Plant x has 2n=4 chromosomes.

i)Sketch various stages of meosis of this plant.

Meiosis

• Specialized kind of cell division that reduces the chromosome number by half (thus called reductional division) resulting in production of haploid daughter cells (gametes)

• Haploid gametes fuse in sexual reproduction to give rise to diploid cells.

• Meiosis I starts after DNA has been replicated in S phase. Meiosis I is followed by meiosis II at the end of which four haploid cells are formed.

Meiosis I

• Meiosis I is divided into 4 phases.

• Prophase I − longest phase and is further sub-divided into 5 phases

  • Leptotene − Condensation makes chromosomes become distinct and compact.

  • Zygotene − Homologous chromosomes start pairing together by a process calledsynapsis to form a complex structure called synaptonemal complex. Two synaptonemal complexes further form a complex called bivalent or tetrad.

  • Pachytene − Longest phase of prophase I
    Recombination nodules appear in this stage at the sites where crossing over has to take place between non-sister chromatids of homologous chromosomes.

  • Diplotene − Synaptonemal complex dissolves and recombinants separate from each other except at crossover sites to form X-shaped structure called chiasmata.

  • Diakinesis − Chiasmata terminalises and chromosomes condense. Mitotic spindle assembles and nucleolus and nuclear envelope disappear.

• Metaphase I − Bivalent chromosomes align on the equatorial plate and spindle fibres appear and attach to the homologous chromosomes.

• Anaphase I − Homologous chromosomes separate; sister chromatids remain attached at their centromeres.

• Teleophase I − Nuclear membranes and nucleolus reappear. Cytokinesis follows.

• Interkinesis − Stage between meiosis I and meiosis II
   

Meiosis II − Resembles Mitosis

• Prophase II − Chromosomes become compact and nuclear membrane disappears.

• Metaphase II − Chromosomes align on equatorial plate and spindle fibres appear and attach to kinetochores of sister chromatids.

• Anaphase II − Centromere of each chromosome splits and sister chromatids move towards opposite poles of cells.

• Telophase II − Nuclear envelope reappears and cytokinesis follows, resulting in formation of a tetrad (4 haploid cells).