6.2 - Cell Cycle and Mitosis

The Cell Cycle

• The cell cycle is the sequence of events involving DNA replication and cell division to produce two daughter cells.
• It consists of interphase and the M phase.

Interphase

• The longest phase of the cell cycle.

• Consists of the $G_1$, $S$, and $G_2$ phases.

• $G_1$ phase:

  • The cell grows.
  • Cell components such as mitochondria and the endoplasmic reticulum are produced.
  • Proteins used in the cell cycle are synthesised.
  • The nucleus appears large; chromosomes are in the form of chromatin.

• $S$ phase:

  • DNA in the nucleus replicates.
  • Each chromosome duplicates into two identical sister chromatids.
  • Both sister chromatids contain identical copies of the DNA molecule and are joined at the centromere.

• $G_2$ phase:

  • The cell continues to grow and remains metabolically active.
  • The cell accumulates energy and makes final preparations for cell division.

M Phase

• Consists of mitosis and cytokinesis.
• Mitosis involves prophase, metaphase, anaphase, and telophase.

Stages of Mitosis

• Mitosis: Division of the parent-cell nucleus into two nuclei, each with the same chromosome number and genetic content as the parent-cell nucleus.

Prophase

• Chromatin shortens and thickens to form chromosomes visible under a light microscope.
• Each chromosome consists of two identical sister chromatids joined at the centromere.
• The nuclear membrane disintegrates and the nucleolus disappears.
• Centrioles move to opposite poles; spindle fibres begin to form.

Metaphase

• Centrioles are at opposite poles.
• Spindle fibres arrange the chromosomes in one line at the equatorial plane.
• Metaphase ends when the centromeres begin to divide.

Anaphase

• Centromeres divide; sister chromatids separate.
• Spindle fibres shorten and contract, pulling sister chromatids to opposite poles.
• Anaphase ends when the chromatids reach the poles.

Telophase

• At opposite poles, the chromatids are called daughter chromosomes.
• Each pole has one complete, identical set of chromosomes.
• Chromosomes become fine chromatin threads again.
• Nucleoli reappear; spindle fibres disappear; new nuclear membranes form.
• Telophase is followed by cytokinesis.

Cytokinesis

• Division of the cytoplasm after the nuclei form at the end of telophase.
• Plant cells lack centrioles but can still form spindle fibres during mitosis.

Cytokinesis in Animal Cells

• The plasma membrane constricts at the middle of the cell between the two nuclei.
• Microfilaments at the constriction contract, forming a cleavage furrow.
• The cell constricts until it separates into two daughter cells.

Cytokinesis in Plant Cells

• Vesicles fuse at the middle of the cell to form a cell plate.
• A new plasma membrane surrounds the cell plate; new cell-wall material forms between the cell plate.
• The cell plate grows outwards and fuses with the plasma membrane.
• Cellulose fibres strengthen the new cell wall.
• Two diploid daughter cells form.

The Necessity of Mitosis

• Embryonic development and organism growth: Ensures a rapid increase in cell number.
• Healing: Produces new cells to replace dead or damaged cells.
• Regeneration: Produces a new tail when a lizard loses its tail.
• Asexual reproduction: Produces new individuals, such as budding in Hydra.