#Question id: 2905
#Unit 2. Cellular Organization
With respect to X-chromosome inactivation, which, if any, of the following statements are not correct?
I. X-chromosome inactivation in mammals begins in the pre-implantation embryo
II. In humans all diploid cells that carry two normal X-chromosomes are subject to a random pattern of X-inactivation.
III. Once a decision has been made to inactivate an X chromosome (either the paternal or maternal X), all descendant cells show the same pattern of X-inactivation.
IV. The inactivated X chromosome becomes a highly condensed Barr body in which genes are silenced across the length of the chromosome.
#Question id: 2906
#Unit 2. Cellular Organization
A closer look at the interactions between the histones and the nucleosomal DNA reveals the structural basis for the binding and bending of the DNA within the nucleosome. Fourteen distinct sites of contact are observed, one for each time the minor groove of the DNA faces the histone octamer. State whether the following set of statements is true/false regarding this association?
I. The majority of these associations are between the proteins and the oxygen atoms in the phosphodiester backbone near the minor groove of the DNA.
II. The association of DNA with the nucleosome is mediated by a large number (about 40) of hydrogen bonds between the histones and the DNA.
III. Maximum numbers of hydrogen bonds are made between the protein side chains and the bases, and all of these are made in the minor groove of the DNA.
IV. The highly basic nature of the histones further facilitates DNA bending by masking the negative charge of the phosphates that ordinarily resists DNA bending.
V. The positively charged nature of the histones facilitates the close juxtaposition of the two adjacent DNA helices necessary to wrap the DNA more than once around the histone octamer.
#Question id: 2907
#Unit 2. Cellular Organization
With respect to histone modifications, which, if any, of the following statements, is true?
#Question id: 2908
#Unit 2. Cellular Organization
The H3.H4 tetramers and H2A.H2B dimers each interact with a particular region of the DNA within the nucleosome. Which of the following statement is CORRECT regarding the characteristic binding of histones with the DNA?
I. Of the 147 bp of DNA included in the structure, the histone-fold regions of the H2A.H2B dimer interact with the central 60 bp.
II. Histone H3.H4 tetramers occupy a key position in the nucleosome by binding the middle and both ends of the DNA.
III. The amino-terminal region of H4 most proximal to the histone-fold region forms a fourth alpha helix that interacts with the final 13 bp at each end of the bound DNA.
IV. The H3.H4 tetramer associate with approximately 30 bp of DNA on either side of the central 60 bp of DNA bound by H2A.H2B dimer.
V. The two H2A.H2B dimers form the bottom part of the histone octamer located across the disc from the DNA ends.
#Question id: 2909
#Unit 2. Cellular Organization
The DNA can be labeled and used to probe a microarray, yielding a map of genomic sequences to which those particular nucleosomes bind. Because microarrays are often referred to as chips, this technique is called a ChIP-chip experiment. This experiment is designed to reveal the genomic DNA sequences to which a particular histone variant binds.
I. Chromatin is isolated from the cells and digested briefly with micrococcal nuclease (MNase).
II. A histone variant with an epitope tag is introduced into a particular cell type, where it is incorporated into nucleosomes.
III. The DNA bound by nucleosomes is protected from digestion, but the linker DNA is cleaved, releasing segments of DNA bound to one or two nucleosomes.
IV. The pattern of hybridization on the microarray reveals the DNA sequences bound by the nucleosomes with the histone variant.
V. Antibody is added, and the nucleosomes containing the epitope-tagged histone variant are selectively precipitated.
VI. The DNA in these nucleosomes is extracted from the precipitate, labeled, and used to probe a microarray representing all or part of the genomic sequences of that particular cell type.
Arrange the following statements in the correct order in which they are done.
#Question id: 2910
#Unit 2. Cellular Organization
With respect to the DNA methylation mechanism in mammalian cells, which of the following statements, if any, is true?
I. The principal role of the DNMT1 DNA methyltransferase is in de novo methylation.
II. The DNMT3A and DNMT3B DNA methyltransferases require hemi-methylated DNA as a substrate and are responsible for methylating nascent DNA strands that are complementary to methylated parental DNA strands.
III. Active DNA demethylation means removal of methyl groups from a hemi-methylated DNA double helix.
IV. DNA methylation is not essential in mammalian development.