Remember that for her move, Ms. Sewsalot packed her threads into thread boxes, and put the thread boxes into moving boxes. All the boxes were packed the same way, regardless of which threads were in them. The packaging of DNA in a eukaryotic cell is done the same way.

While heterochromatin and euchromatin are condensed to different degrees, the basic levels of packaging are the same for heterochromatin and euchromatin. Proteins called histones are responsible for the first level of DNA packaging. Histones are proteins that bind to DNA in chromatin.There are five different types of histones: histone H1, H2, H2B, H3, and H4. Histones are positively charged so they bind to the negatively charged DNA.

DNA wraps around a complex called the histone core, which is made up of two each of histone H2, H2B, H3, and H4. In the "beads on a string" level of packaging, the histone core is the bead. Histone H1 is thought to be on the outside of the bead. The histone core is the "spool" that DNA winds around.

Histones have been highly conserved throughout evolution, and their structure varies little across a wide range of organisms. This makes sense because the structure of DNA is the same in every organism, and histones play the same role in every organism.

The complex of DNA and the histone core is called a nucleosome. The nucleosome is the basic repeating subunit of chromatin. The nucleosome is analogous to the spool and thread. Nucleosomes are packaged into a helical fiber that is 30 nanometers wide. There are six nucleosomes per turn of the helix. This packaging is analogous to placing the spools of thread into thread boxes. The 30-nanometer fiber is organized into large loops, called looped domains.

This level of packaging is like stacking thread boxes on top of each other.

For cell division, the entire chromosome is condensed into a chromosome that's visible under a microscope. Without this packaging, the DNA would become tangled and cell division could not proceed normally. Visible chromosomes are a cell's ''moving boxes" for DNA.

Here are the levels of DNA condensation. Drag the diagrams to place the order of the chromatin components from least to most organized. Drag the corresponding label to the space under each diagram. Click Submit to see if you're correct.

DNA wraps around the histone core forming a nucleosome. A series of nucleosomes along DNA is called "beads on a string." Nucleosomes are packaged into a 30-nanometer fiber, which condenses into looped domains. During cell division, the chromatin condenses into chromosomes that are visible under a microscope.

Because of the different levels of packaging, DNA is condensed when it's not being used in transcription, and decondensed for gene expression. Ms. Sewsalot leaves the threads she uses frequently in thread boxes, but stacks the boxes to keep things organized. If she packs the thread boxes into moving boxes, she can't get to the threads. Similarly, if euchromatin is packaged any further than looped domains, it isn't accessible for transcription. Heterochromatin doesn't need to be accessible, so it's condensed further than euchromatin.

One example of DNA that remains highly condensed all the time is the Barr body, a highly condensed X chromosome normally found in female mammals. The sex of mammals is determined by the X and Y-chromosomes. Females have two X chromosomes, and males have one X and one Y chromosome. Males and females must have the same overall level of gene expression from their X chromosomes. For this to occur, one of the X chromosomes in female mammals is turned off, or inactivated, by keeping it highly condensed all the time.

Early in development, a cell randomly inactivates one of its X chromosomes. As the cell divides, all the daughter cells have the same X chromosome inactivated. This produces patches of cells with different X chromosomes inactivated.

X inactivation causes the coloring of calico cats. Some fur is white, due to a gene not on the X chromosome. The gene that makes the rest of the fur either orange or black is on the X chromosome. Males have only one X chromosome, so their fur is either black or orange, but not both. In females, one X chromosome is randomly inactivated in a group of cells giving patches of either black or orange fur, depending on which X is inactivated.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education