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www.eenadupratibha.net Junior Inter Botany t e Cell Cycle & Cell Division n . a Short Answer Questions h b i Q. Explain Prophase - I of Meiosis. A: It is more complex and divided into 5 phases based on chromosomal behaviour. t a r 1) Leptotene: Chromosomes appear like long, thread like structures (say for example Maize microspore mother cells 2n = 20). p u d 2) Zygotene: Homologous Chromosomes pair with each other under the influence of a protein called synaptinemal complex. Pairing is called a n synapsis. Each pair thus formed is called bivalent (Maize: 10 bivalents). e e . w w 3) Pachylenes: Each chromosome has 2 chromatids. Thus a bivalent is now called tetrad of chromatids (Maize 10 tetrads). Exchange of genetic materi- t e n al occurs between non sister chromatids of homologous chromosomes. It is w enzyme (Recombinase) mediated process. Chiasmata (X - Shape) are formed . a h at the place of recombination nodule (Where non-sister chromatids meet). Quality of the chromosomes changes. Recombinants are formed. b i t 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 a r p of vertebrates. 5) Diakinesis: Chromosomes become more distinct. Chiasmata show u d a terminalisation. Nuclear membrane and necleolus dissolve. Spindle apparatus begins to form. n e e . ww Q. Mention the key features of Meiosis. A: Meiosis, a reductional cell division occurs in diploid cells (Concerned with reproduction) only. w e.g.: 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. R-22-12-14 www.eenadupratibha.net www.eenadupratibha.net 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. t e n . a ★ Quantity of chromosomes is reduced to half in Anaphase - I where centromeres separate and disjunction of homologous chromosomes occurs. h b i Inter Kinesis: 2 daughter cells (n) enter into short Interphase where S - stage is absent. t a r Meiosis - II (Homotype division): The 2 daughter cells (n) participate in it. Each one produces 2 daughter cells similar to their immediate mother cell. p u d Key features: 1. Number of chromosomes is reduced to half. a n 2. Recombinants are formed due to crossing over in Pachytene. It helps in evolution. e e . w w 2n w Meiocyte Meiosis - I (heterotype division) Meiosis - II (= Mitosis) n . a h n n n t e n b i t a r p n n 4 haploid daughter cells u d a 3. It is essential to keep the chromosome number constant for any number of generations. n e e . ww Q. Which division is necessary to maintain constant chromosome number in all body cells of multicellular organisms and why? A: Mitotic cell division is necessary to maintain the chromosome number in all the cells of an organism. The mother cell in mitotic cell division produces 2 daughter cells which are equal to their mother cell both in quantity and quality of chromosomes. Every sexually reproducing diploid organism starts its life with a single cell called Zygote. It under goes numerous mitotic divisions and forms an organism which consists the same chromosome number in all its cells. w www.eenadupratibha.net www.eenadupratibha.net ★ The spore also produces a gametophyte by mitotic divisions due to which multicellular the chromosome number is maintained constant in all the cells. Q. Though redundantly described as a resting phase, interphase does not really involve rest. Comment. t e n . a A: Interphase represents the phase between two successive M phases. Interphase lasts more than 95% of duration of cycle. Thus it is the largest stage. It is a preparatory stage or resting phase. It is also called phase of non - apparent division. Though it is called resting phase, it is actually the most active phase as it involves the following activities. It has 3 phases. h b i t a r 1) G1 (G = Gap): Cytokinesis is followed by G1 p u d ★ The cell is active and shows normal metabolism. Organelle duplication occurs. The amount of DNA in the cell is 2C. a n 2) S (S = Synthesis): e e . w w ★ It is between G1 and G2. ★ It is the largest phase of Interphase. t e n ★ DNA synthesis (replication) occurs in the nucleus 2C DNA becomes 4C. But there is no increase in chromosome number. Parallel to this in animal cells centriole duplicales in cytoplasm. w . a h b i t 3) G2 (G = Gap) ★ Cytoplasm increases. The amount of DNA is 4C. Protein synthesis, ATP synthesis occurs. The cells are ready to participate in Prophase. a r p Writer: Dr. M. Chandraiah u d a n e e . ww w www.eenadupratibha.net