Meiosis – process
Steps in Meiosis
Meiosis is a two step process.
- Division I (Heterotypic division)
- Division II (Homotypic division)
1st Meiotic division
It starts after interphase (as of mitosis). Nucleus absorbs water and swells up. G2 phase is very short or nearly absent.
Includes following sub-stages.
- Leptotene or Leptonema
- Zygotene or Zygonema
- Pachytene or Pachynema
- Diplotene or Diplonema
Chromosomes are extremely thin, long uncoiled, longitudinally single and slender like threads.
Leptotene or Leptonema
Chromosomes begin to condense and assume long thread like shape. However sister chromatids are still so tightly bound that they are indistinguishable from one another. Chromosomes take specific orientation inside the nucleus; the telomeres are oriented towards the nuclear membrane. Hence, this stage is called “bouquet stage”.
Zygotene or Zygonema
Synapsis is the main event. The homologous chromosomes which came from the mother and father are attracted towards each other and pair. Synapsis begins at one or more points along the length of chromosome. Pairing of chromosomes is very exact and specific i.e. gene for gene. Completion of the replication of the remaining 0.3% DNA of each nucleus occurs. Secondary constriction develops. Synaptonemal complex is the one which stabilizes the pairing of homologous chromosomes.
Pachytene or Pachynema
Pair of chromosomes get twisted spirally around each other and can not be distinguished. Each homologous chromosome separates length wise to form two chromatids (just split, replication is already completed in interphase). Each chromatid remains linked at centromere. The two names used to refer these chromosomes are bivalent and tetrad. Bivalent as two chromosomes are paired and tetrad as there are four sister chromatids. The exchange of information between the non-sister chromatids of homologous chromosomes i.e. crossing over occurs at chaisma. Nucleolus is predominant. It is useful stage for study of chromosome morphology as they are longer than mitotic metaphase.
Diplotene or Diplonema
The synaptonemal complex degrades and desynapsis occurs. However, the homologous chromosomes of each bivalent remain tightly bound at chaismata, the regions where crossing-over occurred, so chaismata can be first time observed. Secondary constriction appears to be dissolved.
Bivalent chromosomes get more condensed and even distribution in the nucleus. Terminalization occurs (chaisma moves towards ends of chromosome and diminish). Chromatids still remain connected by terminal chaisma. This is the first point in meiosis where the four parts of the tetrads are actually visible. Nucleolus disappears. The nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.
Chromosomes get arranged at equator. Spindle appears.
Microtubules of the spindle get attached with centromere of each homologous chromosome. Bivalents (tetrads) become aligned in the center of the cell. The physical basis of the independent assortment of chromosomes is the random arrangement of each bivalent along the equatorial plate.
Homologous chromosomes are pulled apart due to kinetochore microtubules shortening. One kinetochore forms on each chromosome instead of on each chromatid as in mitosis; because of this whole chromosomes are pulled towards opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Actual reduction and disjunction occurs at this stage. Two chromatids of chromosome do not resemble each other genetically because of the crossing over.
Haploid set of chromosomes (but each chromosome consists of a pair of chromatids) arrive at poles. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. Chromosomes uncoil. Nucleolus reappears.
Cytokinesis leads to formation of two haploid cells.
2nd Meiotic division
Both cells pass through a short resting phase interphase II. During interphase there is no DNA replication. The cells further pass through following stages and lead to the formation of four haploid cells. Much of the process is similar to that of mitosis and meiosis I.
Nucleolus and nuclear membrane disappears. Chromatids become short and thick.
Chromosomes occupy plane of the equator of the spindle apparatus. The new equatorial metaphase plate is rotated by 90 degrees as compared to meiosis I, perpendicular to the previous plate. Microtubules get attached to centromere. Each centromere contains two kinetochores. Centromere divides into two.
Sister chromosomes (before called as sister chromatids) move towards opposite poles as microtubules attached to the kinetochores pull the sister chromatids apart.
Similar to telophase I. Chromosomes uncoil, nuclear envelope reforms, spindle apparatus disappears.
Cytoplasm separation and cell wall formation eventually produces four haploid daughter cells.