The Cell Cycle and Mitosis Overview

The Cell Cycle

• The cell cycle is a series of events that occur in a cell leading to its division and duplication.
• The cell cycle progresses through distinct stages: G1 (first gap), S (synthesis - DNA replication), G2 (second gap) and M (mitosis) phases.
• These phases are regulated by checkpoints and internal control that ensure proper DNA replication and cell division.
• G1, S, G2 phases are collectively termed as interphase.

Mitosis and Cytokinesis

• M-phase includes both mitosis, nuclear division, and cytokinesis, cytoplasmic division.
• During mitosis, the replicated chromosomes are separated and distributed into two daughter nuclei.
• The phases of mitosis are prophase, prometaphase, metaphase, anaphase, and telophase.
• During prophase, chromosomes condense, and the mitotic spindle begins to form.
• During prometaphase, the nuclear envelope breaks down, and chromosomes attach to the spindle fibers via kinetochores.
• During metaphase, the chromosomes line up at the metaphase plate, midway between the spindle poles.
• During anaphase, sister chromatids separate and move to opposite poles of the cell.
• During telophase, the chromosomes arrive at the poles, the nuclear envelope reforms, and the spindle fibers disappear.
• Cytokinesis overlaps with telophase, and it involves the division of the cytoplasm to produce two daughter cells.
• In animal cells, cytokinesis is achieved by the formation of a cleavage furrow, a contractile ring made of microfilaments that pinches the cell in two.

Regulation of the Cell Cycle

• The cell cycle is regulated by cyclin-dependent kinases (Cdks), enzymes that control the cell cycle by phosphorylating proteins and promoting activity.
• The activity of Cdks is controlled by cyclins, regulatory proteins that bind to Cdks and activate them.
• Different Cdks and cyclins are active at different stages of the cell cycle.
• For example, the cyclin-cdk complex known as M-Cdk is activated at the G2/M checkpoint and triggers entry into mitosis.
• Specific checkpoints within the cell cycle ensure that the cell only progresses through the cycle when all necessary conditions are met.
• These checkpoints monitor the completion of DNA replication, the proper alignment of chromosomes, and the presence of sufficient nutrients and growth factors.

Cell Cycle Checkpoints

• The G1 checkpoint is often called the "restriction point" as it determines whether the cell will continue through the cell cycle. It checks for DNA damage and the availability of growth factors.
• The G2/M checkpoint ensures that DNA replication is complete before initiating mitosis. It also detects and repairs DNA damage before the cell enters mitosis.
• The M checkpoint occurs during metaphase and ensures that all chromosomes are properly attached to the mitotic spindle before proceeding to anaphase.

Cancer and the Cell Cycle

• Uncontrolled cell division is a hallmark of cancer. This is often due to mutations that disrupt the normal regulation of the cell cycle.
• Cancer cells often have defects in genes that regulate the cell cycle, including genes that encode cyclins, Cdks, and checkpoint proteins.
• These defects can lead to the uncontrolled proliferation of cells and the development of tumors.

Cancer Treatment

• Cancer treatments often target the cell cycle, aiming to disrupt the growth and proliferation of cancer cells.
• One strategy is to use inhibitors of Cdks to block the progression of the cell cycle at specific checkpoints.
• Specific inhibitors of different Cdks in the cell cycle have been developed and are being tested in clinical trials.
• Other cancer treatments, including DNA-damaging agents, are also designed to disrupt the cell cycle and induce cell death.

The Mitotic Spindle

• The mitotic spindle is a structure made of microtubules that forms during mitosis and plays a crucial role in chromosome segregation.
• Microtubules are long, thin, hollow cylinders made of the protein tubulin.
• The mitotic spindle is composed of microtubules and associated proteins, including motor proteins that drive the movement of microtubules and chromosomes.
• The mitotic spindle forms a bipolar array anchored at two poles of the cell.
• The spindle fibers attach to the chromosomes via kinetochores, specialized protein structures located at the centromeres of replicated chromosomes.
• The spindle fibers pull the sister chromatids apart, ensuring equal distribution of genetic material to the daughter cells.

Cytokinesis

• Cytokinesis is the division of the cytoplasm, which typically occurs after nuclear division (mitosis).
• In animal cells, cytokinesis is achieved by the formation of a cleavage furrow, a contractile ring made of microfilaments made of the protein actin that pinches the cell in two.
• In plant cells, cytokinesis occurs through the formation of a cell plate, a new cell wall that grows from the center of the cell outwards and eventually divides the cell into two.
• Cytokinesis ensures that each daughter cell receives a complete complement of organelles and cytoplasm.

This quiz explores the cell cycle, including its phases such as G1, S, G2, and M, as well as the processes of mitosis and cytokinesis. Understanding these concepts is crucial for grasping how cells divide and replicate their DNA. Test your knowledge on checkpoints, stages of mitosis, and the significance of each phase.