how 23 pairs of chromosome are made from the chromatin fibre with detail diagram

During interphase and M phase, the chromatin fibres wrap around histone proteins and form nucleosomes. The DNA in nucleosomes condenses further in M phase to form chromosomes. A single strand of double helix DNA or chromatin condenses many folds and forms a single chromosome. As there are 23 pairs of similar single stranded double helix DNA or chromatin fibres in each diploid human cell, these condense to form 23 pairs of homologous chromosomes.

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Meiosis

Specialized kind of cell division that reduces the chromosome number by half (thus called reductional division) resulting in production of haploid daughter cells (gametes)
Haploid gametes fuse in sexual reproduction to give rise to diploid cells.
Meiosis I starts after DNA has been replicated in S phase. Meiosis I is followed by meiosis II at the end of which four haploid cells are formed.

Meiosis I

Meiosis I is divided into 4 phases.

Prophase I longest phase and is further sub-divided into 5 phases
- Leptotene Condensation makes chromosomes become distinct and compact.
- Zygotene Homologous chromosomes start pairing together by a process calledsynapsisto form a complex structure calledsynaptonemal complex. Two synaptonemal complexes further form a complex called bivalent or tetrad.
- Pachytene Longest phase of prophase IRecombination nodules appear in this stage at the sites where crossing over has to take place between non-sister chromatids of homologous chromosomes.
  Actual reason for genetic difference in progenies
  Crossing is mainly responsible for the genetic difference. Crossing over is the exchange of genetic material between two homologous chromosomes with the help of enzymerecombinase. It results in recombinant homologous chromosomes.
- Diplotene Synaptonemal complex dissolves and recombinants separate from each other except at crossover sites to form X-shaped structure calledchiasmata.
- Diakinesis Chiasmata terminalises and chromosomes condense. Mitotic spindle assembles and nucleolus and nuclear envelope disappear.

Metaphase I Bivalent chromosomes align on the equatorial plate and spindle fibres appear and attach to the homologous chromosomes.
Anaphase I Homologous chromosomes separate; sister chromatids remain attached at their centromeres.
Teleophase I Nuclear membranes and nucleolus reappear. Cytokinesis follows.
Interkinesis Stage between meiosis I and meiosis II

Meiosis II Resembles Mitosis

Prophase II Chromosomes become compact and nuclear membrane disappears.
Metaphase II Chromosomes align on equatorial plate and spindle fibres appear and attach to kinetochores of sister chromatids.
Anaphase II Centromere of each chromosome splits and sister chromatids move towards opposite poles of cells.
Telophase II Nuclear envelope reappears and cytokinesis follows, resulting in formation of a tetrad (4 haploid cells).