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10 CELL CYCLE AND CELL DIVISION

118 CELL CYCLE AND CELL DIVISION SCAN CODE Cell Cycle and Cell Division Chromatid A chromatid is one of two identical halves of a replicated chromosome. Centromere The region of the chromosome to which the spindle fiber is attached during cell division. Chromatin fibres Chromatin is composed of DNA and histones that are packaged into thin, stringy fibers. Kinetochore It is a protein complex assembled on the centromeric region of DNA. It serves the attachment point for the spindle. Spindle apparatus The spindle apparatus is a network of microtubules that forms within a dividing eukaryotic cell. Bivalent Bivalent is the homologous chromosome pair, which consists of two chromosomes. Tetrad Bivalent consists of four sister chromatids together. These four sister chromatids are collectively known as a tetrad. Sister chromatids Sister chromatids are pairs of identical copies of DNA joined at the centromere. Homologous chromosomes Homologous chromosomes are chromosome pairs inherited from each parent. They are similar in gene position but may contain different alleles.  All cells reproduce by dividing into two. Each parental cell gives rise to two daughter cells each time they divide.  The newly formed daughter cells can themselves grow and divide, giving rise to a new cell population.  Such cycles of growth and division allow a single cell to form a structure consisting of millions of cells. Fig 10.3: Population growth due to cell division  Cell division is a very important process in all living organisms.  During the division of a cell, DNA replication and cell growth take place.  All these processes, i.e., cell division, DNA replication, and cell growth, hence, have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes.  A cell with a single set of chromosomes is known as haploid (n) while a cell with two sets of chromosomes is called diploid (2n). Fig 10.4: Modes of cell division
CELL CYCLE AND CELL DIVISION 119 SCAN CODE Cell Cycle and Cell Division  The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.  The duration of cell cycle can vary from organism to organism, and from cell type to cell type.  Yeast for example, can progress through the cell cycle in only about 90 minutes.  A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours. The time interval between two cell cycles is called Generation time. 1.1 Phases of Cell Cycle  In the 24-hour average duration of cell cycle of a human cell, cell division properly lasts for only about an hour.  The cell cycle is divided into two basic phases: 1. Interphase 2. M Phase (Mitosis phase) 1.1.1. Interphase  The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.  It is also known as intermitosis.  The interphase lasts more than 95% of the duration of the cell cycle.  The interphase is divided into three further phases: G1 phase S phase or Synthetic phase G2 phase Fig 10.5: Phases of cell cycle 1.1.1.1 G1 Phase  It is also known as the first growth phase or post mitotic gap phase.  It is the longest phase of interphase.  G1 phase corresponds to the interval between mitosis and initiation of DNA replication.  During the G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.