Prophase II is the first stage of the second meiotic division (meiosis II), which occurs in the haploid cells formed after meiosis I. These cells contain only one set of chromosomes (n). In some organisms, a short resting phase called “interkinesis” takes place between the two meiotic divisions. Other organisms enter prophase II directly after meiosis I.
The chromosomes of these haploid cells are still in their duplicated form. Each chromosome is made up of two sister chromatids joined together at the centromere. No DNA replication takes place between meiosis I and meiosis II, even when the cells pass through interkinesis.
Events of Prophase II
The major cellular events occurring during prophase II are as follows-
- Condensation of Chromosomes- During prophase II, the chromosomes start to shorten and thicken again if they had decondensed during telophase I. They become distinct and visible. Each chromosome still consists of two sister chromatids which remain attached at the centromere.
- Disappearance of Nucleolus- The nucleolus disappears during this stage, if it was re-formed after the first meiotic division.
- Breakdown of Nuclear Envelope- The nuclear envelope, if formed during telophase I, starts breaking down into small membrane vesicles. In some organisms, the nuclear envelope does not re-form after meiosis I. These cells enter prophase II without the breakdown of a newly formed nuclear envelope.
- Movement of Microtubule-Organizing Centres- In this step, the microtubule-organizing centres (MTOCs) begin moving away from each other towards the opposite poles of the cell. These centres are referred to as centrosomes in animal cells.
- Formation of Spindle Fibres- As the microtubule-organizing centres move apart, microtubules begin to form a new spindle apparatus. The spindle continues its formation and becomes fully developed during prometaphase II.
Prophase II Diagram
Chromosome Number During Prophase II
During prophase II, the cells contain a haploid (n) number of chromosomes, but each chromosome is still present in its duplicated form. The chromosome consists of two sister chromatids attached together at the centromere. Chromosome number is determined by the number of centromeres, not by the number of chromatids. Each cell contains only one set of chromosomes after the separation of homologous chromosomes during meiosis I.
In humans, the diploid cell entering meiosis I contains 46 chromosomes (2n), which have already undergone DNA replication. These make up 92 chromatids. After the first meiotic division, each cell receives 23 chromosomes, consisting of 46 chromatids. No further DNA replication takes place between meiosis I and meiosis II.
The chromosome and chromatid numbers per cell during human meiosis are as follows-
| Characteristics | Beginning of Meiosis I | Prophase II |
|---|---|---|
| Chromosome number | 46 | 23 |
| Chromatid number | 92 | 46 |
| Chromosome sets | 2 (Diploid) | 1 (Haploid) |
| Chromatids per chromosome | 2 | 2 |
Importance of Prophase II
Some of the important functions of prophase II are as follows-
- Chromosome Condensation- During prophase II, the chromosomes become condensed and visible if they had decondensed after meiosis I. Each chromosome still consists of two sister chromatids joined at the centromere. These condensed chromosomes are subsequently arranged at the equatorial region during metaphase II.
- Formation of Spindle Apparatus- A new spindle apparatus starts forming during prophase II. The microtubule-organizing centres move towards the opposite poles of the cell. The spindle microtubules later attach to the kinetochores of sister chromatids and are involved in their separation during anaphase II.
- Breakdown of Nuclear Envelope- The nuclear envelope, if formed during telophase I, starts breaking down during prophase II. This allows the spindle microtubules to gain access to the chromosomes for their attachment during prometaphase II.
- Preparation of Haploid Cells for Division- Prophase II takes place in haploid cells without any additional DNA replication between meiosis I and meiosis II. The cells contain one set of duplicated chromosomes. During the subsequent stages of meiosis II, the sister chromatids are separated and distributed into the daughter nuclei, each receiving one set of chromosomes.
Consequences of Prophase II Dysfunction
The defects occurring during prophase II can affect the normal organization of chromosomes and their separation during the later stages of meiosis II. Some of the abnormalities and their possible consequences are described below-
- Abnormal Chromosome Condensation- In this condition, the condensation of chromosomes or resolution of sister chromatids is disturbed. The chromosomes may fail to arrange normally during metaphase II. In mouse oocytes, disturbances in condensin proteins have been associated with abnormal chromosome organization and segregation.
- Defective Spindle Formation- The bipolar spindle may fail to develop normally during prophase II, and the spindle microtubules may not form proper attachments with the chromosomes, affecting their alignment in the later stages of meiosis II.
