Cell Cycle and Mitosis Notes

Cell Replication

• Reasons for Cell Replication:

  • A Sexual Reproduction:

    • Asexual reproduction produces offspring genetically identical to the parent.

    • Examples: fungi (like yeast) and protists.

    • Parent cell divides to form two new daughter cells.

  • Growth & tissue renewal

    • Multicellular organisms increase in size or replace cells through cell replication.

    • Somatic cells (body cells, not sperm or eggs) replicate for growth and tissue renewal (e.g., skin, liver, heart, neurons, thyroid).

General Process of Cell Replication

• Cell replication follows a general process:

  1. Replication of DNA.

  2. DNA copies separated.

  3. Cell splits to produce two genetically identical daughter cells.

• Cell cycle in eukaryotic cells (like humans).

• Binary fission (binary “two” + fission “split”) in prokaryotic cells.

Overview of the Cell Cycle (Eukaryotic)

• Three stages:

  1. Interphase

     • DNA replication occurs here.

  2. Mitosis (M-phase)

     • DNA copies separated.

  3. Cytokinesis (M-phase)

     • Division of cytoplasm, resulting in cell splitting in two.

DNA Replication

• DNA replication results in a chromosome with two sister chromatids

• During mitosis, sister chromatids are separated

Interphase: Preparation for Cell Division

• Interphase: Prepares the cell for division.

  • G1 phase (First gap phase):

    • Cell grows physically in size.

    • Makes proteins needed for DNA replication.

  • S phase (Synthesis phase):

    • DNA is replicated.

  • G2 phase (Second gap phase):

    • Cell continues to grow.

    • Makes more organelles.

    • Makes microtubules.

    • Preparation and growth should be nearly finished as the cell transitions to mitosis.

Mitosis

• Five phases (stages) of mitosis: prophase, prometaphase, metaphase, anaphase, and telophase

Prophase

• First phase of mitosis.

  • DNA packs tightly into visible chromosomes (condensation of chromatin).

  • Microtubule spindle starts to assemble on either side of the cell.

  • Centrosomes migrate to poles.

Key Terms

• Chromosome: Packaged and structured DNA molecule.

• Centromere: (centro “center” + mer “part) Holds the two sister chromatids together.

• Kinetochore: (kinetic “movement” + chore “place”) Attaches a sister chromatid to spindle fibers.

Spindle Organization

• The spindle organizes and later separates the replicated chromosomes.

• Spindle Composition:

  1. Microtubules: Form the spindle.

  2. Centrosomes: Organize the spindle.

• Mitotic spindle helps organize cell division

Prometaphase

• Second phase of mitosis.

  • Nuclear envelope breaks down.

  • Microtubules attach to the chromosomes (at the kinetochores).

Metaphase

• Third phase of mitosis.

  • Chromosomes are arranged along an imaginary axis (the ‘metaphase/equatorial plate’) at the cell’s equator.

  • All chromosomes are attached to microtubules via kinetochores.

Anaphase

• Fourth phase of mitosis.

  • Sister chromatids break apart and begin moving towards the poles.

  • Each sister chromatid now becomes a ‘daughter chromosome’.

Telophase

• Fifth phase of mitosis.

  • The chromosomes will loosen (or decondense) into chromatin.

  • A nuclear envelope will form around each set of chromosomes.

  • Cleavage furrow forms in the middle.

Cytokinesis

• Not an actual stage of mitosis (overlaps with telophase).

• Cytokinesis is the division of cytoplasm, resulting in two daughter cells.

• Microfilament ring contracts and pinches cell in half.

Regulation of Cell Division and Checkpoints

• During mitosis, the cell must pass checkpoints to continue the cell cycle properly.

• Failures at checkpoints result in delays or even death of the cell.

Cancer Cells

• Cancer cells are cells that have lost control over cell division and divide uncontrollably.

  • Ignore checkpoints

  • Replicate in the absence of a signal to go

  • Proliferate in an uncontrolled manner

• Cancer can occur in different cell types and differ in the particular failed checkpoint.

Loss of Control

• Cancer cells do not respond normally to the body’s control mechanisms

  • Accumulation of mutations in genes that control the cell cycle (tumor suppressor genes and proto-oncogenes).

• Cancer cells may not need growth factors to grow and divide

  • They may make their own growth factors (signals to grow).

  • They may respond to a growth factor’s signal without the presence of the growth factor.

  • They may have an abnormal cell cycle control system.

Benign vs. Malignant Tumors

• If abnormal cells remain only at the original site and stop growing, the lump (mass of cells) is called a benign tumor. A benign tumor is not considered cancerous.

• A malignant tumor is cancerous. It grows and spreads throughout the body.

• Tumor does not necessarily mean cancer (they are not the same things)

Malignant Tumors

• Malignant tumors invade surrounding tissues and can metastasize, spreading cancer cells to other parts of the body, where they may form additional tumors.

• Metastasis – noun version of the process.

Vocabulary

• Asexual reproduction

• Mitosis

• Binary Fission

• Interphase

• G₁ phase

• S phase

• G₂ phase

• M-phase

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase

• Cytokinesis

• Chromosomes

• Sister chromatids

• Centromeres

• Centrosomes

• Kinetochores

• Mitotic spindle

• Spindle fibers (microtubules)

• Metaphase plate

• Cell cycle checkpoints

• Cancer