Meiosis I: Introduction

Sexual reproduction is a stepwise evolution of eukaryotes; varying from species to species. The basic mechanism of zygote fusion and meiosis regulators are conserved in all eukaryotes; the difference is seen in the method the gametes meet and evolved along the groups.

Pollination in flowering plants is the means of gametic fusion and produces seeds; which germinates on suitable condition to produce plants.

Plants are exposed to varied errors in cell division and few species under adverse conditions; produces spores which on exposure to suitable conditions proliferates.

Eggs and pollens of plants naturally depends on external mechanical factors for the initial fusion of gametes.

The development and maturation of the plants takes place in a sequential manner on appropriate time and nutrient availabilities.

The basic principles of meiosis include the law of independent assortment with which the plant acquires genetic diversity and variability by crossing over forming a recombinant.

Crossing Over and recombination followed by continuous double cellular division give rise to haploid daughter cells; changes to gametes of male and female in respective parts of a flower of same plant or a different plant.

The reductive division from which the name meiosis was given; in Greek meiosis is to diminish, recombination by crossing over where homologous chromosomes come together to interchange genetic material, and independent assortment of genes to the progeny are the main features a meiotic cell provides to make the process more unique and provide genetic diversity.

Meiosis History

Meiosis was first described by Oscar Hertwig, German Biologist in 1876 further research took place in the field where Theodor Boveri, 1888 reported it in Roundworm.

The reason behind such division was later pointed by August Weismann as to maintain equal amount of genetic material transfer from parent to progeny because if the reproductive division follows mitosis a diploid cell becomes tetraploid and increase exponentially over generation and have resulted in the formation of species with infinite genetic material.

Thomas Hunt Morgan in Drosophila found the recombination of the chromosomes (genetic material) to provide evolutionary variability to organism.

Mitosis vs Meiosis

Both are type of cell division taking place in every eukaryote. But the main difference is, Mitosis produces Diploid Somatic cells identical to parent and Meiosis give rise to cells which forms a new progeny which are genetically different from the parent by producing haploid gametes.

In general; all cells are mitotic and undergoes mitotic cell division; even the gametes before meiosis are divided mitotically from their progenitor and enter meiosis.

“The Molecular Switch” present in every cell when induced on appropriate nutrients makes the cell competent to enter meiotic cycle; is the mechanism to “turn on” meiosis at the beginning of Phase G2.

From G2 phase the meiosis proceeds by 2 continuous cell division.

Further; the phases of meiotic cell division differ from the mitotic phase to support complex changes during meiosis.

Meiosis is divided into MEIOSIS I and MEIOSIS II; each meiotic phase has sequence of Prophase, Metaphase, Anaphase and Telophase.

The Prophase of Meiosis I is more significant where the primary feature of meiosis takes place: Pre – leptonema, Leptonema, Zygonema, Pachynema, Diplonema, Diakinesis and are absent in Prophase of Meiosis II.

Meiosis I

Somatic cells and germ line cells are differentiated in this method; specialized and prominent switching of process from mitotic to meiotic phase in testes and ovaries; takes place in Prophase I.

Meiosis generally skips the G2 Phase as soon as the Switch is “ON” to Prophase I.

Significant modification in the genetic material is well accounted in different phases of PROPHASE I; other cellular changes are similar in that of mitosis wherein the nuclear membrane starts disintegrating in the prophase and disintegration of other membrane bound organelles.

The specialized phases of the prophases and the events at each stage are given below:

Prophase I: Pre-Leptonema

The chromosomes are extremely thin to be identified except for the differentiated sex chromosomes which has Heter pyknotic bodies.

Heter pyknotic bodies are regions of either tightly or loosely bound chromatin fibers which are stained more or very less from the rest of the chromosomes.

Prophase I: Leptonema

o Leptonema in Greek means thin thread like structures.

o The chromosomes in the phase are characterized by thin appearance even after the replication.

o Out of all phases of meiosis the PROPHASE I is longer in all eukaryotes with time variation in different species. The phase also constitutes other changes:

o Nucleus enlarges in size occupying most of the cytoplasm signifying the increased genetic content in cells.

o Chromatin starts to form a loop of 5 – 22 µm DNA.

o DNA appear single rather than double as in mitosis because of this the phase has its name LEPTONEMA with thin chromosomal appearance.

o The chromosome starts condensing and has bead like thickened structures called chromomeres present irregularly in a chromosome and the number of chromomeres are not constant.

o Prophase chromosomes forms a telomere bouquet which orients the chromosomes theoretically to form homologous pairs.

o These telomere bouquets attach the chromosome to the inner nuclear membrane making chromosomes easier to pair.

o Synaptonemal Complexes are initiated to form in this phase as a preparatory part of next phase.

