NCERT Solution Class 11th Biology Chapter – 10 Cell Cycle and Cell Division Notes

NCERT Solutions Class 11th Biology Chapter – 10 Cell Cycle and Cell Division

NCERT Solutions Class 11th Biology Chapter – 10 Cell Cycle and Cell Division

?Chapter – 10?

✍Cell Cycle and Cell Division✍

?Notes?

 According to the cell theory, cells arise from pre-existing cells. The process by which this occurs is called cell division.

 Any sexually reproducing organism starts its life cycle from a single-celled zygote.

 Cell division does not stop with the formation of the mature organism but continues throughout its life cycle.

 The stages through which a cell passes from one division to the next are called the cell cycle.

 The cell cycle is divided into two phases called

1. interface a period of preparation for cells division, and
2. Mitosis the actual period of cell division.

 Interphase is further subdivided into G₁, S₁, and G₂ G₁ phase is the period when the cell grows and carries out normal metabolism.

 Most of the organelle duplication also occurs during this phase.

 S phase marks the phase of DNA replication and chromosome duplication. The G₂ phase is the period of cytoplasmic growth.

 Mitosis is also divided into four stages namely prophase, metaphase, anaphase, and telophase. Chromosome condensation occurs during prophase.

 Simultaneously, the centrioles move to the opposite poles.

 The nuclear envelope and the nucleolus disappear and the spindle fibers start appearing.

 Metaphase is marked by the alignment of chromosomes at the equatorial plate.

 During anaphase, the centromeres divide and the chromatids start moving towards two opposite poles.

 Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus, and the nuclear membrane reappear.

 This stage is called telophase.

 Nuclear division is followed by the cytoplasmic division and is called cytokinesis

 Mitosis thus is the equational division, in which the chromosome number of a parent is conserved in the daughter cell.

 In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. It is called the reduction division since it reduces the chromosome number by half while making the gametes.

 In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the parent cell.

 Meiosis is devided into two phases-meiosis 1 and meiosis II. In the first meiotic division the homologous chromosomes pair to form bivalents, and undergo crossing over.

 Meiosis I has long prophase, which is divided further into five phases. These are leptotene, zygotene, pachytene, diplotene and diakinesis.

 During metaphase I the bivalents arrange an equatorial plate. This is followed by anaphase 1 in which homologous chromosomes move to opposite poles with both their chromatids.

 Each pole receives half the chromosome number of the parent cell.

 In telophase I the nuclear membrane and nucleolus reappear.

 Meiosis II is similar to mitosis.

 During anaphase II the sister chromatids separate. Thus at the end of meiosis four haploid cells are formed.

 Equational division – M phase is the most dramatic period of the cell cycle, involving a major reorganization of virtually all cell components. Since the chromosome, number (ploidy) of parent and progeny cell is the same it is also called equational division.

 Cytokinesis – Mitosis accomplishes the segregation of duplicated chromosomes into daughter nuclei (Karyokinesis), but the cell itself is divided into two daughter cells by a separate process called cytokinesis at the end of which cell division is complete.

 Cell-plate – The formation of the new cell wall begins with the construction of a simple precursor, called the cell plate that represents the middle lamella between the walls of two adjacent cells.

 Syncytium – In some organisms’ karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises which is called syncytium.

 Meiosis – The specialized kind of cell division that reduces the chromosome number by half, resulting in the production of haploid daughter cells, is called meiosis.

 Meiosis I and Meiosis II – Meiosis involves two sequential cycles to nuclear and cell division, called meiosis I and meiosis II but only a single cycle of DNA replication.

 Synapsis – Zygotene is the second stage of prophase I during which certain chromosomes start pairing together and this process of association is called synapsis.

 Synaptonemal complex – Electron complex structure called synaptonemal complex.

 Bivalent – The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.

 Interkinesis – The stage between the two meiotic divisions is called interkinesis and is generally short-lived.

 Crossing Over – Exchange of similar segments between non-sister chromatids of homologous chromosomes usually takes place during the Pachytene stage.