Malcolm Rivera
The cell cycle consists of interphase and the mitotic (M) phase. Interphase itself is divided into three phases: G1 (First Gap), S (Synthesis), and G2 (Second Gap). During interphase, the cell grows and prepares for cell division. In the G1 phase, the cell grows and prepares for DNA replication. In the S phase, DNA replication occurs, resulting in two identical copies for each chromosome, known as sister chromatids. In the G2 phase, the cell makes final preparations for division, ensuring that the replicated chromosomes are healthy and ready to be distributed to the daughter cells.
| Characteristics | Values |
|---|---|
| Number of Phases | 3 |
| Phase 1 | G1 (First Gap) |
| Phase 2 | S (Synthesis) |
| Phase 3 | G2 (Second Gap) |
| G1 Phase Characteristics | Cell growth, preparation for DNA replication, protein synthesis, restoration of nucleoli, duplication of organelles and centrioles |
| S Phase Characteristics | DNA replication, duplication of chromosomes, formation of sister chromatids, duplication of centrosomes |
| G2 Phase Characteristics | Final preparations for cell division, replenishing energy, synthesis of new proteins, additional growth, dismantling of the cytoskeleton |
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What You'll Learn
G1 phase: cell growth, DNA replication preparation, protein synthesis
The G1 phase is the first stage of interphase in the cell cycle. During this phase, the cell grows in size and synthesizes mRNA and proteins necessary for DNA synthesis in the subsequent S phase. This growth is fuelled by a high amount of growth factors and a steady rate of protein synthesis. The cell also performs normal metabolic processes and prepares for DNA replication by copying some organelles.
The G1 phase is critical because it determines whether a cell commits to division or exits the cell cycle. The cell assesses whether it has sufficient reserves, adequate cell size, and undamaged DNA to progress to the S phase. If the requirements are not met, the cell enters a state of dormancy called the G0 phase.
In the G1 phase, the cell also produces energy and accumulates the building blocks of chromosomal DNA and associated proteins. It ensures enough energy reserves are available to replicate each chromosome in the nucleus. This phase is marked by high Cdk inhibitor concentrations, which prevent cell-cycle events from occurring out of order.
The G1 phase is the longest phase in the cell cycle for human somatic cells, lasting about 10 hours. However, its duration varies among different cell types. For example, in Xenopus, sea urchin, and Drosophila embryos, the G1 phase is barely existent or defined by the gap between mitosis and the S phase.
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S phase: DNA replication, doubling genetic material
The S phase, or synthesis phase, is the second phase of interphase in the cell cycle, occurring after the G1 phase and before the G2 phase. This phase is crucial as it involves DNA replication and the doubling of genetic material in preparation for cell division.
During the S phase, the cell actively replicates its DNA, ensuring that the genetic material is doubled before the cell enters mitosis or meiosis. This process is highly regulated and tightly conserved to maintain the integrity of the genetic material. DNA replication involves the synthesis of new DNA strands that are identical to the original. An enzyme called DNA polymerase plays a key role in this process by binding nucleotides to single template DNA strands, following the complementary base pairing rule: adenine binds to thymine, and cytosine binds to guanine. This results in the formation of a new double helix of DNA.
The initiation and completion of DNA replication define the beginning and ending of the S phase. The cell must carefully regulate and monitor this process to ensure that the entire genome is duplicated accurately and without errors. The S phase is also important for detecting and correcting any DNA damage. If there is any damage to the DNA, it can be identified and repaired during this phase.
Regulatory pre-replication complexes play a crucial role in the S phase by signalling where DNA synthesis should begin. Additionally, protein p16 and its partners inhibit the cell from entering the S phase until it has reached a sufficient size. The entry into the S phase is controlled by the G1 restriction point, which assesses the nutritional status and DNA integrity of the cell. Once the cell passes this point, it is committed to progressing through the S-phase, even if environmental conditions change.
The S phase is essential for ensuring that the cell has doubled its genetic material before undergoing mitosis or cell division. This phase is highly regulated to maintain the accuracy and integrity of the genetic material, which is crucial for successful cell division.
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G2 phase: final preparations for cell division, energy replenishment
The G2 phase, or the second gap phase, is the third phase of interphase in the cell cycle. During this phase, the cell completes its preparation for mitosis or cell division. It is a crucial phase as it ensures that the cell is fully prepared to enter the process of division.
