• 1.  Basics of Genetics- explanation i...  
   • 2.  DNA Packaging-explanation in mala...  
i. G1-Phase or Gap-1 phase
 It is the longest phase of the cell cycle and is followed by the M phase of the previous cell cycle.
 It is also termed as the “resting phase” as no DNA synthesis takes place during this phase.
 The cell grows in size due to active biosynthesis, several cell organelles increase in size and cell rapidly synthesizes different types of RNA and structural and functional proteins.
 This phase lasts for about 11 hours
ii. S-Phase or Synthetic phase
 S-phase involves the replication of nuclear DNA and the synthesis of histone proteins takes place which covers DNA.
 Thus, at the end of the S phase, each chromosome has two DNA molecules and a duplicate set of genes.
 This phase lasts for about 8 hours.
iii. G2-Phase or Gap-two phase or Second growth phase
 G2 phase is termed the second gap phase or resting phase of the interphase.
 During this phase, the synthesis of RNA and proteins required for the cell continues.
 By the end of this phase, the cell enters the division or M-phase of the cell cycle.
 Centrioles get replicated (in case of animal cell)
 Synthesis of spindle proteins takes place.
 This phase lasts for about 4 hours
iv. M phase Karyokinesis
 This phase lasts for about 1 hour
 Karyokinesis is the division of the nucleus.
 It consists of the following four phases.
i. Prophase
 Prophase is the first stage of mitosis which is characterized by the appearance of thin-thread like condensing chromosomes.
 The nucleolus disappears, and the nuclear membrane breaks down.
 Spindle fibers begin to form.
 Each chromosome starts to splits longitudinally into two sister chromatids. These sister chromatids are attached with each other at centromere.
 The chromosome in the prophase is composed of two coiled filaments, the chromatids, which are the result of the replication of DNA during the S phase.
 As prophase progresses, the chromatids become shorter and thicker, and two sister chromatids of each chromosome are held together by a special DNA-containing region, called the centromere.
 The nuclear membrane and nucleolus starts to disappear and by the end it will completely disappear.
ii. Metaphase
 Nuclear membrane and nucleolus completely disappears and spindle fibres appear
 Spindle fibres attached to the centromere of chromosome.
 The centromeres of the sister chromatids occupy the plane of the equator forming a metaphase plate, and the arms remain directed towards the poles - metaphasic plate.
 During metaphase, the chromosomes are shortest and thickest.
iii. Anaphase
 Sister chromatids separate
 Spindle fibers begin to shorten, pulling chromatids toward opposite sides of the cell.
 The centromere of each chromosome splits into two sister chromatids and forms two daughter chromosomes.
 In anaphase, there is a movement of chromatids towards the pole due to the shortening of the microtubules.
 During their poleward migration, the centromeres remain forward so that the chromosomes characteristically appear U, V or J- shaped.
 At the end of anaphase, each pole will get one set of daughter chromosomes.
 It is shortest phase and is also known as migratory phase.
iv. Telophase
 A nuclear membrane forms around the chromatin.
 Chromosomes begin to unwind.
 Spindle fibers begin to break down.
 Two identical cells form.
 During telophase, the events of prophase occur in reverse sequence.
 A nuclear envelope reassembles around each group of chromosomes to form two daughter nuclei.
 The daughter chromosomes reach respective poles and uncoil and become thin, long and visible.
 The spindle fibres start disappearing and finally disappear.
 The nuclear membrane and the nucleolus reappear.
 Two nuclei are formed at the end of telophase. Both the nuclei have the same number of chromosome as parent cell.
3. Cytokinesis
 Cytokinesis is the division of cytoplasm which is followed by mitosis, resulting in the formation of two separate daughter cells.
 Cytokinesis usually begins in anaphase and continues through telophase and into interphase.
 In animals, cytokinesis occurs through constriction and furrow formation.
 The constriction grows more in-depth from the outside to the inside, and ultimately a cell divides into two daughter cells.
SIGNIFICANCE OF MITOSIS:
 Mitosis produces 2 genetically identical cells, so mitosis maintains the genetic stability of organisms.
 DNA remains constant, so mitosis keeps the chromosomes number constant in a species.
 Mitosis helps in the development of multicellular organism.
 Mitosis helps in replacement of old, dead or damaged cells by new one.
 It helps in the recovery of wounds and injury of the body by formation of new cells..
 Mitosis causes maturation and multiplication of germ cells and makes them ready for meiosis.