Wednesday, August 12, 2020

Cell cycle and cell division - Notes | Class 11 | Part 3: Meiosis

10. CELL CYCLE AND CELL DIVISION 

MEIOSIS

It is the division of diploid germ cells that reduces the chromosome number by half forming haploid daughter cells (gametes). 

It occurs during gametogenesis. 

It leads to the haploid phase in the life cycle of sexually reproducing organisms. 

Fertilisation restores diploid phase.

Key features of meiosis 
  • It involves two cycles (meiosis I & meiosis II) but only a single cycle of DNA replication.
  • It involves pairing of homologous chromosomes and recombination between their non-sister chromatids.
  • Meiosis I begins after replication of parental chromosomes to form identical sister chromatids at the S phase.
  • 4 haploid cells are formed at the end of meiosis II.

Meiosis I

Meiosis II

Prophase I

Prophase II

Metaphase I

Metaphase II

Anaphase I

Anaphase II

Telophase I

Telophase II


Meiosis I 

Prophase I:

It is typically longer and more complex.

It includes 5 phases based on chromosomal behaviour: Leptotene, Zygotene, Pachytene, Diplotene & Diakinesis.

a. Leptotene (Leptonema):
  • Chromatin fibres become long slender chromosomes. 
  • Nucleus enlarges.
b. Zygotene (Zygonema): 
  • Chromosomes become more condensed. 
  • Similar chromosomes start pairing together (synapsis) with the help of a complex structure called synaptonemal complex. 
  • The paired chromosomes are called homologous chromosomes. 
  • Each pair of homologous chromosomes is called a bivalent.
c. Pachytene (Pachynema): 
  • Comparatively longer phase. 
  • Bivalent chromosomes split into similar chromatids. This stage is called tetrads. During this, recombination nodules appear at which crossing over occurs. It leads to genetic recombination on homologous chromosomes.
Crossing over: The exchange of genetic material between non-sister chromatids of two homologous chromosomes in presence of an enzyme, recombinase.
  • Recombination is completed by the end of pachytene.
d. Diplotene (Diplonema): 
  • Dissolution of the synaptonemal complex occurs. 
  • The recombined homologous chromosomes of the bivalents separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata. 
  • In oocytes of some vertebrates, diplotene lasts for months or years.
e. Diakinesis: 
  • Terminalisation of chiasmata. 
  • Chromosomes are fully condensed. 
  • The meiotic spindle fibres originate from the poles to prepare the homologous chromosomes for separation. 
  • Nucleolus & nuclear envelope disappear.
Metaphase I:
  • Spindle formation is completed. 
  • The chromosomes align on the equatorial plate. 
  • The microtubules from the spindle attach to the pair of homologous chromosomes.
Anaphase I:
  • The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
Telophase I:
  • The nuclear membrane and nucleolus reappear and 2 haploid daughter nuclei are formed. This is called diad.
  • After this, cytokinesis may or may not occur.
  • After a short interphase, it is followed by meiosis II.
  • This short stage between the two meiotic divisions is called interkinesis. DNA replication does not occur in this phase. 
Meiosis II

It resembles the mitosis. It has the following phases:

Prophase II:
  • It is initiated immediately after cytokinesis. 
  • The chromosomes again become compact.
  • Nucleolus and nuclear membrane disappear in both nuclei.
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.
Anaphase II:
  • It begins with the simultaneous splitting of the centromere of each chromosome (which was holding sister chromatids together). 
  • Thus they move toward opposite poles of the cell by shortening of microtubules attached to kinetochores.
Telophase II:
  • The two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., 4 haploid daughter cells.
Significance of meiosis 
  • It conserves the chromosome number of each species.
  • It causes genetic variation (due to crossing over) in the population of organisms. It is important for evolution.
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