USA: +1-585-535-1023

UK: +44-208-133-5697

AUS: +61-280-07-5697

Meiosis

MEIOSIS

This type of cell division is found only in sexually reproducing organisms, where the male and female gametes (or positive and negative gametes in primitive organisms) fuse to form the zygote. This union of gametes in each generation would lead to successive doubling of chromosome number, causing unlimited number of chromosomes in the nucleus, which is undesirable. In such sexually reproducing organisms from generation to generation, the constant normal chromosome number is maintained by the specific type of division, meiosis, in which the chromosome number in daughter cells (gametic cells) is ‘reduced to half. Thus, the meiosis involves two divisions of the parent cell, whereas, the DNA of the chromosomes replicates only once, during S-phase ofInterphase. This produces 4 daughter cells having haploid number of chromosomes (n) which differ not only among themselves with respect to the distribution of paternal and maternal chromosomes and exchange of genetic material (crossing over or genetic recombination), but also from the parent cell that has diploid number of chromosomes (2n). The crossing over causes the variation among various individuals within a species, that is necessary the evolution process.

In advanced organisms (e.g. higher plants, mammals, etc.), meiosis occurs during the formation -haploid gametes(e.g. eggs in females and sperms in males) that unite to form the diploid zygote having similar chromosome number as in somatic cells of normal male and female individuals. The zygote, thereafter, divides by mitosis to form the adult individual

Whereas, in primitive organisms (e.g.: green alga-Chlamydomonas, fungus-Neurospora,Bryophytes, Pteridophytes, etc.), where the normal body is haploid (n), at the time of sexual reproduction they produce male and female gametes (n) that unite to form the diploid zygote (2n). To attain the normal haploid chromosome number, the zygote undergoes meiosis

Thus, meiosis involves two divisions, where the first division causes reduction in chromosome number in the two daughter nuclei (without dividing the sister chromatids of each chromosome) and the second division is similar to the mitosis that leads to the splitting and separating of the sister chromatids of individual chromosomes to the separate cells.

Like mitosis, meiosis is also a continuous process and has been divided into various stages. The meiotic cell cycle has been divided into following stages:

1. Interphase: It is similar to that of mitosis, divided into G1, Sand G2 phases, where the DNA replication takes place during S-phase.

2. Meiosis: The cell division process, Meiosis involves two divisions:

I Division

The complex division is the reduction division that is again divided into four phases:

(i)                 Prophase I: In animals, the two centrioles separate and get arranged at the two opposite poles. The complex Prophase I is again divided into five stages:

(a) Lepotene: The individual chromosomes as long, thin, thread-like structures due tocontinually increasing level of coiling appear as loosely entangled mass of wool.

 (b) Zygotene: The homologous chromosomes (similar chromosomes one maternal and the other paternal) undergo pairing (synapsis) in a zipper-like fashion to form bivalents having four chromatids (tetrads). During S-Phase of Interphase, total DNA replication does not occur and 0.3% DNA replicates during Zygotene that controls the synapsis

 (c) Pachytene: The chromosomes become thicker and shorter and exchange of segments(crossing over or recombination) of non-sister chromatids in some of the bivalents may take place by breakage and reunion, resulting in the formation of cross-like structures, called chiasmata (singular is chiasma) (Fig. 5.11 see Appendix). Number of chiasmata in a bivalent is directly proportional to the length of paired homologous chromosomes. Chiasmata are not the cause of crossing over butare formed due to crossing over.

 (d) Diplotene: The chromosomes become more shortened and thickened and the homologous chromosomes start separating from one another, where the separation starts at centromeres and travels towards the ends. This process is called terminalization.

 (e) Diakinesis: It is similar to Diplotene, except for more contracted bivalents due to increasing level of coiling. Moreover, the nuclear membrane and the nucleolus disappear.

(ii) Metaphase I: The spindle fibres start appearing, and they become attached to the centromeres of the chromosomes of bivalents. The bivalents now get arranged at the equatorial plate in the cell. At this stage, the degree of coiling is highest and the chromosomes become thickest and shortest, therefore, chromosome counting is generally performed at this stage. [Colchicine obtained from Colchicum autumnale, inhibits the spindle fibre formation, causing polyploidy.]

