PART - 1 - Unit III : CH - 10 Cell Cycle and Cell Division — BIOLOGY STD 11 Science — Question
Rajasthan BoardEnglish MediumSTD 11 ScienceBIOLOGYPART - 1 - Unit III : CH - 10 Cell Cycle and Cell Division4 Marks
Question
Describe the following: (i) Synapsis, (ii) Bivalent (iii) Chiasmata.
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Answer
(i) Synapsis: Homologous chromosomes are attracted towards each other in the zygotene stage. Pairing of homologous chromosomes is called synapsis. One chromosome of each pair is maternal type and the other is paternal type. There are three types of synapsis: (a) Procentric: If the pairing starts from the centromere and goes towards both the ends then it is called procentric synapsis. (b) Proterminal: If the pairing starts from the ends of the chromosomes and moves towards the centromere, then it is called proterminal synapsis. (c) Random: If pairing starts from any part of the chromosomes, then it is called random synapsis. (ii) Bivalent: Chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. (iii) Chiasmata: At the beginning of diplotene, the fourth phase of meiosis-I, the synaptonemal complex disintegrates and the homologous chromosomes of the pair start separating from each other except the exchange point. The X shaped structure at the exchange point is called Chiasmata. Chiasmata are formed between both homologous chromosomes. Any chromatid of one homolog can form any number of chiasmata with any chromatid of the other homolog.
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