Text on screen: Recall that DNA is a double helix before packaging Text on screen: Recall that DNA is a double helix before packaging. However, in cells DNA is rarely found in this extended form.

Movement: show the histones coming together to form the octamer Text on screen: Normally in cells, Eight histones come together to make the histone octamer which begins the process of packaging the DNA.

Core particle nucleosome Movement: If possible show the DNA being attracted to the histone octamer and wrapping around. Then zoom out to show lots of these nucleosomes linked (see next slide) Text on screen: The histone octamer is positively charged and DNA is negatively charged, so the DNA is strongly attracted to the octamer. This results in the DNA wrapping around the octamer. DNA usually wraps around the octamer 1 ¾ times, the remaining 50 base pairs form a linker which extends from the core particle to the next nucleosome.

11 nm Continue zooming out to show many of these on screen not just three. Text on screen: This packaging level is called the “beads-on-a-string” form of DNA.

H1 histone This arrow is indicating how the parts should move in the animation. Movement: from previous slide to this one, zoom in to show this image of the globular H1 protein on the “bead”, then out to show at least 3 beads in the frame with H1 proteins, now show the DNA which is between the beads being pulled closer to the beads which should make the “beads” get closer together. Text on screen: A 9th histone, named H1 is needed to further compact the DNA. The globular portion of this histone pulls the linker DNA closer to the nucleosome as it exits the nucleosome pulling all the nucleosomes core particles closer to one another.

For clarity I made these nucleosomes different colors Movement: Zoom out more to show this image of many “beads” which are getting pulled closer and closer. They should “stack” in a circular fashion. Imagine you had three marbles which fit in the bottom of a tube then put three more on top, they would rotate positions. On a clock: 12, 4, 8 then 2, 6, 10 then 12, 4, 8 etc.. Text on screen: Like actual beads would align in a column the nucleosomes are pulled into a stacked array called…

This slide should show the stacked array in good detail on the left then fade away into the far distance. I tried to show that but it looks a little ugly. Movement to next slide: fade out to very long section of straightish chromatin fiber Text on screen: the 30 nm chromatin fiber. This is the form of DNA most often found in cells. Sometimes further condensing is needed though, such as for mitosis.

Chromatin Movement: show long fiber being pulled (arrows in image of to indicate direction of movement) from straightish into the next slide image. The arrows should not be shown in the actual animation Text on screen: When further packaging is required special proteins called scaffolding proteins are believed to pull the 30 nm chromatin fiber into large loops.

300 nm Movement: continuation from previous image to this image. Text on screen: The details of the proteins and mechanisms involved are not known but the final structure resembles this and is 300 nm thick.

Movement: The animation should start with the previous image then zoom out to the X shape but start coiling from the middle and move outwards in both the up and down directions (along the arms of the X) Text on screen: After protein scaffolding the structure begins to coil around its self into the well known metaphase chromosome structure. The fully packed DNA chromosome is 10,000 times shorter than the same length of extended double helix.

Movement: These changes should occur while the words are appearing on screen. Text on screen: Enzymes facilitate the tight packing of DNA wrapped histones. This highly packed section is called heterochromatin and little to no transcription occurs here. Then different proteins facilitate the unpacking of DNA for transcription and replication. This structure is called euchromatin.

Text on screen: DNA Packaging Overview This slide is meant to show an overview of the entire packaging steps. It would be nice for it to be a running animation from the left to right. Text on screen: DNA Packaging Overview