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

http://www.biotec.or.th/Genome/chromosome.JPG

http://www.efa.org/images/research/chromosome.gif

http://www.gifs.net/animate/webl2.htm