines that help the cell function properly. Imagine the cell gathering all the tools and supplies it needs for the trip ahead.
S Phase (Synthesis Phase): In the S phase, the cell copies its DNA. DNA is like a recipe book that contains all the instructions for making the cell and controlling its functions. It's essential for the cell to have a full and accurate copy of its DNA to pass on to the new cells it will create later. Think of it as making a backup passport - it has all the important identifying information for the cell.
G2 Phase (Gap 2 Phase): Now that the cell has grown and made a copy of its DNA, it checks everything to make sure it's all good to go. It verifies if there are any errors in the DNA copy and repairs them if needed. The cell also continues to produce more proteins and organelles. This phase is like doing a final check before starting a big adventure to ensure everything is ready.

Once the cell completes the G1, S, and G2 phases, it is fully prepared for the most complicated part of the cell cycle, which is the mitosis phase. In mitosis, the cell will divide into two new cells, each with the same set of DNA and all the necessary materials to function and grow.

Mitosis

Image attribution: Brandon9993, CC BY-SA 4.0, via Wikimedia Commons

Mitosis is the process that allows a single cell to divide into two identical daughter cells. Mitosis is crucial for growth, development, tissue repair, and maintaining the proper functioning of our bodies. Mitosis can be divided into several distinct stages, each with its own specific tasks:

Prophase: This is like the preparation stage for mitosis. The cell's DNA, which is usually spread out and thin like spaghetti, starts to condense and coil up tightly. It becomes visible as distinct chromosomes under a microscope. Each chromosome is a structure that contains one long piece of DNA with many genes. Genes are like individual recipes while the DNA is the entire recipe book.
Metaphase: During metaphase, the chromosomes line up in the middle of the cell. They arrange themselves in a single file along an imaginary line called the "metaphase plate." It's like the chromosomes are getting ready to be evenly distributed between the two new cells.
Anaphase: In anaphase, the real magic happens! The chromosomes are split apart at the center and pulled toward opposite ends of the cell. It's like pulling apart two halves of a zipper. Each side of the cell now contains one copy of each chromosome.
Telophase: This is the final act of mitosis. The separated chromosomes reach the opposite ends of the cell, and new membranes start forming around them. This process creates two new nuclei, one for each of the future daughter cells. It's like each cell is getting its own control center.
**Cytokinesis**: Though not technically part of mitosis, cytokinesis happens right after. It's like the grand finale! The cell's cytoplasm, which is the jelly-like substance inside the cell, divides into two parts, each surrounding one of the new nuclei. This division creates two separate daughter cells, each identical to the original cell.

And there you have it! The cell has completed the amazing process of mitosis, resulting in two new cells ready to continue their own journeys through the cell cycle. This cycle of cell division is vital for the growth, repair, and maintenance of our bodies, allowing us to function and thrive. At the end of cytokinesis there are two identical cells that begin their journey in interphase. Whenever your body needs new cells the process can begin again.

Image attribution:
Kelvinsong, CC0, via Wikimedia Commons

G0

G0 (pronounced "G zero") is a resting phase or quiescent state that some cells enter instead of continuing through the cell cycle. In simpler terms, it's like a timeout or a break period for cells, where they temporarily stop dividing and stay in a non-dividing state.
When a cell enters the G0 phase, it means that it has left the active cell cycle and is not actively preparing to divide or replicate. Instead, the cell can perform its specialized functions without the intention of dividing further. Cells in G0 are often considered to be in a state of "cellular retirement" or "resting phase" because they have exited the normal cell cycle progression.
Not all cells enter the G0 phase; some cells continue through the cell cycle repeatedly. However, certain types of cells, like nerve cells (neurons) and muscle cells (myocytes), tend to enter the G0 phase and stay there for extended periods or even indefinitely. These specialized cells have reached a stage where their main focus is performing specific functions within the body rather than undergoing continuous cell division.
Cells can transition back and forth between the G0 phase and the active cell cycle phases (G1, S, G2, M) depending on signals from the environment or the body's needs. If the cell receives the right signals or stimuli, it can exit the G0 phase and re-enter the cell cycle to divide and replicate.
The G0 phase is essential for maintaining the proper balance of cell growth and tissue maintenance in our bodies. It allows cells to avoid unnecessary division and helps preserve their functions and stability. This way, different types of cells can work together harmoniously to support the overall health and functioning of our bodies.

Checkpoints

It's incredibly important that the cell cycle occurs perfectly each time a cell divides. The process is highly regulated by cell cycle checkpoints. These checkpoints are like immigration checkpoints during the cell cycle journey, they act as control points to ensure that the cell is ready to move on to the next phase and that the cell has everything it needs and hasn't made any mistakes along the way. If the cell doesn't pass these checkpoints, it may undergo repairs or stop progressing altogether.