Meiosis I Prophase Diagram

Prophase I: Zygonema

o Zygonema in Greek means Adjoining. The threaded chromosomes pairs with its homologous chromosome.

o Maternal chromosome and paternal chromosome of same functions are segregated and paired to each other for recombination in next phases.

o Chromatin loops concentrate further in ZYGONEMA.

o To make sure the homologous pairing corresponds with the similar DNA sequences in both homologs.

o Recombination Complex breaks the double strand at specific sites and join the similar part of the chromosome.

o This process takes place before the synapsis of the 2 chromosomes called presynaptic complex.

o Chromosomal pairing by the synaptonemal complex.

o The homologous chromosomes pairing causes the formation of synapsis.

Synaptonemal Complex

o Synaptonemal Complex is a highly complexed structure involving proteins similar to histones to form rail road like or zipper like filaments across both chromosomes along their peripheral axis.

o Synaptonemal Complex has two kinds of filaments two Lateral and 2 transverse or Medial filaments.

o Synaptonemal Complex prevents the complete fusion of homologous chromosome with 100nm gap.

o The initiation of the complex is random and starts at any point of the pair; guided by the telomere bouquet attachment to the inner nuclear membrane.

o At the point of attachment of telomere to the nuclear membrane the Synaptonemal Complex deposits to form a thick fixation plate.

o These fixation plate attracts the re – formation of nuclear pore annuli at the region of attachment.

o For a homologous chromosome to form the process must be made possible by multitudinal involvement of various parts and process of the nuclear complex and Synaptonemal Complex forms a skeleton to support the complex formation and recombination of the chromosome providing stability.

Prophase I: Pachynema

o Pachynema refers to thick chromosome in Greek. The stage is significant because of the crossing over and recombination of the maternal and paternal chromosome results in genetically different species.

o SC is complete

o Chromatin loop is well concentrated making the genetic material to have brush like appearance.

o The number of chromosomes reduces to half forming bivalents or tetrads.

o The region of connection between the bivalents are termed as chiasma where the homologs forms X – shaped connection to hold each paternal and maternal chromosome.

o Chromatids of homologue becomes 8 with 8 kinetochores on each chromatid.

o SC ensures the homologous pairing of all chromosomes in the nucleus before proceeding to next process.

o The SC remains intact throughout the pachytene.

o Crossing over between homologous pairs takes place.

o Crossing over is regulated by components and are determined to provide structural support and genetic variability and diversity among the species.

o Crossing over regulations takes before crossing over ensures the chromosome to attain more than one recombination and restricts the closely related genes from crossing over.

o A separate rule prevails to conserve the integrity of the chromosome.

o The chromosome is divided into “Hotspots” and “Cold spots” based on the recombination sites.

o Telomeric and heterochromatin centromere regions are prevented from crossing over.

o Other regions are exposed to the crossing over for the recombination of the genetic materials among the maternal and paternal genes.

o Recombination also takes place in Pachytene stages indicated by the formation of Recombination Nodules which has intact SC to ensure the cuts which are produced to recombine does not eliminate the region from the chromosome which leads to errors in cell division.

o The recombination nodule forms a bar like structure across the chromatids to reach its corresponding pair and exchange its DNA material.

Prophase I: Diplonema

o The crossed over chromosomes are separated in the phase but are held by chiasmata.

o SC is removed at the stage after the crossing over.

o Chiasmata is intact and separation of paternal maternal chromosomes at most of the sites takes place.

o The removal of cross overs and recombination sites leaving a single site makes the four tetrads are visible.

o Diplonema is more significant because of long duration it takes for all chromosomes to separate.

o In certain species; the chromosomes have a specialized appearance of lamp brush.

Prophase I: Diakinesis

o Diakinesis is the terminal process of prophase I where the chromosome’s chiasmata are completely lost except at the end region, this process is the TERMINALIZATION.

o Diakinesis in Greek means breaking across; where the chromosomes are cut across each other and marks the end of Prophase I

Pro-Metaphase

o Disintegration of Nuclear membrane

o Complete condensation of chromosomes

o The kinetochores of homologous chromosomes attach to microtubules

o Sister kinetochores maintains the integrity as a functional unit.

Meiosis I Diagram

Metaphase

o Similar to mitosis Metaphase I has similar functions

o The metaphase arranges the bivalents at the equator by the microtubules

o The main difference is the chiasma between the homologous chromosome remains intact

Prophase I: Anaphase I

o The Cohesins in the chromosomal arms are removed to break free the homologous pairing and tension created because of the separation makes the microtubule to contract towards the organizing center.

o The chromosomes are segregated to the poles.

Prophase I: Telophase I

Telophase is simple and marked by formation of the nuclear membrane by furrow in animals and phragmoplasts in plants and the cell separation – Cytokinesis.