The cell cycle is a series of events that occur in a cell leading to its division and duplication. It consists of three main stages: G1, S, G2, and Mitosis.
(Source - Quizlet)
G1 Phase - The G1 phase, also known as the first gap phase, is the first stage of the cell cycle. During this stage, the cell grows and synthesizes new proteins and organelles. The G1 phase is the longest phase of the cell cycle and can vary in length depending on the cell type and environmental conditions.
S Phase - The S phase, also known as the synthesis phase, is the second stage of the cell cycle. During this stage, DNA replication occurs, resulting in the duplication of the cell's genetic material. Each chromosome is duplicated to form two sister chromatids, which are attached to each other at a region called the centromere.
(Source - ResearchGate)
G2 Phase - The G2 phase, also known as the second gap phase, is the third stage of the cell cycle. During this stage, the cell prepares for mitosis or cell division. The cell checks for DNA damage and makes any necessary repairs before proceeding to mitosis. The G2 phase is also important for the assembly of the spindle fibers that will help separate the chromosomes during cell division.
Mitosis
Mitosis is a complex process that involves a series of coordinated events that ensure the equal distribution of genetic material between the two daughter cells. The process of mitosis is essential for the growth and development of organisms, tissue repair, and asexual reproduction in some organisms.
· Prophase: During prophase, chromatin condenses into visible chromosomes, and the nuclear envelope begins to break down. The centrosomes move towards opposite poles of the cell, and spindle fibers begin to form from each centrosome. The spindle fibers radiate out from the centrosomes and attach to the kinetochores, which are located at the centromeres of the chromosomes. The kinetochores act as attachment sites for the spindle fibers, and they ensure that the chromosomes will be separated correctly during the later stages of mitosis.
· Prometaphase: During prometaphase, the nuclear envelope completely disintegrates, and the spindle fibers continue to grow and extend toward the chromosomes. The spindle fibers that attach to the kinetochores of the chromosomes are called kinetochore microtubules, and they begin to pull the chromosomes toward the center of the cell, aligning them at the metaphase plate. As the kinetochore microtubules attach to the chromosomes, they begin to exert tension on the centromeres, causing the chromosomes to move toward the center of the cell.
(Source - The Biology Notes)
During prometaphase, the spindle fibers that do not attach to the chromosomes, called polar microtubules, push against each other, causing the poles of the cell to move further apart. The result is the elongation of the cell and the stretching of the spindle fibers, allowing the kinetochore microtubules to pull the chromosomes toward the center of the cell.
· Metaphase: In metaphase, the spindle fibers align the chromosomes at the equator of the cell, called the metaphase plate. The chromosomes are pulled toward the equator by the spindle fibers, which attach to the kinetochores of each chromosome. The alignment of the chromosomes at the metaphase plate ensures that each daughter cell will receive an identical copy of the genetic material.
· Anaphase: During anaphase, the spindle fibers shorten, pulling the sister chromatids apart at their centromeres. The spindle fibers attached to the chromosomes are called kinetochore microtubules, while the spindle fibers not attached to the chromosomes are called polar microtubules. The polar microtubules elongate the cell, while the kinetochore microtubules shorten, pulling the chromosomes toward the poles of the cell.
· Telophase: In telophase, the spindle fibers begin to break down, and the chromosomes begin to decondense. A new nuclear envelope forms around each set of chromosomes, forming two new nuclei. The cell then begins to divide into two daughter cells by a process called cytokinesis.
· Cytokinesis: It is the final stage of mitosis and involves the physical separation of the two daughter cells. In animal cells, a contractile ring made of actin filaments forms around the cell, pinching it in the middle, and dividing it into two. In plant cells, a cell plate forms in the middle of the cell, which eventually develops into a new cell wall, dividing the cell in two.
Written by - Narayanamanikandan B
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