You isolate nuclei from three different eukaryotic species. You treat the sample

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:

Approximate size of bands (in base pairs)

200, 400, 600, 800

180, 360, 540, 720

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?

Approximate size of bands (in base pairs)

200, 400, 600, 800

180, 360, 540, 720

190, 380, 570, 760

Explanation / Answer

Hi,
The 3 species DNA is given same enzymati digestion treatment. The enzyme micrococcus nuclease cleaves the DNA where the histones are not bound. That means it cuts where the linker DNA is present. It gives information about the chromatin structure. Here we have species 1 with 200bp distinct bands, so at each 200bp histones are bound to DNA. SImilary in others the histone is bound at 180bp and 190bp intervals. The linker space is different among the species.
The core-DNA is same in all the species, so 146bp standard DNA is bound to core. THe reminiang linker region is cut if the digestion is thoroughly carried out. So band sizes would be 200 - 146 =54bp ; 180-146 = 34bp; 190-146 =44bp. these bands would occur along with common 146bp region.

The typical DNA associated with histone is 146bp, the new specie core DNA has only 120bp associated with histones. The new species lack H3. The nee histone H7 may not be repplacing the H3 in the core because it is presnt in all the chromatin along with H1. These observations suggest a model in hich the core has only 6 histones, two each of H2a, H2B and H4, this also explains the smaller 120bp DNA associated with histones. The longer DNa per nucleosome is exlained by the additional H7 along with H1or in the spacer between nucleosomes.

Related Questions

Navigate

• Browse (All)
• Browse Y
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.