Topic F: Cell and Nuclear Division

Describe the events that occur during the mitotic cell cycle.

(Similar to what occurs during Meiosis II) 

• Prophase: 

  • Chromatin condense into chromosomes. 

  • Nuclear envelope disintegrates, and nucleolus disperses.

  • Centrosome arrange microtubules into spindle fibres.  Centrosome migrate to opposite poles of the cell by lengthening spindle fibres. Radial arrangement of centrioles around centrosomes are called asters. 

• Metaphase:

  • Spindle fibres attach to the kinetochore of the chromosome’s centromere. 

  • Spindle fibres align chromosomes singly along metaphase plate. 

  • Non kinetochore microtubules will interact with one another. 

• Anaphase: 

  • Spindle fibres shortern, thus pulling sister chromatidsof a chromosome apart to opposite poles, with the centromere leading the way.

  • Once fully separated, sister chromatids are now daughter chromosomes.

• Telophase:

  • Nuclear envelope reforms around each set of daughter chromosomes.

  • Nucleolus reappears.

  • Spindle fibres disintegrate.

  • Chromosomes decondense into chromatin.

• Cytokinesis:

  • Cell surface membrane invaginates towards the metaphase plate.

  • Ring of actin microfilaments interact with myosin molecules, causing the cleavage furrow to deepen, and cleave parent cell into 2 genetically identical daughter cells. 

Describe the events that occur during the meiotic cell cycle.

• Prophase I:

  • Nuclear envelope disintegrates. Nucleolus disperses.

  • Chromatin condense into chromosomes.

  • Centrosome arrange microtubules into spindle fibres.

  • Centrosomes will move towards opposite poles of the cell.

  • Homologous chromosomes pair up as bivalents, and there is crossing over between non-sister chromatids of homologous chromosomes, resulting in the formation of chiasmata that will exchange corresponding DNA segments and give rise to new allelic combinations. 

• Metaphase I 

  • Spindle fibres will attach themselves to the kinetochore of the centromere of chromosomes and line up pairs of homologous chromosomes side by side along the metaphase plate. 

  • Independent assortment of homologous chromosomes. 

• Anaphase I: 

  • Spindle fibres will shortern, and pull chromosomes away from one another, to opposite poles of the cell.

  • Chromosomes in a pair of homologous chromosomes separate.

• Telophase I: 

  • Nuclear envelope reforms around a haploid set of chromosomes. Nucleolus reappears. 

  • Spindle fibres depolymerise

  • Chromosomes decondense into chromatin.

• Cytokinesis I:

  • Cell surface membrane invaginates towards the metaphase plate.

  • Ring of actin microfilaments interact with myosin molecules and cause cleavage furrow to deepen, cleaving parent cell into 2 genetically different daughter cells.

Explain the significance of the mitotic cell cycle.

• Asexual reproduction.

• Replacement of damaged cells by genetically identical cells.

• Growth.

Explain the significance of the meiotic cell cycle.

• Variation.

  • During Prophase I of the Meiosis cycle, there is crossing over between non-sister chromatids of homologous chromosomes, to form chiasmata and exchange corresponding DNA segments, thus resulting in new allelic combinations.

  • During Metaphase I of meiosis, there is independent assortment of homologous chromosomes. 

• Reduction Division

  • Gametes will have haploid of chromosomes number. Thus, when sperm and egg fuse during fertilisation, it restores diploid condition in the zygote, and prevents chromosomal number from doubling in every generation.

• Random fertilisation (not apart of meiosis) 

  • A random egg and random sperm will fuse together during fertilisation, resulting in the combination of genetic material from 2 individuals. 

Outline what occurs prior to Prophase (not a part of the cell cycle).

• Interphase.

• G1 Phase:

  • Extensive synthesis of ribosomal RNA for ribosomes.

  • Extensive synthesis of cell organelles.

• S Phase:

  • Semiconservative replication occurs here, resulting in the doubling of DNA content.

  • Synthesis of histone proteins.

• G2 Phase:

  • Extensive synthesis of proteins, organelles and ATP.

Outline what occurs at each cell cycle checkpoint, as well as the importance of it.

• Cell cycle checkpoints are key in ensuring that the cell undergoes normal growth and development. Dysregulation of checkpoints can result in unlimited proliferation and cell division of cells, and can lead to cancer.

• G1 Checkpoint:

  • Checks to ensure sufficient nutrients for cells.

  • Checks to ensure DNA are not damaged.

  • Occurs at the end of G1 phase.

• G2 Checkpoint:

  • Checks to ensure sufficient nutrients for the cell.

  • Checks to ensure DNA was replicated accurately during SCR of Synthesis stage of Interphase.

  • Occurs right before the start of cell cycle.

• M Phase:

  • Checks to ensure that spindle fibres are correctly attached to the centromere of the chromosomes.