Cell Cycle, DNA Replication, and Mitosis Explained

Posted on Mar 21, 2025 in Biology

The Cell Cycle

The cell cycle involves:

  • Doubling of hereditary material, which is then equally divided between daughter cells.
  • Division of the cytoplasm into two.

The cell cycle is a series of changes a cell undergoes from one division to the next, ultimately resulting in two daughter cells.

Cell Cycle Control

Cell cycle control occurs at the molecular level, regulated by cytoplasmic proteins, including cyclins and kinases. Checkpoints determine if the cell receives a signal to divide; if not, it enters a state of quiescence.

Interphase

Interphase is the period between successive mitoses, occupying most of the cell cycle. The cell grows and duplicates its genetic material. It consists of three phases:

  1. G1 Phase

    The longest phase, where proteins are synthesized for cell growth, and DNA replication begins. Cells that don’t undergo mitosis enter G0, a state of quiescence. Damaged DNA is repaired, and mRNA and proteins are synthesized, lasting until DNA replication starts.

  2. S Phase

    DNA replication occurs, and histones are synthesized. Centrioles form, and mRNA and protein synthesis continues (mainly histones).

  3. G2 Phase

    A short phase where the cell grows larger. Protein-coding genes are transcribed and translated. Centrioles are duplicated. This phase ends with chromosome condensation, initiating mitosis.

DNA Replication

DNA replication, essential for cell division, occurs during the S phase of interphase.

Watson and Crick proposed that the double helix opens, separating nucleotide chains, with each chain serving as a template for a new, complementary strand. Three models were proposed:

  • Conservative Model: Preserves the original strands, with a new double helix formed from newly synthesized strands.
  • Dispersive Model: Each daughter strand contains segments of both original and newly synthesized fragments.
  • Semiconservative Model: Proposed by Watson and Crick, where each double helix retains one original strand and synthesizes a new one.

The Meselson-Stahl experiment with E. coli confirmed that replication is semiconservative, supporting the Watson and Crick model.

Mitosis and Cytokinesis

Mitosis

Mitosis, also called karyokinesis, occurs in all somatic cells. Duplicated hereditary material from the S phase is divided into successive stages:

Prophase

  • Chromatin condenses, making chromosomes visible, each attached at the centromere.
  • Centrioles separate, forming polar microtubules (the mitotic spindle).
  • The nuclear membrane and nucleolus disappear.
  • Kinetochores form at the centromeres, from which kinetochore microtubules originate.

Metaphase

  • Chromosomes reach maximum condensation.
  • The spindle apparatus forms.
  • Kinetochore microtubules move chromosomes to the spindle equator, forming the metaphase plate.
  • Centromeres align, with each chromatid facing a pole.

Anaphase

Sister chromatids separate and move to opposite poles, pulled by shortening kinetochore microtubules. Polar microtubules elongate, separating the poles further.

Telophase

Nucleoli and nuclear membranes reappear around each group of chromosomes.