Cell Division: Mitosis

Overview of Cell Division

• Organisms reproduce, distinguishing living entities from nonliving matter.

• The continuity of life relies on cell division, allowing for reproduction of cells.

Chromosomes and Cell Division

• Every eukaryotic species has a specific number of chromosomes in each cell nucleus.

  • Somatic cells: Non-reproductive cells.

  • Gametes: Reproductive cells (sperm and eggs).

  • Eukaryotic chromosomes are made of chromatin.

Distribution of Chromosomes During Eukaryotic Cell Division

• Cell division involves DNA replication and chromosome condensation.

• Each duplicated chromosome has two sister chromatids, connected at the centromere.

Homologous Chromosomes

• Features of homologous chromosomes:

  • Same length and centromere position.

  • Same banding pattern and gene locations, but may differ in alleles.

  • One chromosome of each pair is inherited from each parent (e.g., ovum and sperm).

The Cell Cycle

• Interphase (90% of the cell cycle) comprises three subphases:

  • G1 phase: First gap; cell growth.

  • S phase: Synthesis; DNA replication.

  • G2 phase: Second gap; final growth and preparation for division.

• Mitosis is divided into five phases:

  • Prophase

  • Prometaphase

  • Metaphase

  • Anaphase

  • Telophase

• Cytokinesis: Division of the cytoplasm occurs alongside mitosis.

Phases of Mitosis

Prophase

• Chromosomes condense and become visible; two sister chromatids appear.

• Spindle apparatus begins to form.

• The nuclear envelope breaks down.

• The cytoskeleton disassembles.

Prometaphase

• Transition occurs after nuclear envelope disassembly.

• Kinetochore microtubules attach to sister chromatids.

• Chromosomes begin to move towards the center of the cell.

Metaphase

• Chromosomes align at the metaphase plate, the future axis of cell division.

• Each chromosome is oriented with kinetochores connected to opposite spindle poles.

Anaphase

• Centromeres split, allowing sister chromatids to be pulled apart to opposite poles.

• Proteins holding the centromeres degrade, freeing individual chromosomes.

Telophase

• Spindle apparatus disassembles; nuclear envelope reforms around the sets of chromosomes.

• Chromosomes begin to uncoil, preparing for interphase.

Cytokinesis

• In animal cells, it occurs via cleavage furrow formation.

• In plant cells, a cell plate forms as part of cytokinesis.

Cell Cycle Control System

• The cell cycle is regulated by a control system, influenced by internal and external factors.

• Specific checkpoints decide if the cell cycle continues or stops:

  • G1/S checkpoint: Determines if the cell will divide.

  • G2/M checkpoint: Assesses DNA replication success.

  • Late metaphase checkpoint: Ensures all chromosomes are properly attached.

Growth Factors and Checkpoints

• Growth factors influence the G1 checkpoint; if a signal is received, the cell progresses through the cycle. If not, it enters G0, a non-dividing state.

• Cancer cells often bypass normal growth controls, leading to unregulated division and tumor formation.

Cancer Cell Characteristics

• Cancer cells can grow without growth factors, produce their own growth signals, or have dysfunctional cell cycle controls.

• Tumors are masses of abnormal cells that can be benign (localized) or malignant (invading other tissues).

• Malignant cells can metastasize, forming secondary tumors elsewhere in the body.

Chromosomes consist of DNA tightly coiled around proteins called histones, forming a structure that carries genetic information. They play a crucial role in ensuring accurate distribution during cell division.

Chromosome vs. Chromatids:

• A chromosome is made up of two sister chromatids when duplicated, which are identical copies connected at a region called the centromere. A chromatid exists when the chromosome has been copied but before the sister chromatids separate during cell division.

Homologous Chromosomes:

• Homologous chromosomes are pairs of chromosomes that have the same length, centromere position, and gene locations, but may differ in alleles. One chromosome of each pair is inherited from each parent (e.g., one from the mother and one from the father).

Cell Cycle Phases:

1. Interphase (90% of the cell cycle)

   • G1 Phase: The first gap where the cell grows.

   • S Phase: Synthesis phase where DNA replication occurs.

   • G2 Phase: The second gap for final growth and preparation for division.

2. Mitosis: Divided into five phases:

   • Prophase: Chromosomes condense and become visible; the spindle apparatus forms, and nuclear envelope breaks down.

   • Prometaphase: The nuclear envelope fully disassembles, and kinetochores attach to microtubules.

   • Metaphase: Chromosomes align at the metaphase plate, oriented for division.

   • Anaphase: Sister chromatids are pulled apart to opposite poles.

   • Telophase: The nuclear envelope reforms and chromosomes begin to uncoil.

3. Cytokinesis: Division of the cytoplasm occurs alongside mitosis; in animal cells, it involves cleavage furrow formation, while in plant cells, a cell plate forms.

Spindle:

• The spindle apparatus is made of microtubules that help separate sister chromatids during mitosis. It connects to the kinetochores of chromosomes, facilitating their movement toward opposite poles of the cell.

Plant vs. Animal Cells During Mitosis:

• In animal cells, cytokinesis occurs via cleavage furrow formation, while in plant cells, a cell plate forms as a part of cytokinesis.

Checkpoints in the Cell Cycle:

• G1/S Checkpoint: Determines if the cell will divide.

• G2/M Checkpoint: Assesses whether DNA replication has been successful.

• Late Metaphase Checkpoint: Ensures all chromosomes are properly attached to the spindle apparatus before division proceeds.

Cancer Cells:

• Cancer cells can grow without external growth factors, produce their own growth signals, or have dysfunctional cell cycle controls. This leads to unregulated division and the formation of tumors, which can be benign or malignant. Malignant cells may metastasize, invading other tissues and forming secondary tumors