Cell Cycle and Mitosis

Introduction to the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. This process is fundamental to the development, growth, and maintenance of all living organisms.

• Cell division is the process by which new cells are produced from pre-existing cells.

• Observations in plants and animals confirm that:

  • All multicellular organisms start life as single-celled embryos.

  • Growth occurs through a series of cell divisions.

• There are two main types of cell division:

  • Mitosis: Produces most cells, called somatic cells.

  • Meiosis: Produces the gametes (egg and sperm).

Mitosis vs. Meiosis

• Mitosis produces cells that are genetically identical to the original cell and to each other.

  • Genetic material is copied and divided equally between two cells.

  • Responsible for growth, repair, and maintenance in multicellular organisms.

• Meiosis produces cells that are genetically different from the parent cell.

  • Genetic material is divided in half, resulting in gametes with half the chromosome number.

  • Occurs only in specialized reproductive cells.

Phases of the Cell Cycle

Overview of Cell Cycle Phases

The cell cycle alternates between two main phases: Interphase and M phase (mitotic or meiotic phase).

• Interphase (G1, S, G2): Non-dividing phase.

  • The cell grows and fulfills its specialized functions.

  • Genes on chromosomes are transcribed to make proteins needed by the cell.

  • Chromosomes are replicated only before proceeding to M phase.

• M phase (Mitosis or Meiosis): Dividing phase.

  • The cell separates chromosomes and divides to create two cells.

Chromosomes and Genetic Material

• A chromosome contains a single long double helix of DNA wrapped around proteins called histones.

• Each species has a characteristic number of chromosomes in each somatic cell (e.g., humans have 46).

• A gene is a specific segment of DNA on a chromosome that codes for a specific mRNA and, ultimately, a specific protein.

Chromosome Replication

• Before a cell divides, each chromosome is replicated during the S phase of interphase.

• The two identical copies are called sister chromatids, attached at a region called the centromere.

• Chromosomes condense and become visible only when the cell begins mitosis.

Mitosis: Mechanism and Phases

Overview of Mitosis

Mitosis is a continuous process divided into five sub-phases, followed by cytokinesis. It ensures equal distribution of genetic material to daughter cells.

1. Prophase: Chromosomes condense, and the spindle apparatus begins to form.

2. Prometaphase: The nuclear envelope breaks down, and microtubules contact chromosomes at the kinetochore.

3. Metaphase: Chromosomes align at the middle of the cell (metaphase plate).

4. Anaphase: Sister chromatids separate and are pulled to opposite poles of the spindle apparatus.

5. Telophase: Nuclear envelope reforms around each set of chromosomes, and chromosomes de-condense.

• Cytokinesis follows mitosis, dividing the cytoplasm and resulting in two daughter cells.

Structures Involved in Mitosis

• Centrosomes: Organize the spindle apparatus; each contains a pair of centrioles in animal cells.

• Spindle apparatus: Made of microtubules that separate chromosomes.

• Kinetochore: Protein complex at the centromere where spindle fibers attach.

Movement of Chromosomes During Anaphase

During anaphase, microtubules attached to kinetochores shorten by depolymerizing at the kinetochore end, pulling sister chromatids toward opposite poles.

Cytokinesis

• In animal cells, a contractile ring of actin and myosin filaments forms beneath the plasma membrane, pinching the cell in two (cleavage furrow).

• In plant cells, vesicles deliver materials to form a cell plate that develops into a new cell wall.

Summary Table: Mitosis Phases and Key Events

Key Terms and Definitions

• Somatic cells: All body cells except gametes.

• Gametes: Reproductive cells (egg and sperm).

• Sister chromatids: Identical copies of a chromosome, attached at the centromere.

• Centromere: Region where sister chromatids are joined and spindle fibers attach.

• Kinetochore: Protein structure on the centromere for spindle attachment.

• Spindle apparatus: Structure made of microtubules that segregates chromosomes.

Example: Human Cell Division

• Human somatic cells have 46 chromosomes (23 pairs).

• After DNA replication, each chromosome consists of two sister chromatids.

• During mitosis, each daughter cell receives 46 chromosomes, identical to the parent cell.

Practice Questions (from notes)

• At what phase are chromosomes not replicated?

• When do chromosomes first condense?

• When do sister chromatids first attach to the spindle?

• At what stage are sister chromatids no longer attached to each other?

• When are there two nuclei in one cell?

Additional info: For further study, students are encouraged to watch time-lapse videos of mitosis in animal and plant cells, as referenced in the notes.