Pulling the Wool Over Mitosis... or, how you can help students understand the difference between chromosomes and chromatin.

We have two ideas about DNA Think DNA... … you see a double helix.

The other picture of DNA... Think “chromosome” and you see X’s (and an occasional Y).

Chromosomes DNA is not neatly wrapped in chromosomes during most of the cell cycle Only during mitosis does DNA condense into “chromosomes.”

Chromatin During the rest of the cell cycle, DNA is in the uncoiled or CHROMATIN form.

Chromatin Model Teased sheep’s wool can model chromatin in a cell This is the wool as purchased from a crafts store

Wool-as-chromatin Pull the wool loosely apart... … explain that during Interphase DNA must be loose in order to be transcribed into RNA.

Chromatin Condensing In prophase, the chromatin must condense into chromosomes. In this model, the “chromatin” is spun into a thread.

Chromosome Taking Shape As you twirl the fibers between your fingers, the “chromosome” takes shape. Limitation: Here many fibers form one “chromosome.” In a cell, one long DNA molecule condenses to form one single chromosome.

Chromosomes Coil As you continue spinning, the “chromosome” will coil and supercoil on itself. Limitation: This model leaves out histone proteins and the specific levels of coiling and supercoiling.

chromatin supercoiling condensing

Credits and acknowledgements