December 22, 2014

Junior Inter Botany - Cells - Important Questions for AP and Telangana Students

Q: Explain Prophase I of Meiosis.
A: It is more complex and divided into 5 phases based on chromosomal behaviour.

1) Leptotene: Chromosomes appear like long, thread like structures (say for example Maize microspore mother cells 2n = 20).

2) Zygotene: Homologous Chromosomes pair with each other under the influence of a protein called synaptinemal complex.
Pairing is called synapsis. Each pair thus formed is called bivalent (Maize: 10 bivalents).

3) Pachylenes: Each chromosome has 2 chromatids. Thus a bivalent is now called tetrad of chromatids (Maize 10 tetrads). Exchange of genetic material occurs between non sister chromatids of homologous chromosomes.
It is enzyme (Recombinase) mediated process. Chiasmata (X-Shape) are formed at the place of recombination nodule (Where non-sister chromatids meet).
Quality of the chromosomes changes. Recombinants are formed.

4) Diplotene: Synaptonemal complex dissolves. Chromosomes of the bivalents repel or separate except in the region of X-ma. It lasts long time in oocytes of vertebrates.

5) Diakinesis: Chromosomes become more distinct. Chiasmata show terminalisation. Nuclear membrane and necleolus dissolve. Spindle apparatus begins to form.

Q: Mention the key features of Meiosis.
A: Meiosis, a reductional cell division occurs in diploid cells (Concerned with reproduction) only.
Ex: Zygote in Algae and Fungi; Spore mother cells (of anther and ovule in angiosperms.) In higher plants the meiocyte (2n) divides by Meiosis to form 4 haploid of cells (n), which in turn produce gametes by mitosis. It is divided into 2 parts.
Meiosis I (Hetero type division): Meiocyte divides into 2 haploid daughter cells.
Quality of chromosomes changes in Pachentene of Prophase I due to crossing over. Recombinants are formed.
Quantity of chromosomes is reduced to half in Anaphase I where centromeres separate and disjunction of homologous chromosomes occurs.
Inter Kinesis: 2 daughter cells (n) enter into short Interphase where S-stage is absent.
Meiosis II (Homotype division): The 2 daughter cells (n) participate in it. Each one produces 2 daughter cells similar to their immediate mother cell.

Key features:
1. Number of chromosomes is reduced to half.
2. Recombinants are formed due to crossing over in Pachytene. It helps in evolution.
3. It is essential to keep the chromosome number constant for any number of generations.

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