Jonah Patel
The cell cycle is a process by which cells replicate and produce two new cells. This sequence consists of four coordinated processes: cell growth, DNA replication, distribution of the duplicated chromosomes to daughter cells, and cell division. The cell cycle can be divided into two main parts: interphase and mitosis. Interphase, which includes the G1, S, and G2 phases, is the longest phase of the cell cycle, accounting for about 90% of the total cycle. During this phase, the cell grows in size, forms more organelles, and replicates its DNA. The second phase, mitosis, includes the division of the nucleus, followed by the division of the cytoplasm, called cytokinesis.
| Characteristics | Values |
|---|---|
| Two main phases | Interphase and mitosis |
| Interphase | The cell grows at a steady rate and is the longest phase of the cell cycle, accounting for about 90% of the total cycle |
| Mitosis | Includes division of the nucleus, and then division of the cytoplasm, called cytokinesis |
| Interphase sub-phases | G1, S, G2 |
| G1 | Interval between mitosis and initiation of DNA replication |
| S | DNA replication takes place |
| G2 | Cell growth continues and proteins are synthesized in preparation for mitosis |
| Mitosis sub-phases | Prophase, metaphase, anaphase, telophase, cytokinesis |
| Mitosis | M phase of the cycle |
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What You'll Learn
- Interphase: cell growth, DNA replication, and cell division preparation
- Mitosis: division of the nucleus, then cytoplasm, forming two daughter cells
- G1 phase: interval between mitosis and DNA replication, cell metabolically active
- S phase: DNA replication, DNA content doubles
- G2 phase: cell growth, protein synthesis, preparation for mitosis
Interphase: cell growth, DNA replication, and cell division preparation
The cell cycle is divided into two basic parts: interphase and mitosis. Interphase accounts for about 90% of the total cycle. During interphase, the cell grows in size by forming more organelles, and replicates its DNA, resulting in the duplication of the cell's chromosomal matter. The cell grows at a steady rate throughout interphase, with most dividing cells doubling in size between one mitosis and the next. DNA is synthesized during only a portion of interphase, specifically the S phase.
The interphase itself consists of three phases: G1, S, and G2. G1 is the stage where the cell is preparing to divide. The cell then moves into the S phase, where it copies all its DNA. After the DNA is copied, the cell moves into the G2 stage, where it organizes and condenses the genetic material and prepares to divide. During G1, the cell is metabolically active and continuously grows but does not replicate its DNA. G2 is a shortened growth period in which many organelles are reproduced or manufactured, and parts necessary for mitosis and cell division are made.
Following interphase is the M phase, which corresponds to mitosis. Mitosis includes the division of the nucleus, followed by the division of the cytoplasm, called cytokinesis. Mitosis has several stages: prophase, metaphase, anaphase, and telophase, which are finally followed by cytokinesis. During metaphase, the chromosomes align at the metaphase plate, an imaginary plane in the middle of the cell. In anaphase, the sister chromatids of each chromosome separate and are pulled toward opposite poles of the cell by the spindle fibers. During telophase, the chromosomes decondense, and new nuclear membranes form around each set. Cytokinesis is the final stage of the cell cycle, during which the cytoplasm divides, forming two genetically identical daughter cells.
In bacteria, cell growth and DNA replication take place throughout most of the cell cycle, and duplicated chromosomes are distributed to daughter cells in association with the plasma membrane. However, in eukaryotes, the cell cycle is more complex and consists of four discrete phases. Although cell growth is usually continuous, DNA is synthesized during only one phase of the cell cycle, and the replicated chromosomes are then distributed to daughter nuclei before cell division.
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Mitosis: division of the nucleus, then cytoplasm, forming two daughter cells
The cell cycle consists of two main phases: interphase and mitosis. Interphase is the longest phase, accounting for about 90% of the total cycle. During this phase, the cell recovers from previous division, performs its normal functions, and grows in size by forming more organelles and replicating its DNA.
Mitosis is the second phase of the cell cycle, during which the nucleus divides, followed by the division of the cytoplasm, resulting in the formation of two daughter cells. This process is critical for life and is carefully controlled by specific genes to prevent health issues such as cancer. Mitosis can be further divided into several stages: prophase, metaphase, anaphase, telophase, and cytokinesis.
Prophase is the initial stage of mitosis, where chromosomes recruit condensin and undergo a condensation process that continues until metaphase. During this stage, spindle fibers also begin to form as centrioles move to opposite poles, and microtubules polymerize from duplicated centrosomes. The breakdown of the nuclear membrane is essential for spindle assembly.
In metaphase, chromosomes align themselves on the metaphase plate, an imaginary plane in the center of the cell, with the help of spindle fibers. This ensures the proper alignment of each chromosome.
Anaphase begins with the separation of sister chromatids, which are pulled toward opposite poles by spindle fibers. As the chromosomes reach the poles, the cell elongates.
Telophase is the final stage of mitosis, where chromosomes reach the poles, and the nuclear membrane reforms. The chromosomes begin to decondense, and new nuclear membranes form around each set of chromosomes. The spindle fibers disassemble, and the cell prepares for cytokinesis.
Cytokinesis is the division of the cytoplasm, resulting in the formation of two genetically identical daughter cells. This stage can begin during telophase and is the final step in the cell cycle, leading to the irreversible separation of the two daughter cells.
