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ClassNotes UNDERSTANDING THE CONCEPTS Q#1 What is cell cycle and what are its main phases? Ans: Cell Cycle: The cell cycle is the series of events from the time a cell is produced until it completes mitosis and produces new cells. Main phases of cell cycle: The cell cycle consists of two major phases i. interphase ii. mitotic phase (M phase) The mitotic phase is a relatively short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for division. Interphase is the time when a cell's metabolic activity is very high, as it performs its various functions. Phases of inter phase: It is divided into three phases. 1. G1 (first gap), 2. S (synthesis), 3. G2 (second gap). G1 phase: After its production, a cell starts its cell cycle in G1 phase. During this phase, the cell increases its supply of proteins, increases the number of many of its organelles (such as mitochondria, ribosomes), and grows in size. This phase is also marked by the synthesis of various enzymes that are required in the next phase i.e. S phase for DNA replication. Cells that have temporarily or permanently stopped dividing are said to have entered a state of quiescence called GO phase. www.classnotes.xyz Biology ClassNotes, Inc. © 2020 1
ClassNotes S phase: In this phase the cell duplicates its chromosomes. The DNA molecule of each chromosome is copied, and new protein molecules are attached. The result is that each chromosome consists of two sister chromatids, which contain identical genes. G2 phase: In the G2 phase the cell prepares proteins that are essential for mitosis mainly for the production of spindle fibres. After the G2 phase of interphase, the cell enters the division phase le. M phase. It is characterized by mitosis, in which the cell divides into the two daughter cells. G0 phase: In multicellular eukaryotes, cells enter the G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). Some cell types in mature organisms, such as some cells of the liver and kidney, enter the G0 phase semi-permanently and can only be induced to begin dividing again under very specific circumstances. Other cells, such as epithelial cells, do not enter G0 and continue to divide throughout an organism's life. Q#2 The S-phase of interphase is important and a cell can never divide without it Justify? Ans: S-phase of interphase: Mitosis is the type of cell division in which a cell divides into two daughter cells, each with the genetic equivalent of the parent cell i.e. same number of chromosomes as were present in the parent cell. Because each resultant daughter cell should be genetically identical to the parent cell, the parent cell www.classnotes.xyz Biology ClassNotes, Inc. © 2020 2
ClassNotes must make a copy of each chromosome before mitosis. This occurs during S phase of interphase. Q#3 How would you state the events of prophase? Ans: Events of Prophase: Chromatin: Normally, the genetic material in the nucleus is in a loose thread-like form called chromatin. Chromosomes: At the one of prophase, chromatin condenses into highly ordered structures called chromosomes. Since the genetic material has already been duplicated earlier in S phase, each chromosome is made of two sister chromatids, bound together at the centromere. Each chromosome has kinetochore at the centromere. A kinetochore is a complex protein structure that is the point where spindle fibers attach. Centrosome: Close to the nucleus are two centrioles collectively called a centrosome. Each centriole replicates and thus two daughter centrosomes are formed. Each daughter centrosome acts as a coordinating center for the cell's microtubules. Mitotic Spindle: The two centrosomes give rise to microtubules by polymerizing (joining monomers to form polymers) the tubulin proteins present in the cytoplasm. The microtubules thus formed are called spindle fibers, and the complete set of the spindle fibers is known as mitotic spindle. During the formation of mitotic spindle, the centrosomes migrate to opposite side of the nucleus. The nucleolus and the nuclear envelope have degraded, and spindle fibers have invaded the central space. www.classnotes.xyz Biology ClassNotes, Inc. © 2020 3
ClassNotes Aggregation of Tubulin Proteins: In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before prophase. The cells of plants lack centrioles. Instead, spindle fibers are formed by the aggregation of tubulin proteins on the surface of the nuclear envelope during prophase. Q.4 Make a list of the events of mitosis. Ans: Events of Mitosis: The process of mitosis is complex and highly regulated. The sequence of events is divided into major phases. 1. Karyokinesis: The division of the nucleus known as karyokinesis. 2. Cytokinesis: The division of the cytoplasm known as cytokinesis. Events of Karyokinesis: The division of the nucleus is further divided into four phases. i. prophase, ii. metaphase, iii. anaphase iv. telophase. Q#5 How is mitosis significant? Ans: Significance of mitosis: 1. Maintenance of the chromosomal set: The importance of mitosis is the maintenance of the chromosomal set i.e. each daughter cell receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell. www.classnotes.xyz Biology ClassNotes, Inc. © 2020 4