Question

You isolate nuclei from three different eukaryotic species. You treat the samples in exactly the same way (adding same amount of enzyme, buffer and time) to partially digest the chromatin with micrococcal nuclease, extract the DNA, and run it on a gel. You see the pattern below:

Lane	Approximate size of bands (in base pairs)
1	200, 400, 600, 800
2	180, 360, 540, 720
3	190, 380, 570, 760

a-2pts) Knowing that the core-DNA in all cell types is the same what is your explanation for the difference in size in the patterns you observe (shown below):

b- 2pts) If you digested each of the three samples more thoroughly, what would the pattern look like? [Be specific and indicate what the size of the bands would be]

c-3pt) An explorer discovers a strange new species of plant and sends some of the plant tissue to a geneticist to study. The geneticist performs the kinds of experiments as described above in (b) except that: After digestion with nuclease 120-bp fragment of DNA are seen.
Analysis of the histone core reveals histones in the following proportions:

H2A 33.3%
H2B 33.3%
H3 0% [no histone H3 found]

H4 33.3%

On the basis of these observations, what conclusions could the geneticist make about the probable structure of the nucleosome in the chromatin of this plant? Be specific in describing the nature of the nucleosome: which histones form the core (1pt), how many of each are in there (1pt) and how much core-DNA is around it (1pt).

d-2pt) The geneticist also found H1 and a new histone H7 when histones were extracted from all of the chromatin (not just the nucleosome). What do you think the role of the new histone H7 would be in this new plant species?

Answer 1

Answer a) We have obtained these nuclei from 3 different eukaryotic species. though, their core-DNA is similar in all cells. we know that micrococcal nuclease cleaves the chromosomal DNA in the histone-free region of the nucleosome. so, cleavage with micrococcal nuclease shows the minimum length of the nucleosome.

From species 1, 200, 400, 600 and 800 represents single nucleosome, two nucleosomes, three nucleosomes and four nucleosomes respectively. where single nucleosome DNA length is 200bp.

From species 2, 180, 360, 540 and 720 represent single nucleosome, two nucleosomes, three nucleosomes and four nucleosomes respectively. where single nucleosome DNA length is 180bp.

From species 3, 190, 380, 570 and 760 represent single nucleosome, two nucleosomes, three nucleosomes and four nucleosomes respectively. where single nucleosome DNA length is 190bp.

Answer b) As we know that Single nucleosome contains core DNA of 146bp length and remaining linked DNA. it means more thoroughly digestion would result in core-DNA and linked DNA of every single nucleosome fall apart.

this way, Species 1 would show 200bp band (or core DNA).

also, Species 2 would show 180bp band (or core DNA).

similarly, Species 3 would show 190bp band (or core DNA).