(iii) Anaphase I: The maternal and paternal chromosomes of each bivalent start moving towards the opposite poles, where each chromosome is made up of two chromatids. (Unlike mitoticMetaphase, the two sister chromatids of each chromosome remain attached to the centromere)

(iv)Telophase I: Two complete sets of chromosomes at the two opposite poles of the cell get surrounded by the nuclear membrane to form nuclei, each containing a nucleolus. Each nucleus, though; contains half the chromosome. number, but each chromosome consists of two DNA molecules (chromatids). The chromosomes become thin and thread-like.

(v) Cytokinesis I: It mayor may not be present.

After the first division, the resting phase Interphase mayor may not be present, but if present, does not involve any DNA replication.

II Division

This division is simply the mitotic division where the two cells of Meiosis I divide to produce four cells:

(i) Prophase II: The chromosomes become distinct as long, thin, thread-like structures and the nuclear membrane and the nucleolus disappear at the end.

(ii) Metaphase II: The spindle fibres start appearing and the chromosomes, each made up of two chromatids, get arranged at the equatorial plate.

(iii) Anaphase II: Each chromosome is splits at the centromere and the two sister chromatids start moving towards the opposite poles.

(iv) Telophase II: Ultimately, the four complete sets of chromosomes separately get surrounded by a nuclear membrane to form four nuclei, each containing a nucleolus. The chromosomes become thin and thread-like.

(v) Cytokinesis II: The cytoplasm of the parent cell divides into four daughter cells, each containing half the number of chromosomes (e.g. 11 = 2 chromosomes) and DNA content (e.g. 2C) in comparison to the parental cell (e.g. with 2n = 4 chromosomes and DNA content = 4C, which at S-Phase of Interphase I becomes 8C due to DNA replication) (Fig. 5.12 see Appendix).

At the time of Metaphase I, the arrangement of maternal and paternal chromosomes in a pair of homologous chromosomes is independent of the other pairs. Thus, 2ncombinations become possible to give 2n types of daughter cells, where n is the number of pairs of homologous chromo- somes, e.g., a cell with one pair of homologous chromosomes would give 2n= 2 x 1 = 2 types of products, with 2 pairs giving 2n= 22 = 2 x 2 = 4 combinations, with 3 pairs giving 2n= 23 = 2 x 2 x 2 = 8 combinations, etc.,

Synaptonemal Complex

 It is a three-layered structure formed between the paired homologous chromosomes of a bivalent at the time of Prophase I (Zygotene and Pachytene) of meiosis. It facilitates the proper alignment of the paired homologous chromosomes that is necessary for the exchange of DNA segments (recombination). At the time of Zygotene, 0.3% replication of DNA is somehow involved in the formation of synaptonemal complex and, thus, if the cell is treated with the inhibitor of DNA synthesis at this time, it arrests the proper formation and functioning of the synaptonemal complex

This complex was discovered by Moses (1956) and consists of three electron dense layers (two lateral elements and one central element) alternating with two electron lighter layers. The two lateral elements are made up of mainly DNA with some RNA and proteins, whereas, the central element consists of mainly RNA and proteins with a little DNA

King (1970) gave a hypothesis, called synaptomere-zygosome hypothesis, where the two lateral elements are made up of about 10 nm (l00 A) fibres, called synaptomeres, whereas, the central element is ladder-like consisting of zygosomes

Each synaptomere in the two lateral elements has three segments A, B and C, where A and C segments are involved in pairing with the similar types of segments of the adjacent synaptomeres, e.g. A segment of one synaptomere pairs with the A segment of other synaptomere, similarly C of one synaptomere pairs with C segment of other synaptomere. The B segments face towards the central element and are attached to the zygosomes.

The zygosomes in the central element are rod-shaped, each having a head that is attached to the B-segment of synaptomere and a tail with charged sites that bind laterally in a ladder-like fashion with the tail of opposite zygosome (Fig. 5.14).

 Comparison of Mitosis and Meiosis 

Differences between mitosis and meiosis

QUESTIONS

  1.  Describe the details of meiosis with particular emphasis on Prophase I. What is the significance of meiosis ?
  2.  What do you understand by reductional (disjunctional) and equational divisions. Demonstrate that first and second meiotic divisions are partly reductional and partly equational.
  3.  Explain why do you expect differences among cells arising from meiosis and not from mitosis.
  4.  Write a short note on synaptonemal complex.
  5.  Describe mitotic cell division and its significance.
  6.  Compare and contrast mitosis with meiosis.