- Tardigrade
- Question
- Biology
- Match list-I with list-II and select the correct answer using the codes given below the lists. List-II A. Pairing of homologous chromosomes. 1. Chiasmata B. Actual inter change of complex segments between two homologous chromosomes. 2. Synaptonemal complex C. Protein body formed between paired homologous chromosomes. 3. Synapsis D. The cross shaped configuration visible at diplotene between homologous chromosomes 4. Crossing over
Q.
Match list-I with list-II and select the correct answer using the codes given below the lists.
List-II A. Pairing of homologous chromosomes. 1. Chiasmata B. Actual inter change of complex segments between two homologous chromosomes. 2. Synaptonemal complex C. Protein body formed between paired homologous chromosomes. 3. Synapsis D. The cross shaped configuration visible at diplotene between homologous chromosomes 4. Crossing over
| A. | Pairing of homologous chromosomes. | 1. | Chiasmata |
| B. | Actual inter change of complex segments between two homologous chromosomes. | 2. | Synaptonemal complex |
| C. | Protein body formed between paired homologous chromosomes. | 3. | Synapsis |
| D. | The cross shaped configuration visible at diplotene between homologous chromosomes | 4. | Crossing over |
Solution:
Prophase– I of meiosis I is comparatively longer and more eventful than prophase of mitosis. It is divided into five sub-phases.
(a) Leptotene: Due to condensation and dehydration of chromatin material, chromosomes become gradually visible. Initially, they are present in monad form. These chromosomes are attracted towards centrioles and show the convergence of chromosomal ends towards centrioles, this arrangement is called the bouquet stage. During this phase, centrioles start moving towards opposite poles.
(b) Zygotene: Pairing of chromosomes takes place; this process is called synapsis. During this pairing, a synaptonemal complex is formed between two chromosomes and after pairing such chromosomes are called homologous chromosomes. Pair of synapsed chromosomes is also called bivalent or tetrad.
(c) Pachytene: Four chromatids of each bivalent are visible. Recombination nodules appear on chromatids of non-homologous chromosomes, at which crossing-over takes place in the presence of recombinase enzyme.
(d) Diplotene: Dissolution of synaptonemal complex and then separation of chromosomes of homologous pair occur. Due to this, some X-shaped structures are visible called chiasmata. In some vertebrates, during oogenesis, this diplotene stage remains continued for several months or years.
(e) Diakinesis: Characterised by terminalisation of chiasmata. Up to this phase, chromosomes become completely condensed. The nucleolus, nuclear membrane and cell organelles are disappeared, and formation of spindle fibre has been started.
(a) Leptotene: Due to condensation and dehydration of chromatin material, chromosomes become gradually visible. Initially, they are present in monad form. These chromosomes are attracted towards centrioles and show the convergence of chromosomal ends towards centrioles, this arrangement is called the bouquet stage. During this phase, centrioles start moving towards opposite poles.
(b) Zygotene: Pairing of chromosomes takes place; this process is called synapsis. During this pairing, a synaptonemal complex is formed between two chromosomes and after pairing such chromosomes are called homologous chromosomes. Pair of synapsed chromosomes is also called bivalent or tetrad.
(c) Pachytene: Four chromatids of each bivalent are visible. Recombination nodules appear on chromatids of non-homologous chromosomes, at which crossing-over takes place in the presence of recombinase enzyme.
(d) Diplotene: Dissolution of synaptonemal complex and then separation of chromosomes of homologous pair occur. Due to this, some X-shaped structures are visible called chiasmata. In some vertebrates, during oogenesis, this diplotene stage remains continued for several months or years.
(e) Diakinesis: Characterised by terminalisation of chiasmata. Up to this phase, chromosomes become completely condensed. The nucleolus, nuclear membrane and cell organelles are disappeared, and formation of spindle fibre has been started.