One of the key functions of the G2 phase is energy replenishment. The cell replenishes its energy stores to ensure it has sufficient energy for the upcoming events of cell division. High energy levels are required for mitosis, and the G2 phase helps to meet this energy demand.
In addition to energy replenishment, the G2 phase involves protein synthesis. The cell synthesizes proteins, such as cyclins, that are essential for chromosome manipulation and movement. These proteins play a critical role during mitosis in separating sister chromatids and ensuring proper distribution to the daughter cells.
The G2 phase also includes organelle duplication. Certain organelles, such as mitochondria, are duplicated to provide energy for the daughter cells after division. This duplication ensures that each new cell will have the necessary organelles to function properly.
Another important aspect of the G2 phase is the dismantling of the cytoskeleton. The cytoskeleton provides structure to the cell, but it is dismantled during this phase to allow for the reorganization of internal structures in preparation for cell division. The G2 phase serves as a quality control point where the cell checks its DNA for errors and ensures that the replicated chromosomes are healthy and ready for division.
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Gap phases: cell growth, synthetic processes
The cell cycle consists of four coordinated processes: cell growth, DNA replication, distribution of duplicated chromosomes to daughter cells, and cell division. The cell cycle can be divided into two parts: interphase and mitotic (M) phases. Interphase, during which the cell spends most of its life, can be further divided into three phases: G1, S phase, and G2. The G phases are the gap phases, and the S phase is the synthesis phase.
The G1 phase is the first gap phase. During this phase, the cell grows rapidly and performs normal metabolic processes. It also prepares for DNA replication by synthesizing proteins and copying some organelles. The length of the G1 phase can vary greatly depending on external conditions and extracellular signals from other cells. If extracellular conditions are unfavourable, cells may delay progress through G1 or even enter a specialized resting state known as G0, in which they can remain for extended periods of time.
The S phase, or synthesis phase, is the second phase of interphase. During this phase, DNA replication occurs, and the cell duplicates its chromosomes. This results in two identical copies for each chromosome, known as sister chromatids. The S phase requires 10-12 hours and occupies about half of the cell cycle time in a typical mammalian cell.
The G2 phase, or second gap phase, is the third phase of interphase. During this phase, the cell makes final preparations for division. It checks its DNA for errors to ensure that the replicated chromosomes are healthy and ready to be distributed to the daughter cells. The cell also continues to grow and synthesizes proteins in preparation for mitosis.
The gap phases serve as more than just time delays to allow for cell growth. They also provide time for the cell to monitor its internal and external environment and ensure that conditions are suitable before committing to the major upheavals of the S phase and mitosis.
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Synthesis phase: longest phase, genetic material duplication
The synthesis phase, also known as the S phase, is the second phase of interphase in the cell cycle. During this phase, DNA replication occurs, and the cell duplicates its chromosomes. This results in two identical copies of each chromosome, known as sister chromatids. The S phase is crucial for ensuring that the cell has enough genetic material to divide successfully.
The synthesis phase is the longest phase of interphase, during which the cell actively copies its DNA. This process of DNA replication is essential for the cell to carry out its regular functions and prepare for cell division. While the cell grows at a steady rate throughout interphase, DNA synthesis only occurs during the S phase. This timing divide the cycle of eukaryotic cells into four distinct phases.
The progression between the stages of the cell cycle is controlled by a regulatory apparatus that coordinates the different events and links the cell cycle with extracellular signals controlling cell proliferation. This ensures that the cell is ready to undergo mitosis, which follows interphase.
During the synthesis phase, the cell's primary function is to replicate its DNA accurately. The cell duplicates each chromosome, creating identical sister chromatids that will be distributed to the daughter cells during cell division. This process ensures that the genetic material is preserved and passed on accurately to the next generation of cells.
The S phase is preceded by the G1 phase (First Gap) and followed by the G2 phase (Second Gap). While the cell grows and prepares for DNA replication in G1, it makes final preparations for division in G2. These three phases of interphase are essential for the cell to be ready for mitosis, which is the subsequent phase of the cell cycle.
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Frequently asked questions
The three phases of the cell cycle that constitute interphase are G1, S, and G2.
During the G1 phase, the cell grows and prepares for DNA replication by synthesizing proteins and copying some organelles.
In the S phase, DNA replication occurs, resulting in two identical copies for each chromosome, known as sister chromatids.