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G1 phase: interval between mitosis and DNA replication, cell metabolically active
The cell cycle consists of two main parts: interphase and mitosis. Interphase, which accounts for about 90% of the total cycle, is further divided into three phases: G1, S, and G2. The G1 phase, also known as "gap 1", is the interval between mitosis and the initiation of DNA replication. During this phase, the cell is metabolically active and continuously grows, but does not replicate its DNA. Animal cells in G1 are diploid, containing two copies of each chromosome, and their DNA content is referred to as 2n.
The G1 phase is a critical period for the cell as it prepares for the upcoming DNA replication and cell division processes. The cell grows steadily during this phase, forming more organelles and increasing in size. This growth is necessary to ensure that the cell has sufficient resources and energy to undergo the demanding process of DNA replication and subsequent cell division. The duration of the G1 phase can vary depending on the type of cell. For example, in a typical rapidly proliferating human cell with a 24-hour cycle time, the G1 phase might last about 11 hours. However, other cells, such as budding yeasts, can progress through the G1 phase much more rapidly, completing it in a shorter duration.
Following the G1 phase is the S phase, or synthesis phase, during which DNA replication occurs. This phase involves the replication of the cell's DNA content, increasing it from 2n to 4n. After the completion of DNA synthesis, the cell enters the G2 phase, or "gap 2", where it continues to grow and synthesize proteins in preparation for mitosis. This phase ensures that the cell is ready for the upcoming division process, providing the necessary proteins and cellular components.
The G1 phase is tightly regulated by a conserved regulatory apparatus that controls the progression of the cell cycle. This regulatory mechanism coordinates the various events of the cell cycle, ensuring that DNA replication and cell division occur at the appropriate times. Additionally, it links the cell cycle with extracellular signals that control cell proliferation, allowing the cell to respond to environmental cues and maintain proper growth and division control. Overall, the G1 phase plays a crucial role in the cell cycle, providing a period of metabolic activity and growth that prepares the cell for the upcoming DNA replication and cell division processes.
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S phase: DNA replication, DNA content doubles
The cell cycle consists of two main parts: interphase and mitosis. Interphase, which accounts for about 90% of the total cycle, is further divided into three phases: G1, S, and G2. During the S phase, DNA replication takes place, and the cell's DNA content doubles.
The S phase is a critical period in the cell cycle, as it involves the replication of DNA. This process ensures that there is enough genetic material to be distributed to the daughter cells during cell division. The S phase is tightly regulated by various mechanisms to ensure its success and maintain genome stability. For instance, regulatory pre-replication complexes, including protein p16, prevent the cell from entering the S phase prematurely, only allowing progression when the cell has grown sufficiently and passed the G1 checkpoint.
During the S phase, the DNA molecule, in the form of a double helix, is unwound by an enzyme called helicase. This unwinding creates two single DNA strands, which then serve as templates to synthesize two identical double DNA strands. The replication process is facilitated by the activation of MCM (Mini-chromosome Maintenance Complex) complexes, which form the core of the replicative helicases. These MCM complexes are loaded onto the DNA during the G1 phase through the sequential action of licensing factors, including ORC (Origin Recognition Complex), CDC6, and CDT1.
The successful completion of DNA replication in the S phase is essential for maintaining the integrity of the genome and ensuring the accurate transmission of genetic information to daughter cells. Errors during this phase can have severe consequences, leading to genetic abnormalities, diseases, or even cell death. Therefore, the cell cycle is equipped with regulatory mechanisms, such as the activation of the tumor-suppressing protein p16, to safeguard against premature entry into the S phase and ensure the fidelity of DNA replication.
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G2 phase: cell growth, protein synthesis, preparation for mitosis
The cell cycle consists of two main parts: interphase and mitosis. Interphase, which accounts for about 90% of the total cycle, is further divided into G1, S, and G2 phases. During interphase, the cell recovers from previous division and carries out its normal functions. It grows in size by forming more organelles and replicating its DNA, resulting in the duplication of the cell's chromosomal matter.
The G2 phase is the second growth phase, where the cell prepares for entry into the mitosis phase. During this phase, the cell continues to grow and synthesize proteins in preparation for mitosis. Energy is replenished, and new proteins are synthesized, with additional cell growth occurring. The integrity of the DNA is checked, and double-strand breaks in one sister chromatid may be repaired by homologous recombinational repair using the other intact sister chromatid as a template. The G2 phase ends when the cell enters mitosis, which includes the division of the nucleus, followed by the division of the cytoplasm, called cytokinesis.
The G2 phase is critical for ensuring the successful division of cells. If the necessary proteins and organelles are not synthesized during this phase, the cell would be halved at every cell division until there is nothing left to divide. This is because DNA replication and the subsequent division cannot occur without the necessary components synthesized in the G2 phase.
The progression of the cell cycle is tightly regulated by a conserved regulatory apparatus. This regulatory system coordinates the different events of the cell cycle, such as growth, DNA replication, and cell division, and also links the cycle with extracellular signals that control cell proliferation. The cell cycle is vital for the continued existence of all eukaryotes and prokaryotes.
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Frequently asked questions
The two main phases of the cell cycle are interphase and mitosis.
During interphase, the cell grows in size by forming more organelles and replicating its DNA, resulting in the duplication of the cell's chromosomal matter.
The M phase of the cycle corresponds to mitosis, which is usually followed by cytokinesis, resulting in the formation of two daughter cells.
The G2 phase is a shortened growth period where the cell continues to grow and synthesize proteins in preparation for mitosis.
During mitosis, the nucleus of the cell divides, followed by the division of the cytoplasm, known as cytokinesis.
