Questions and Answers
In which stage of meiosis II do the chromosomes align at the equator?
What is the main characteristic of interkinesis?
What happens to the sister chromatids during Anaphase II?
What happens to the nuclear membrane during Prophase II?
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What happens to the chromosomes during Telophase I?
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What is the result of cytokinesis during Telophase I?
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Which stage of prophase I is characterised by the appearance of recombination nodules?
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What is the result of crossing over between non-sister chromatids of homologous chromosomes?
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What is the name of the enzyme involved in the process of crossing over?
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What is the stage of meiotic prophase I marked by the dissolution of the synaptonemal complex?
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What is the final stage of meiotic prophase I marked by?
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During which stage do the bivalent chromosomes align on the equatorial plate?
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What is the result of the terminalisation of chiasmata during diakinesis?
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What is the stage that follows diakinesis?
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What happens during the telophase II stage of meiosis?
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Which stage of meiosis II is characterized by the separation of sister chromatids?
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What is the main difference between prophase I and prophase II?
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What is the term for the process of association between homologous chromosomes during prophase I?
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What is formed during the zygotene stage of prophase I?
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What is the term for the complex formed by a pair of synapsed homologous chromosomes?
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During which stage of meiosis II do the chromosomes line up at the center of the cell?
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What is the phase that occurs between meiosis I and meiosis II?
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Study Notes
Meiosis I
- Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
- It involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes.
- Prophase I is further subdivided into five phases based on chromosomal behavior: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
- During Leptotene, chromosomes become gradually visible under the light microscope and compaction of chromosomes continues.
- In Zygotene, chromosomes start pairing together, forming homologous chromosomes, and a complex structure called synaptonemal complex is formed.
- A pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
Pachytene Stage
- During Pachytene, the four chromatids of each bivalent chromosome become distinct and clearly appear as tetrads.
- This stage is characterized by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes.
- Crossing over is an enzyme-mediated process, involving the enzyme recombinase, and leads to recombination of genetic material on the two chromosomes.
Diplotene Stage
- The beginning of Diplotene is recognized by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers.
- These X-shaped structures are called chiasmata.
- In oocytes of some vertebrates, Diplotene can last for months or years.
Diakinesis Stage
- Diakinesis is marked by terminalization of chiasmata.
- During this phase, the chromosomes are fully condensed, and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.
- By the end of Diakinesis, the nucleolus disappears, and the nuclear envelope also breaks down.
Metaphase I and Anaphase I
- In Metaphase I, the bivalent chromosomes align on the equatorial plate, and the microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes.
- In Anaphase I, the homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
Telophase I
- The nuclear membrane and nucleolus reappear, and cytokinesis follows, resulting in a dyad of cells.
Meiosis II
- Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated.
- Meiosis II resembles a normal mitosis.
Prophase II, Metaphase II, and Anaphase II
- In Prophase II, the nuclear membrane disappears by the end of the phase.
- In Metaphase II, the chromosomes align at the equator, and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
- In Anaphase II, the centromere of each chromosome splits, allowing them to move toward opposite poles of the cell by shortening of microtubules attached to kinetochores.
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Description
Learn about the stages of Meiosis I, including prophase I, and the process of recombination between non-sister chromatids of homologous chromosomes.
