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NEET Biology Cell Cycle and Cell Division

Cell Cycle and Cell Division PDF Notes, Important Questions and Synopsis

SYNOPSIS

  • The sequence of events by which a cell duplicates its genome and synthesises all other cell contents and eventually divides into two daughter cells is called the cell cycle.
  • Interphase involves a series of changes which take place in a newly formed cell and its nucleus before it gets ready for division again. It is also called intermitosis. It is further divided into
    • First gap or G1 phase: Interval phase between mitosis and the initiation of DNA replication. The cell grows to its maximum size to prepare for DNA replication.
    • Synthetic or S phase: Synthesis or replication of DNA occurs on the template of the existing DNA strand. The amount of DNA per cell doubles, but there is no change in the chromosome number of the cell.
    • Second gap or G2 phase: Synthesis of RNA and proteins continues. Spindle protein synthesis and aster formation take place.
  • Phases of Mitosis


Phase

Description

Karyokinesis (Division of the nucleus)

Prophase

  • Chromatin fibres become shorter and thicker, nuclear envelope disintegrates.

  • Centriole begins to move towards the opposite poles at the end of prophase.

  • Fine radiating microtubules appear around each pair of centrioles to form astral rays.

Metaphase

 

  • The chromosomes appear to be made of two sister chromatids held together by the centromere.

  • Small disc-shaped structures called kinetochores serve as sites of attachment of spindle fibres to the chromosomes.

  • All the chromosomes come to lie at the equator and get aligned along the metaphase plate.

Anaphase

 

  • The centromere of each chromosome splits into two, resulting in the production of daughter chromatids.

  • Chromatids migrate towards the respective poles.

  • As the chromatids move apart, the polar fibres of the spindle elongate and the cell becomes larger.

Telophase

 

  • Daughter chromosomes undergo decondensation and uncoiling at each pole.

  • Chromatin gets surrounded by discontinuous segments of the nuclear membrane.

  • Spindle fibres and astral rays gradually disintegrate and disappear.

Cytokinesis (Division of the cytoplasm)

In animals – cell furrow method

In plants – cell plate formation

  • Phases of Meiosis
    • Meiosis-I or Heterotypic Division
 

Phase

Description

Prophase I

  • It is divided into five sub-stages—leptotene, zygotene, pachytene, diplotene and diakinesis.
  •  Leptotene/Leptonema
  • Duplicated centrioles start moving apart and aster formation takes place.
  • Chromatin fibres appear in the form of long, thin condensed filamentous chromosomes.
  •  Zygotene/Zygonema

 

  • Homologous chromosomes come to lie in pairs.

  • Pairing of homologous chromosomes is called synapsis. The paired chromosomes are called bivalents.

  •  Pachytene/Pachynema
  • Bivalent chromosomes become more thickened, shortened and condensed.

  • Two visible chromatids of a chromosome are referred to as a dyad.

  • A group of four homologous chromatids is called a tetrad.

  • At the tetrad stage, thecrossing over occurs between the non-sister chromatids of the homologous chromosomes.

  •  Diplotene/Diplonema
  • The recombinant homologous chromosomes begin to separate. This phenomenon is called desynapsis.

  • The chromosomes remain attached to one or more points where crossing over has occurred. These points of attachment are called chiasmata.

  •  Diakinesis
  • Nucleoli and the nuclear membrane disintegrate and disappear.

  • Astral rays and asters become fully developed.

  •  Metaphase-I
  • The bivalents align themselves on the equator of the bipolar spindle.
  • Anaphase-I

 

  • One chromosome from each homologous pair moves to the opposite poles with recombined characters of both paternal and maternal chromosomes.
  • Telophase-I

 

  • The haploid chromosomes form a chromatin network and the nuclear envelope develops.

  • The astral rays and spindle fibres disintegrate and disappear, and two daughter nuclei are formed.

Cytokinesis (division of the cytoplasm)

In animals – cell furrow method

In plants – cell plate formation

By the end of cytokinesis, each diploid parental cell divides into two daughter cells each with a haploid number of chromosomes but double the content of DNA.

  • Meiosis-II or Homotypic Division

  • In this division, the two chromatids of each chromosome separate from each other and go to separate daughter cells.
  • The number of chromosomes remains the same as produced by meiosis-I.
  • Meiosis-II consists of four stages—prophase-II, metaphase-II, anaphase-II and telophase-II.
  • Cytokinesis follows karyokinesis of meiosis-II.
  • At the end of cytokinesis, two daughter cells are formed, each with half the number of chromosomes and half the amount of nuclear DNA of the parent cell.