- Premature Separation of Sister Chromatids- The sister chromatids are normally held together by centromeric cohesion until anaphase II. Loss of this cohesion before anaphase II can cause their premature separation. The arrangement of sister kinetochores may also get disturbed, increasing the chances of incorrect spindle attachment.
- Abnormal Chromosome Segregation- Improper kinetochore-microtubule attachment or a defective spindle apparatus may cause errors in chromosome segregation during anaphase II. Some chromosomes lag behind during the separation of sister chromatids. The failure of sister chromatids to separate is referred to as “nondisjunction”. Such abnormalities have been observed during meiosis II in mammalian oocytes.
- Formation of Aneuploid Gametes- When the chromosomes are not distributed properly during meiosis II, gametes with an extra or missing chromosome may be formed. This condition is known as “aneuploidy”. A gamete with such an abnormality, when involved in fertilization, may produce an embryo containing an abnormal chromosome number, which can lead to early pregnancy loss or certain chromosomal disorders. Some chromosome abnormalities detected during meiosis II may also originate from the first meiotic division rather than prophase II.
Difference Between Prophase I and Prophase II
Prophase I and prophase II are the first stages of meiosis I and meiosis II, respectively. Prophase I involves the pairing of homologous chromosomes and genetic recombination, whereas prophase II takes place in haploid cells without the pairing of homologous chromosomes.
The major differences between prophase I and prophase II are as follows-
| Characteristics | Prophase I | Prophase II |
|---|---|---|
| Position in meiosis | It is the first stage of meiosis I, which begins after DNA replication during interphase. | The first stage of meiosis II. It occurs after meiosis I, sometimes following a short interkinesis phase. |
| Ploidy | Diploid (2n). The cell contains two sets of chromosomes, each in its duplicated form. | Haploid (n), containing only one set of duplicated chromosomes. |
| Homologous chromosome pairing | The homologous chromosomes come together and undergo pairing, which is referred to as “synapsis”. | No pairing of homologous chromosomes takes place. |
| Synaptonemal complex | A protein structure called the synaptonemal complex is formed between the paired homologous chromosomes. It is later disassembled during prophase I. | Absent. No synaptonemal complex is formed during this stage. |
| Crossing over | Normal meiotic crossing over occurs between the non-sister chromatids of homologous chromosomes. It results in the exchange of genetic material. | Crossing over does not normally occur during this stage. |
| Chromosome organization | The duplicated homologous chromosomes form bivalents (tetrads). Each bivalent consists of four chromatids. | The chromosomes occur individually without forming bivalents. Each chromosome still contains two sister chromatids joined at the centromere. |
| Spindle formation | A spindle apparatus begins to form for the first meiotic division. | A new spindle apparatus starts forming in each haploid cell. |
| Preparation for chromosome segregation | The homologous chromosome pairs are prepared for their separation during anaphase I. Sister chromatids remain attached. | Spindle formation begins in preparation for the separation of sister chromatids during anaphase II. |
Prophase II at a Glance
The following are the main characteristics of prophase II-
| Characteristics | Description |
|---|---|
| Stage | First stage of meiosis II. |
| Preceding stage | Meiosis I, sometimes followed by a short interkinesis phase. |
| Type of cell | Haploid (n), containing only one set of chromosomes. |
| Chromosome number | 23 chromosomes per cell in humans. |
| Chromatid number | 46 chromatids per cell in humans. Each chromosome contains two sister chromatids joined at the centromere. |
| DNA replication | Absent. No DNA replication takes place between meiosis I and meiosis II. |
| Chromosome condensation | The chromosomes condense again if they had decondensed during telophase I. |
| Nuclear envelope | If formed during telophase I, it starts breaking down. |
| Microtubule-organizing centres (MTOCs) | They move towards the opposite poles of the cell. |
| Spindle apparatus | A new spindle starts forming during this stage. |
| Synapsis and crossing over | Absent. Unlike prophase I, homologous chromosomes do not pair up or undergo crossing over. |
| Next stage | Prometaphase II, followed by metaphase II. |
References
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th ed.). Garland Science. https://www.ncbi.nlm.nih.gov/books/NBK26840/
- Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2022). Anatomy and physiology 2e. OpenStax. https://openstax.org/books/anatomy-and-physiology-2e/pages/3-5-cell-growth-and-division
- Clark, M. A., Douglas, M., & Choi, J. (2018). Biology 2e. OpenStax. https://openstax.org/books/biology-2e/pages/11-1-the-process-of-meiosis
- Cooper, G. M. (2000). The cell: A molecular approach (2nd ed.). Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK9901/