Electron microscopy of a eukaryotic cell during interphase of the cell cycle shows 10-nm thick chromatin fibers with a "beads on a string" appearance. These chromatin fibers are extracted and treated with an endonuclease, which preferentially cleaves the "string" portions of the chromatin. Further evaluation of the "beads" reveals that they are composed of DNA wrapped around a core of proteins. Which of the following proteins is most likely found outside of this core and helps promote chromatin compaction?

Question

Electron microscopy of a eukaryotic cell during interphase of the cell cycle shows 10-nm thick chromatin fibers with a "beads on a string" appearance.  These chromatin fibers are extracted and treated with an endonuclease, which preferentially cleaves the "string" portions of the chromatin.  Further evaluation of the "beads" reveals that they are composed of DNA wrapped around a core of proteins.  Which of the following proteins is most likely found outside of this core and helps promote chromatin compaction?

Quizplus · Accepted Answer

11ec160b_3763_ef18_9249_e1cd47c26c8b_MD0001_00 Nucleosomes are structural subunits within the nucleus of eukaryotic cells that are composed of DNA wrapped around a core of histone proteins. There are 5 major subtypes of histones: H1, H2A, H2B, H3, and H4. Each histone core is made up of 8 histone proteins: 2 molecules each of H2A, H2B, H3, and H4 (Choices B and C). During the initial steps of DNA packaging into chromatin, the negatively charged DNA double helix wraps around the positively charged histone core twice. In contrast to the other histone proteins, H1 is located outside of the histone core. H1 histones bind the linker segments of DNA that lie between nucleosomes and facilitate packaging of nucleosomes into more compact structures. DNA associated with histones has the appearance of "beads on a string." This structure undergoes further rounds of coiling and associates with other proteins (eg, nuclear scaffold protein), ultimately forming chromosomes. (Choice D) Small nuclear ribonucleoproteins (snRNPs) help to splice out introns from pre-mRNA, forming mature mRNA. (Choice E) During prokaryotic DNA replication, topoisomerase II (DNA gyrase) relieves the tension created during DNA strand unwinding by introducing negative supercoils into the DNA. (Choice F) Ubiquitin is a small protein present in the cytoplasm and nucleus of all eukaryotes. It is typically covalently attached to various intracellular proteins to signal for their degradation by the proteasome (ubiquitin-proteasome pathway). Educational objective: Nucleosomes are composed of DNA wrapped around a core of 8 histone proteins (2 molecules each of H2A, H2B, H3, and H4). H1 histone is located outside of this histone core and helps package nucleosomes into more compact structures by binding and linking the DNA between adjacent nucleosomes. __________ References: H1 histones: current perspectives and challenges. (http://www.ncbi.nlm.nih.gov/pubmed/23945933)

Electron Microscopy of a Eukaryotic Cell During Interphase of the Cell