Cell Division and Reproduction Quiz

Study Notes

New cells are needed for growth, replacement of damaged or lost cells, and reproduction.
There are two types of reproduction: asexual and sexual.

Asexual Reproduction:
- One cell divides into two identical cells.
- Examples: mitosis (in eukaryotes) and binary fission (in prokaryotes).
Sexual Reproduction:
- Two cells (an egg and a sperm) join to produce a new cell (zygote) that is not identical to the parent cells.
- Example: meiosis.
Prokaryotic Cell Divison (Binary Fission):
- Prokaryotes have a single circular DNA molecule.
- The DNA molecule replicates and each copy is attached to the cell membrane.
- The cell pulls apart, separating the replicated DNA molecule and the original DNA molecule.
- Two identical daughter cells are formed.
Eukaryotic Cell Division:
- Eukaryotes divide their cells when there is a need for new cells due to growth, damage, or other reasons.
- Cell division starts when the cell reaches an appropriate size with enough protein and energy.
- The Cell Cycle describes the stages of a cell's life from its division to the next division.

The cell cycle is divided into three phases:
- Interphase (G1, S, G2):
  - G1: Cell grows to an appropriate size, increases protein supply, and increases organelles.
  - S: DNA replicates.
  - G2: The cell grows further and synthesizes proteins and organelles needed for division.
- M Phase (Mitosis): Division of the nucleus.
- C Phase (Cytokinesis): Division of the cytoplasm.
Mitosis:
- Occurs only in eukaryotes.
- Some cells (nerve, brain) do not undergo mitosis.
- Four phases of mitosis: Prophase, Metaphase, Anaphase, Telophase.
Prophase:
- Chromatin condenses into chromosomes.
- Nucleolus and nuclear membrane disappear.
- Mitotic spindle forms from centrioles.
Metaphase:
- Chromosomes attach to spindle fibers and line up at the cell equator.
Anaphase:
- Spindle fibers pull sister chromatids apart, separating the chromosomes.
Telophase:
- Chromosomes decondense into chromatin.
- Nuclear envelope reforms.
- Nucleolus becomes visible.
- Spindle breaks apart.
Cytokinesis:
- The cytoplasm is divided into two daughter cells through a cleavage furrow in animal cells.
- The cleavage furrow is formed by actin and myosin contractile rings.

The cell cycle is regulated to prevent uncontrolled cell division.
Cyclins and Cyclin-Dependent Kinases (CDKs):
- Regulatory molecules involved in cell cycle control.
- Cyclins (regulatory subunits) and CDKs (catalytic subunits) form a complex that phosphorylates and activates or deactivates target proteins.
Checkpoints:
- Monitor and regulate the progress of the cell cycle.
- Prevent cell cycle progression at specific points, ensuring proper processes and DNA repair.
- G1 Check Point: Ensures everything is ready for DNA replication.
- G2 Check Point: Ensures the cell is ready for mitosis.
- Metaphase Checkpoint: Ensures the cell is ready to complete cell division.

Loss of cell cycle control can lead to uncontrolled cell division and tumor formation.
A damaged p53 gene can lead to cancer cells that do not respond to cell cycle control signals.
Cancer cells with defective p53 continue dividing uncontrollably, forming tumors.

Cancer is a failure of cell division control causing uncontrolled tissue growth.
This uncontrolled growth occurs due to the loss of checkpoint stops.
The p53 gene plays a key role in the G1/S checkpoint, halting cell division if damaged DNA is detected, but cancer cells often shut down p53 activity.

Necrosis (Unregulated Cell Death):
- Cell death in an organ or tissue due to disease, injury, blood supply failure.
Apoptosis (Regulated Cell Death):
- An energy-dependent programmed cell death that eliminates unwanted cells.

Apoptosis helps to eliminate excess cells during embryonic development, tissue remodeling, and to remove cells that are no longer needed.
Examples:
- Removing excess cells between fingers and toes during embryonic development.
- Eliminating cells during the metamorphosis of a tadpole into a frog.
- Removing cells that are no longer needed in the mammary gland after lactation.
- Removing cells in the prostate gland regulated by testosterone.

Too Much Apoptosis:
- Can lead to tissue atrophy, neurodegeneration, and thin skin.
Too Little Apoptosis:
- Can lead to hyperplasia, atherosclerosis, and cancer.