Thursday, April 10, 2014

CHAPTER 11 : CELL CYCLE AND CELL DIVISION

CHAPTER   11 : CELL CYCLE AND CELL DIVISION

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Cell cycle

It is a series of events that takes place in a cell, leading to the formation of two daughter cells from a single mother cell.

Cell cycle is divided into two basic phases: Interphase and M phase Phases of cell cycle.

·         Interphase

·         M  phase (mitosis phase) karyokinesis and cytokinesis

Interphase

o   G1 phase

o   S phase

o   G2 phase

o   Go phase-quiescent stage

Mitotic phase

·         Karyokinesis (nuclear division): – Prophase, Metaphase, Anaphase and Telophase.

·         Cytokinesis (division of cytoplasm)

Interphase

Interphase involves a series of changes that prepares the cell for division. It involves the period of cell growth and cell division in an orderly manner.

It is divided into three phases:

  • G1 phase – It involves growth of cell and preparation of DNA for replication.
  • S phase – It involves DNA synthesis. The amount of DNA doubles, but the chromosome number remains the same.
  • G2 phase – It involves protein synthesis and further growth of cell, which prepares it for division.
  • G0 phase or Quiescent phase – It is the stage when metabolically active cell remains quiescent for long period of time.
 

 

I Mitosis

  • It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. Hence, it is also called equational division.
  • The process of mitosis keeps the chromosome number equal in daughter as well as parental cell.
  • Mitosis usually takes place in somatic cells.
Mitosis involves four stages:

Prophase

  • It involves initiation and condensation of chromosomes.
  • Nucleolus and nuclear membrane disappears.
Metaphase

  • Chromosomal material condenses to form compact chromosomes that get aligned in the middle of nucleus at equatorial plate.
Anaphase

  • Centromere splits and chromosomes move apart towards two opposite poles due to shortening of spindle fibres.
Telophase

  • Chromosomes finally reach their respective poles.
  • Nuclear envelope assembles around each chromosome clusters.
  • Nucleolus and other organelles reform.
 

Karyokinesis and Cytokinesis

  • Karyokinesis is the division of nucleus during mitosis or meiosis which is followed by cytokinesis.
  • Cytokinesis involves the division of cytoplasm of a cell.
  • Cytokinesis is achieved in animal cell by cleavage, which deepens and divides the cell into two.
  • It is achieved in plant cell by cell plate formation.
  • When karyokinesis is not followed by cytokinesis, a multinucleated condition arises. This is called Syncytium.
 

Significance of mitosis

  • Results in formation of diploid genetically identical daughter cells
  • Growth of the body takes place by mitosis.
  • Cell repair and replacement of worn out tissues
  • Maintenance of nucleo-cytoplasmic ratio
  • Vegetative reproduction in plants takes place by mitosis.
 

II Meiosis

  • It is the process which involves the reduction in the amount of genetic material.
  • It mainly occurs in germ cells.
  • At the end of meiosis II, four haploid cells are formed.
  • It is comprised of two successive nuclear and cell division with a single cycle of DNA replication.
  • The phases of meiosis are as shown below-
Meiosis I

1. Prophase I – It comprises of 5 stages:

i. Leptotene

  • Chromosomes start condensing.
ii. Zygotene

  • Pairing of chromosomes called synapsis occurs.
  • A pair of synapsed homologous chromosomes is called bivalent or tetrad.
iii. Pachytene

  • Exchange of genetic material (crossing over) between non-sister chromatids occurs.
  • Chiasmata formation
iv. Diplotene

  • Bivalents formed during pachytene separate from each other (except at chiasmata) due to dissolution of synaptonemal complex.
v. Diakinesis

  • Terminalisation of chiasmata can be observed.
  • By the end of this stage, the nucleolus disappears and the nuclear envelope breaks.
2. Metaphase I

  • Bivalents (tetrad) get aligned along metaphase plate through spindle fibres.
3. Anaphase I

  • Homologous chromosomes separate while chromatids remain attached at their centromere.
4. Telophase 

  • Nucleolus and nuclear membrane reappear around chromosome clusters at each pole.
  • Inter-kinesis – It is the stage between two meiotic divisions.
Meiosis II

1. Prophase II

  • Chromosomes become compact.
  • Nuclear membrane disappears.
2. Metaphase II

  • Chromosomes align at the equator.
  • Kinetochores of sister chromatids attach to spindle fibres at each pole.
3. Anaphase II

  • Chromatids separate by splitting of centromere.
  • As a result, chromatids move towards their respective poles in the cell.
4. Telophase II

  • Nuclear envelope and nucleolus reform around the chromosome clusters.
Cytokinesis:

  • After meiosis II, the process of cytokinesis results in the formation of four haploid cells.
Significance of meiosis:

  • It results in reduction of chromosome number by half in gametes, which again doubles during fertilization. Therefore, it helps to conserve the chromosome number of species from generation to generation.
  • Crossing-over, occurring in pachytene stage of meiosis I, is a source of genetic variation in sexually reproducing organisms.
  • The variation thus formed helps in evolution.

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