Cell Cycle and Mitosis Notes

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).

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.

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.

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

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.

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.

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

Second phase of mitosis.
- Nuclear envelope breaks down.
- Microtubules attach to the chromosomes (at the kinetochores).

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.

Fourth phase of mitosis.
- Sister chromatids break apart and begin moving towards the poles.
- Each sister chromatid now becomes a ‘daughter chromosome’.

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.

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 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.

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.

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 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.