Researchers studying molecular biology observe a eukaryotic cell via electron microscopy. During interphase of the cell cycle, they notice 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 reveals that the "beads" are composed of DNA wrapped around a core of proteins. Which of the following proteins is most likely found outside this core and promotes chromatin compaction?
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Chromatin is made up of DNA (negatively charged) wrapped twice around an octamer of histone proteins (positively charged). This core of histones is composed of 2 molecules each of histones H2A, H2B, H3, and H4. Together, these histone proteins and wrapped DNA are known as a nucleosome. Nucleosomes are separated by a stretch of DNA, or linker DNA, giving them the appearance of beads on a string. This loose structure of DNA organization is about 10 nm in diameter and provides a high degree of transcriptional access (euchromatin).
The degree to which nucleosomes can be compacted is dynamic and changes based on epigenetic modification (eg, histone acetylation) and binding of additional DNA structural proteins. One such protein is histone H1, which, in contrast to the other histone proteins, is located outside the nucleosome core. Histone H1 binds to both the nucleosome and adjacent linker DNA, which facilitates packaging of chromatin into a thicker (30-nm), more compact structure (heterochromatin) that limits transcriptional access to the DNA.
During cell division, chromatin interacts with additional proteins (eg, nuclear scaffold protein) and undergoes further rounds of coiling, ultimately forming condensed chromosomes.
(Choices B and C) Histones H3 and H4 are components of the nucleosome core.
(Choice D) Small nuclear ribonucleoproteins help splice out introns from pre-mRNA, forming mature mRNA.
(Choice E) During DNA replication, topoisomerase relieves the tension created during DNA strand unwinding by introducing negative supercoils into the DNA. It does not promote chromatin compaction.
(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 histone proteins. Histone H1 is unique in that it is located outside this histone core and helps package nucleosomes into more compact structures, limiting transcriptional access to DNA.