There are three main checkpoints in the cell cycle:

G1 Checkpoint: This checkpoint occurs at the end of the G1 phase, before the cell enters the S phase. At this checkpoint, the cell checks if it has grown enough and if its environment is suitable for cell division. It also checks if the DNA is healthy and free from any damage. If everything looks good, the cell gets the green light to proceed to the S phase, where it will copy its DNA. However, if there are any issues or if the cell receives signals to delay division, it might temporarily or permanently stop dividing.
G2 Checkpoint: This checkpoint takes place at the end of the G2 phase, just before the cell enters mitosis (M phase). At this point, the cell verifies if the DNA replication in the S phase was successful and checks for any damages or errors in the copied DNA. The cell also ensures that it has enough resources and organelles to support the upcoming cell division. If everything is in order, the cell proceeds to enter the M phase. But if there are any issues detected, the cell might pause to fix the problems before moving forward or it may self destruct and die if the mistakes are unable to be resolved.
M Checkpoint: This checkpoint happens during the M phase, specifically during metaphase, when the chromosomes line up at the metaphase plate. The cell checks if all the chromosomes are correctly attached to the spindle fibers, which are like tiny ropes that help move the chromosomes during cell division. This ensures that each daughter cell will get the correct number of chromosomes. If any attachment problems are found, the checkpoint will delay the separation of the chromosomes until the issues are resolved.

Cell cycle checkpoints are crucial for maintaining the stability and health of the cell. They help prevent errors and mutations from passing on to new cells and play a vital role in regulating cell growth and division. If there are significant issues at any of these checkpoints, the cell may undergo programmed cell death (apoptosis) to prevent the development of abnormal or damaged cells. This process helps to protect the body from potentially harmful cells and maintains the balance of cell division and growth.

Cancer

Cancer is a group of diseases characterized by the uncontrolled and abnormal growth of cells. In a healthy body, cells grow, divide, and die in an orderly manner, helping the body to function properly and repair damaged tissues. However, in cancer, this normal process of cell growth and death is disrupted.

Unlike normal cells that receive a growth signal that prompts them to actively begin division, cancer cells continually move through the cell cycle making more and more cells even when no new cells are needed. Cancer cells don't enter the G0 phase which means they don't performed the specialized jobs we need our cells to perform in order to have a functioning body.

In addition to always being "turned on", cancer cells also have the ability to bypass the cell cycle checkpoints. Cancer cells don't properly check for and fix mistakes in the DNA replication process. These mistakes, known as mutations, are passed down to the resulting cells. Over time, cancer cells develop more and more mutations in their DNA.

Lastly, another issue with cancer cells is that they are able to avoid apoptosis - programmed cell death. Normally, when a cell makes mistakes that it is unable to fix it is programmed to die. Cancer cells are able to avoid this programmed cell death and as a result they continue to divide and create more cells with more mistakes.

The cell cycle is controlled by groups of genes, primarily broken into four groups:

Oncogenes: Oncogenes are normal genes involved in promoting cell growth and division. However, when these genes are mutated or overexpressed, they become oncogenic, meaning they can drive uncontrolled cell growth and contribute to the development of cancer. Mutations in oncogenes can lead to the continuous activation of cell growth pathways. As a result, the cell moves through the cell cycle continuously as the signal to divide is constantly turned "on".

Tumor Suppressor Genes: Tumor suppressor genes are responsible for regulating cell growth and division by acting as "brakes" on the cell cycle. They help prevent cells with damaged DNA from growing and dividing uncontrollably. When tumor suppressor genes are mutated or inactivated, their ability to control cell growth is compromised, allowing abnormal cells to divide without restraint.
DNA Repair Genes: DNA repair genes are essential for fixing mistakes and damage in the DNA sequence. Mutations in these genes can lead to an accumulation of further mutations in other critical genes, including oncogenes and tumor suppressor genes. The loss of proper DNA repair mechanisms can contribute to the progression of cancer.
Apoptosis-Related Genes: Apoptosis is a natural process of programmed cell death that helps remove damaged or unwanted cells from the body. Mutations in genes involved in apoptosis can prevent cancer cells from undergoing cell death, allowing them to survive and continue growing despite having abnormal characteristics.

In cancer cells, several genes must have mutations, the combination of mutations leads to the uncontrolled cell growth and division characteristic of the disease. Mutations are changes in the DNA sequence of genes, and when these mutations affect certain critical genes, they can disrupt the normal regulation of cell growth and contribute to the development of cancer.

It's important to note that cancer is a highly complex and varied disease. Different types of cancer can have distinct sets of mutated genes that drive their development and progression. Additionally, the specific mutations in these genes can vary between individuals with the same type of cancer.

Understanding the genetic mutations present in cancer cells is crucial for the development of targeted therapies and personalized medicine approaches. Advances in genomic research have allowed scientists and medical professionals to identify specific genetic mutations in cancer cells and develop therapies that target these mutated genes or the proteins they produce, leading to more effective and precise cancer treatments.

Cancer begins when certain cells in the body start to grow and divide uncontrollably, forming a mass of abnormal cells called a tumor. Tumors can be benign (non-cancerous) or malignant (cancerous). Benign tumors do not spread to other parts of the body and are usually not life-threatening. On the other hand, malignant tumors are cancerous and can invade nearby tissues and spread to other parts of the body through the bloodstream or the lymphatic system, a process known as metastasis.