D2.1.6 Condensation and movement of chromosomes as shared features of mitosis and meiosis Notes

Chromosomes Need to be Condensed and Moved

1. Human DNA stretches over 50,000 μm but must fit into a nucleus less than 5 μm wide.
2. Without condensation, DNA would tangle or break during cell division.

Histones Drive DNA Condensation Through Supercoiling

1. Histones are proteins that act as spools around which DNA wraps, forming nucleosomes, the first level of condensation.
2. Nucleosomes coil into a thicker fiber called chromatin, which further coils and folds into highly condensed chromosomes visible during cell division.
3. This process is called supercoiling.

Microtubules and Motor Proteins Move Chromosomes

Once condensed, chromosomes must be precisely moved to ensure each daughter cell receives the correct genetic material.

1. The Structure
   1. Microtubules are hollow tubes made of tubulin subunits that extend from opposite poles to form the spindle apparatus.
   2. Kinetochores are protein complexes at the centromere where microtubules attach to chromosomes.
2. The Mechanism
   1. Motor proteins walk along microtubules:
      1. Motor proteins in the kinetochore use ATP to walk toward the pole, pulling the chromosome.
   2. Microtubules shorten:
      1. Tubulin subunits are removed from microtubule ends at the kinetochore.
      2. This depolymerization shortens the microtubule, pulling the chromosome toward the pole.
3. The Process
   1. Attachment: Microtubules from opposite poles attach to kinetochores on sister chromatids.
   2. Alignment: Motor proteins and microtubule dynamics position chromosomes at the cell's equator during metaphase.
   3. Separation: During anaphase, cohesin proteins break down. Motor protein movement and microtubule shortening pull chromatids to opposite poles.

Shared Features of Mitosis and Meiosis

1. Condensation: Both use histone-mediated supercoiling to condense chromosomes.
2. Spindle formation: Both use microtubules to form a spindle apparatus.
3. Movement mechanism: Both use kinetochores, motor proteins, and microtubule shortening to move chromosomes.