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.

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