DNase I cuts DNA that is not protected by bound proteins but is unable to cut DNA that is complexed with proteins. Human DNA is isolated, stripped of its nonhistone proteins, and mixed with DNase I. Samples are removed after 30 minutes, 1 hour, and 4 hours and run separately in gel electrophoresis. The resulting gel is stained to make all DNA fragments in it visible, and the results are shown in the figure. DNA fragment sizes in base pairs (bp) are estimated by the scale to the left of the gel. Draw a conclusion about the organization of chromatin in the human genome from this gel.

Key Concepts

Chromatin Structure

Chromatin is a complex of DNA and proteins found in the nucleus of eukaryotic cells. It exists in two forms: euchromatin, which is loosely packed and accessible for transcription, and heterochromatin, which is tightly packed and generally inactive. The organization of chromatin influences gene expression and DNA accessibility, making it crucial for understanding how DNase I interacts with DNA.

DNase I Activity

DNase I is an enzyme that cleaves DNA, specifically targeting regions that are not protected by proteins. Its activity is indicative of the accessibility of DNA within chromatin. When DNA is complexed with proteins, such as histones, DNase I cannot cut it, which helps researchers infer the structural organization of chromatin based on the presence or absence of DNA fragments in gel electrophoresis.

Gel Electrophoresis

Gel electrophoresis is a laboratory technique used to separate DNA fragments based on their size. When DNA samples are subjected to an electric field, smaller fragments move faster through the gel matrix than larger ones. By analyzing the resulting banding pattern, researchers can determine the sizes of DNA fragments, which provides insights into the effects of DNase I treatment and the organization of chromatin in the human genome.