Question
Explain the various phases in Cell Cycle.
✓
Answer
The different phases of cell cycle are as follows:
1. Interphase: Longest part of the cell cycle, but it is of extremely variable length. At first glance the nucleus appears to be resting but this is not the case at all. The chromosomes previously visible as thread like structure, have dispersed. Now they are actively involved in protein synthesis, at least for most of the interphase. C - Value is the amount in picograms of DNA contained within a haploid nucleus.
2. $G_1$ Phase: The first gap phase $-2 C$ amount of DNA in cells of $G_1$ The cells become metabolically active and grows by producing proteins, lipids, carbohydrates and cell organelles including mitochondria and endoplasmic reticulum. Many checkpoints control the cell cycle. The checkpoint called the restriction point at the end of $G_1$ it determines a cells fate whether it will continue in the cell cycle and divide or enter a stage called $G_0$ as a quiescent stage and probably as specified cell or die. Cells are arrested in $G_1$ due to:
3. Nutrient deprivation: Lack of growth factors or density dependant inhibition. Undergo metabolic changes and enter into $G _0$ state. Biochemicals inside cells activates the cell division. The proteins called kinases and cyclins activate genes and their proteins to perform cell division. Cyclins act as major checkpoint which operates in $G _1$ to determine whether or not a cell divides.4. $G_0$ Phase: Some cells exit $G_1$ and enters a quiescent stage called $G_0$, where the cells remain metabolically active without proliferation. Cells can exist for long periods in $G_0$ phase. In $G_0$ cells cease growth with reduced rate of RNA and protein synthesis. The $G_0$ phase is not permanent. Mature neuron and skeletal muscle cell remain permanently in $G_0$. Many cells in animals remains in $G_0$ unless called on to proliferate by appropriate growth factors or other extracellular signals. $G_0$ cells are not dormant.5. S phase - Synthesis phase - cells with intermediate amounts of DNA. Growth of the cell continues as replication of DNA occur, protein molecules called histones are synthesised and attached to the DNA. The centrioles duplicate in the cytoplasm. DNA content increases from $2 C$ to $4 C .6$. $G_2$ - The second Gap phase - 4 C amount of DNA in cells of $G_2$ and mitosis. Cell growth continues by protein and cell organelle synthesis, mitochondria and chloroplasts divide. DNA content remains as 4C. Tubulin is synthesised and microtubules are formed. Microtubles organise to form spindle fibre. The spindle begins to form and nuclear division follows. One of the proteins synthesized only in the $G_2$ period is known as Maturation Promoting Factor (MPF). It brings about condensation of interphase chromosomes into the mitotic form. DNA damage checkpoints operates in $G _1 S$ and $G _2$ phases of the cell cycle.
1. Interphase: Longest part of the cell cycle, but it is of extremely variable length. At first glance the nucleus appears to be resting but this is not the case at all. The chromosomes previously visible as thread like structure, have dispersed. Now they are actively involved in protein synthesis, at least for most of the interphase. C - Value is the amount in picograms of DNA contained within a haploid nucleus.
2. $G_1$ Phase: The first gap phase $-2 C$ amount of DNA in cells of $G_1$ The cells become metabolically active and grows by producing proteins, lipids, carbohydrates and cell organelles including mitochondria and endoplasmic reticulum. Many checkpoints control the cell cycle. The checkpoint called the restriction point at the end of $G_1$ it determines a cells fate whether it will continue in the cell cycle and divide or enter a stage called $G_0$ as a quiescent stage and probably as specified cell or die. Cells are arrested in $G_1$ due to:
3. Nutrient deprivation: Lack of growth factors or density dependant inhibition. Undergo metabolic changes and enter into $G _0$ state. Biochemicals inside cells activates the cell division. The proteins called kinases and cyclins activate genes and their proteins to perform cell division. Cyclins act as major checkpoint which operates in $G _1$ to determine whether or not a cell divides.4. $G_0$ Phase: Some cells exit $G_1$ and enters a quiescent stage called $G_0$, where the cells remain metabolically active without proliferation. Cells can exist for long periods in $G_0$ phase. In $G_0$ cells cease growth with reduced rate of RNA and protein synthesis. The $G_0$ phase is not permanent. Mature neuron and skeletal muscle cell remain permanently in $G_0$. Many cells in animals remains in $G_0$ unless called on to proliferate by appropriate growth factors or other extracellular signals. $G_0$ cells are not dormant.5. S phase - Synthesis phase - cells with intermediate amounts of DNA. Growth of the cell continues as replication of DNA occur, protein molecules called histones are synthesised and attached to the DNA. The centrioles duplicate in the cytoplasm. DNA content increases from $2 C$ to $4 C .6$. $G_2$ - The second Gap phase - 4 C amount of DNA in cells of $G_2$ and mitosis. Cell growth continues by protein and cell organelle synthesis, mitochondria and chloroplasts divide. DNA content remains as 4C. Tubulin is synthesised and microtubules are formed. Microtubles organise to form spindle fibre. The spindle begins to form and nuclear division follows. One of the proteins synthesized only in the $G_2$ period is known as Maturation Promoting Factor (MPF). It brings about condensation of interphase chromosomes into the mitotic form. DNA damage checkpoints operates in $G _1 S$ and $G _2$ phases of the cell cycle.Need a full question paper?
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