Cell Cycle Overview

Cell Cycle

• The cell cycle is a series of events that prepare a cell for division into two daughter cells.
• Cell multiplication occurs through the division of existing cells.
• Cell multiplication is a key feature in embryonic development and is essential for growth and replacing dead cells after birth.
• Daughter cells need to have chromosomes with identical numbers and genetic content to the parent cell.
• Mitosis is a type of cell division.
• Meiosis consists of two successive divisions (first and second).
• In meiosis, the number of chromosomes is reduced to half the normal number.
• Genetic information in various gametes produced during meiosis is not identical.

Cell Cycle Stages

• The cell cycle is divided into two major events:
  • Interphase: A long period of time that includes growth and DNA replication.
    • Cells increase in size during Interphase.
    • Cells replicate their genetic material during Interphase.
  • Mitosis: A shorter period, where the cell divides its nucleus first, then the cytoplasm, producing two daughter cells.
    • Cytokinesis is the division of the cytoplasm
    • Cells may permanently cease mitosis (e.g., muscle and nerve cells).
    • Cells that leave the cell cycle are in a stable phase (G0 phase) or a resting stage.

Interphase

• Interphase is subdivided into three phases:
  • G1 (Gap 1) phase: synthesis of macromolecules essential for DNA replication; cell volume is restored to normal; nucleoli are re-established, and centrioles begin to duplicate.
  • S (Synthesis) phase: DNA is duplicated (autosomal cells double DNA amount from 2n to 4n); all necessary required nucleoproteins are manufactured, and integrated with DNA into chromatin; cell contains twice the normal amount of DNA.
  • G2 (Gap 2) phase: the cells undergo preparation for mitosis, synthesize RNA and proteins essential for cell division, stored energy for mitosis, and synthesize tubulin to assemble microtubules required for mitosis.

Mitosis

• Mitosis (M) occurs at the conclusion of the G2 phase, completing the cell cycle.
• Mitosis is the division of the nucleus.
• Cytokinesis follows karyokinesis, which is the division of the cytoplasm.
• Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase

Meiosis

• Meiosis is a special type of cell division resulting in the formation of gametes (sperm or ovum).
• Meiosis reduces the chromosome number from diploid (2n) to haploid (1n).
• Meiosis begins at the conclusion of interphase and produces germ cells (ova and sperm).
• Meiosis results in two crucial outcomes:
  • Reduction of chromosome number from diploid (2n) to haploid (1n)
  • Genetic recombination to ensure genetic variability and diversity.
• Meiosis is divided into two main stages (Meiosis I and II).

Meiosis I (Reductional Division)

• Separate homologous pairs of chromosomes and reduce the number from diploid to haploid.

• The DNA amount is doubled to 4n but the chromosome number remains at 2n (46 chromosomes).

• Divided into Prophase I, Metaphase I, Anaphase I, and Telophase I

  • Prophase I: homologous chromosomes line up, synapse, form tetrads, and exchange genetic material (crossing over).
  • Metaphase I: homologous chromosomes line up at the metaphase plate.
  • Anaphase I: homologous chromosomes separate and move to opposite poles.
  • Telophase I: nuclear envelopes reform, cytoplasm divides, and the two resulting cells (each with 23 chromosomes) are separated.

Meiosis II (Equational Division)

• Similar to mitosis, resulting in four haploid daughter cells, each with a single set of chromosomes.