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Question
Arrange the following events of meiosis in correct sequence:
I. Terminalisation
II. Crossing over
III. Synapsis
IV. Disappearance of nucleolus
Arrange the following events of meiosis in correct sequence:
I. Terminalisation
II. Crossing over
III. Synapsis
IV. Disappearance of nucleolus
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Solution
The correct option is B III, II, I, IV
III. Synapsis, II. Crossing over, I. Termination of chiasmata, IV. Disappearance of the nucleolus
There are 5 stages in Prophase I:
The first stage is leptotene, during which condensation of the DNA takes place. Close examination of these threads, which continue to shorten and thicken, shows that they are already doubled.
This is followed by the Zygotene stage, in which these chromosomes pair up (synapsis) with their complementary chromosomes. Such chromosomes are called homologous pair and the synaptonemal complex is formed between them. Each homologous pair is now called a tetrad (as it has four chromatids in all).
The Pachytene stage follows Zygotene. Homologous chromosomes are in very close contact, and the physical association between the DNA molecules of the homologous chromosomes is very strong at certain points. It is at these points that exchange of chromosomal segments by crossing over, takes place. The points at which crossover happens are called chiasmata (singular: chiasma).
This is followed by the Diplotene stage, during which the dissolving and break down of the synaptonemal complex, and the separation of the individual components of the two sets of chromosomes occur.
Diakinesis - More and more packing of DNA with histones and proteins makes the chromosomes even shorter and more densely packed. This is the recondensation or final stage of Prophase I. All of the chromosomes are now at their maximum density and degree of packing, and the chiasmata move along the length of each structure until they reach the ends of the chromosomes, by a process called terminalisation.
It is during this last stage of Prophase I that the nucleoli disappear and the nuclear membrane breaks down so that the cell can enter the next stages of meiosis I.
III. Synapsis, II. Crossing over, I. Termination of chiasmata, IV. Disappearance of the nucleolus
There are 5 stages in Prophase I:
The first stage is leptotene, during which condensation of the DNA takes place. Close examination of these threads, which continue to shorten and thicken, shows that they are already doubled.
This is followed by the Zygotene stage, in which these chromosomes pair up (synapsis) with their complementary chromosomes. Such chromosomes are called homologous pair and the synaptonemal complex is formed between them. Each homologous pair is now called a tetrad (as it has four chromatids in all).
The Pachytene stage follows Zygotene. Homologous chromosomes are in very close contact, and the physical association between the DNA molecules of the homologous chromosomes is very strong at certain points. It is at these points that exchange of chromosomal segments by crossing over, takes place. The points at which crossover happens are called chiasmata (singular: chiasma).
This is followed by the Diplotene stage, during which the dissolving and break down of the synaptonemal complex, and the separation of the individual components of the two sets of chromosomes occur.
Diakinesis - More and more packing of DNA with histones and proteins makes the chromosomes even shorter and more densely packed. This is the recondensation or final stage of Prophase I. All of the chromosomes are now at their maximum density and degree of packing, and the chiasmata move along the length of each structure until they reach the ends of the chromosomes, by a process called terminalisation.
It is during this last stage of Prophase I that the nucleoli disappear and the nuclear membrane breaks down so that the cell can enter the next stages of meiosis I.
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